NBSIR 75-819

SENSITIVE ISOTROPIC ANTENNA WITH FIBER-OPTIC

LINK TO A CONVENTIONAL RECEIVER

E.B. Larsen

J.R. Andrews

£.E. Baldwin

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

September 1976

Final Report

ECOM Contract No. 74-93095

Prepared for

U.S. Army Electronics Command

AMSEL-NL-C

Fort Monmouth, New Jersey

NBSIR 75-819

SENSITIVE ISOTROPIC ANTENNA WITH FIBER-OPTIC

LINK TO A CONVENTIONAL RECEIVER

E.B. Larsen

J.R. Andrews

E.E. Baldwin

Electromagnetics Division . Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

September 1976

Final Report i ECOM Contract No. 74-93095

Prepared for

U.S. Army Electronics Command

AMSEL-NL-C

Fort Monmouth, New Jersey

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward 0. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

CONTENTS

Page

1. INTRODUCTION 1 1.1 Background to the Measurement Problem 1 1.2 Objectives of the ECOM/NBS Contract 3 1.3 Characteristics of an Ideal rf Probe 4 2. TECHNICAL APPROACH 5 3. OPTICAL TRANSMISSION LINKS 7 3.1 Discussion of Infrared Fiber Optics 7 3.1.1 System Considerations [16-22] 7 3.1.2 Infrared Sources [23] 7 3.1.3 Fiber Transmission Lines [24, 25 and 26] 9 3.1.4 Infrared Photodetectors 10

3.2 Tests of Fiber-Optic Components 10

3.3 Measurements of Two Infrared Optoelectronic Links 12

3.3.1 Performance of a Radiated System 12

3.3.2 Performance of a Fiber-Optic System 13

4. FIELD STRENGTH AND ANTENNA CALIBRATIONS 14

4.1 Discussion of Field Strength Meters 14

4.2 Auxiliary Calibration of the Receiver as a Tunable rf

Voltmeter 15

4.3 Standard Field Setup Used to Calibrate the Active Antenna 16

5. DESIGN OF A S INGLE -DIPOLE FIBER-OPTIC ACTIVE ANTENNA 17

5.1 Description Of the Dipole Unit and APD Circuit 17

5.2 Preliminary Tests of the Active Dipole System 18

5.3 Effects of Noise and Receiver Bandwidth on Sensitivity

[30 and 31] 18

5.4 Measurements of the System Sensitivity 2 0 5.4.1 Sensitivity of the Active Dipole 20

5.4.2 Effect of Using an Antenna Resonating Inductor 21

5.4.3 Effect of Non-coherence in the Fiber Bundle 22

5.5 Overall Antenna Performance vs. Frequency: Tangential

Sensitivity, Linearity, Dynamic Range and Antenna Factor 23

5.6 Overloading of the Active Antenna System by a Strong

Interfering Signal 24

iii
CONTENTS (Continued)

Page

6. DESIGN AND FABRICATION OF THE COMPLETE ISOTROPIC ANTENNA AND

CONTROL UNIT 25

6.1 Description of the System 25

6.2 Avalanche Photodiode Circuitry 26

6.3 rf/IR Calibrator Circuit 27

6.4 Interface with the EMC-2 5 Receiver 28

6.5 Vector Magnitude Analog Computer 29

6.6 X-Y-Z Measurement Sequencer • 31

6.7 Auxiliary Equipment 32

7. SUMMARY AND CONCLUSIONS 33

8. RECOMMENDATIONS AND FUTURE PLANS 36

9. ACKNOWLEDGMENTS 37

10. REFERENCES 37

iv
LIST OF FIGURES

1 Typical test setup for measuring radiated emissions,

MIL-STD-461A, RE02 41

2 Required measurement sensitivity for MIL-STD-461A — 42

3 Block diagram of the broadband, isotropic, fiber-optic

antenna system developed at NBS 43

4 Typical emission spectra or response of various optical

sources and detectors ' 46

5 Intensity of radiation vs. forward current for a typical

infrared LED ' 46

6 Measured performance of avalanche photodiode No. 2 at an ambient temperature of 28 °C 48

7 Instrumentation for an experimental radiated communications

link 49

8 Performance of the experimental radiated link at a signal

frequency of 25 MHz 50

9 Simplified schematic of the instrumentation for measuring rf

attenuation of the fiber-optic link -1-

: — 51

10 Attenuation vs. frequency of the fiber-optic system 52

11 Block diagram of a typical EMI receiver 53

12 Nominal antenna factor for a thin A/2 dipole and 50

receiver 55

13 Instrumentation for calibrating a receiver to measure CW

voltage 56

14 Linearity of the receiver used (EMC-25, Band 11, narrow

bandwidth, 25 MHz) 57

15 Instrumentation used to establish a known E field in the TEM

cell for calibrating the active dipole 58

16 TEM transmission cell used to generate a known EM field for

calibrating transfer probes and active antennas 59

17 Schematic diagram of the rf amplifier and LED circuit in

each active dipole antenna 60

18 Equivalent circuit of the active antenna 61

19 Simplified schematic of the avalanche photodiode circuitry— 62

20 Test setup for observing the system gain, frequency response

and step response 6 3

v
LIST OF FIGURES (Continued)

Figure Page

21 Signals received in the laboratory by a prototype active

dipole. (Horizontal scan from DC to 200 MHz , spectrum

analyzer bandwidth = 3 kHz.) 64

22 Degradation of TV picture caused by the fiber-optic link 6 5

23 Measured sensitivity of the i 31 cm active dipole and

fiber-optic link, before optimization of the rf amplifier.

(Length of fiber guide = 2 m, LED current = 5 mA, APD

current = 80 yA) — 67

24 Tangential sensitivity of the active dipole system; comparison of APD and PMT infrared sensors. (Length of fiber

guide = 2 m, LED current = 5 mA) 6 8

25 Measured sensitivity of the active dipole system using the

optimized rf amplifier of figure 17. (Length of fiber

guide = 2 m, LED current = 5 mA, APD current = 80 yA) 69

26 Theoretical insertion loss caused by connecting a 7 pH

inductor and resistor across the center gap of the 31 cm

dipole. (See sketch on page 21) 70

27 Tangential sensitivity of the active dipole system; effect

of connecting an inductor across the dipole gap 71

28 Overall performance of the active dipole and fiber-optic

link at four signal frequencies. (Length of fiber

guide = 10 m, LED current = 16 mA, APD current = 40 yA) 74

29 Tangential sensitivity of the active dipole system.

(Length of fiber guide = 10 m, LED current = 16 mA, APD

current = 40 yA) 75

30 Dynamic range or antenna factor of the active dipole system.

(Length of fiber guide = 10 m, LED current = 16 mA, APD

current = 40 yA) 76

31 Instrumentation for measuring strong-signal desensitization

of the active dipole system 77

32 Photograph of the isotropic antenna 79

33A Block diagram of the isotropic antenna control unit

(Part 1) —- — ——. . _ — 80

33B Block diagram of the isotropic antenna control unit

(Part 2) 81

34 Photograph of the antenna control unit chassis 82

35 Photograph of the complete fiber-optic antenna measurement

system including the crossed dipoles, fiber guides, control

unit and conventional receiver 83

36 Schematic of each avalanche photodiode circuitry 84

37 Bias regulator for each avalanche photodiode '— 85

vi
LIST OF FIGURES (Continued)

Figure Page

38 Schematic of the rf/IR calibrator circuit 86

39 Interface circuitry between the control unit and the EMC-25

receiver 87

40 Measured response of the EMC-25 receiver to pulses for the

three possible detection modes. 88

41 Schematic of the vector magnitude "analog computer," board

1 of 3 89

42 Schematic of the vector magnitude "analog computer," board

2 of 3 90

43 Schematic of the vector magnitude "analog computer," board

3 of 3 91

44 Schematic of the measurement sequencer circuitry 93

45 Timing diagrams of the measurement sequencer control

signals 94

46 Timing diagrams of the measurement sequencer interval

signals 95

47 Schematic of the charger for the NiCd batteries 96

4 8 Power supplies on the control unit chassis 97

49 Schematic of the ± 15 volt regulated power supply 98

50 Interconnecting cables between the control unit, EMC-25

receiver and the rf/IR calibrator 99

vii
LIST OF TABLES

Table Page

1 Sensitivity requirements for measuring narrowband signals

according to MIL-STD-461/462 — — ———- 44

2 Characteristics of fiber-optic and radiated infrared com- munication links 45

3 Specified CW characteristics of some typical LED's — 47

4 Specified characteristics of some typical photodetectors 47

5 Specified characteristics of the EMC-25 receiver- 54

6 Bandwidth of the EMC-25 FIM receiver in narrowband selectivity position — — 54

7 Frequency response test of the prototype antenna system.

(Input signal = -80 dBm, Spectrum analyzer bandwidth =

3 kHz) 64

8 Required E field for the given receiver indication (S+N)/N.

(Frequency = 25 MHz, fiber waveguide = type no. 1, 2m long) 66

9 Description of the parallel-resonating coils used with the

final active antenna' — 72

10 Measured performance of the fiber-optic antenna system 72

11 Measured performance of the fiber-optic antenna system with

an inductor across the dipole gap 73

12 Desensitization of the active dipole system; measurements

of a "desired" signal at 75 MHz in the presence of an "interfering" signal at 100 MHz — — 78

13 "Analog computer" voltages for various antenna input

voltages — 92

viii
SENSITIVE ISOTROPIC ANTENNA WITH FIBER-OPTIC

LINK TO A CONVENTIONAL RECEIVER

ABSTRACT

A broadband, active, isotropic receiving antenna was developed at NBS for the frequency range 15 kHz to 150 MHz. It was designed for use with a conventional receiver to measure weak,

near-zone electric fields of unknown polarization, such as leakage

emanations from electronic equipment placed within a shielded

enclosure. The antenna system consists of three mutuallyorthogonal active dipoles, each 31 cm long by 3.5 cm diameter.

The entire frequency range of each of the three field components

is amplified and used to modulate a high-speed light emitting

diode (LED) located inside the dipole. The modulated infrared

(IR) signals are guided through glass fibers 10 meters long which

connect the "isolated" dipoles to avalanche photodiodes at the

far end of the fiber guides. These photodetectors recover the

rf modulation from the IR carrier for input to the receiver.

The fiber-optic antenna system described in this paper has

high sensitivity (down to 10 yV/m) and fast response time (rf

modulations up to 150 MHz) . The readout indication at each

receiver frequency is proportional to the Hermitian magnitude of

E, which is the root-sum-square value of three orthogonal E field

components at the measurement point. The linear dynamic range of

the antenna system depends on the receiver bandwidth and signal

frequency. It is 70 to 75 dB at frequencies between 0.02 and

2 MHz (for 0.5 kHz receiver bandwidth), 60 to 70 dB between 2 and

25 MHz (for 5 kHz receiver bandwidth) and 40 to 55 dB between 25

and 200 MHz (for 50 kHz bandwidth)

.

Key words: Electromagnetic compatibility; fiber optics; field

strength measurement; isolated EMC antenna; sensitive isotropic

probe

.

1. INTRODUCTION

1 . 1 Background to the Measurement Problem

Improved receiving antennas are required in order to make meaningful

i measurements of near-zone fields and permit rapid surveying without requiring

time-consuming multiple orientations of the pickup antenna. The radio frequency (rf) sensor should be small and operate without metal transmission

lines to minimize distortion of the field being measured. This would permit

more accurate measurement of electromagnetic interference (EMI) radiated by

nearby sources within a shielded enclosure, avoiding errors caused by large

antennas located close to equipment racks or enclosure walls. As reported

in the literature, errors of 40 to 50 dB can occur when measuring radiated

emissions inside screen rooms [1 and 2]

.

1

figures in brackets indicate the literature references at the end of this paper.
The errors in measuring fields inside shielded rooms are caused mainly

by conductive surfaces which set up multipath reflections and standing-wave

resonances in these high Q "cavities." Also, the impedance characteristics

and effective length of the pickup antennas used are altered by close proximity to the equipment under test and to the metal enclosure. Limited success

at solving this measurement problem has been reported for frequencies, above

200 MHz by using absorbent walls and "hooded" antennas [3 and 4] . Measurements below 200 MHz may suffer large errors but improved techniques are being

studied [5 and 6] . Figure 1 shows a typical setup for measuring radiated

emissions inside a shielded room. Two recent developments now permit more accurate measurements of

electric field strength in the near zone of antennas or the leakage field

of unknown sources. The first development is an "isolated" antenna system,

that is, one having electric isolation (no metallic conductors) between the

antenna and receiver. The other development also features electrical isolation but combines it with an rf sensor having isotropic response [7] . The

latter property is desirable for making measurements in shielded enclosures

because the pickup is independent of field orientation. Thus it is not

necessary to determine experimentally the antenna orientation of maximum

response, or map an elliptically polarized field, or make separate measurements of three orthogonal electric field components.

Two different techniques are presently employed to fabricate an isolated

rf probe. One uses a high resistance transmission line such as carbonimpregnated plastic to convey the detected antenna pickup to a high impedance,

direct current (dc) readout meter [8] . This type of probe is generally used

to map high level fields and search for possible rf hazards in the range of

1 to 1000 V/m. In this case the total rf pickup, at all frequencies, is

detected at the center of the dipole and no attempt is made to measure the

amplitude as a function of frequency.

A second and newer technique for achieving electrical isolation between

an (active) antenna and receiver makes use of a fiber-optic transmission

link [9] . An earlier fiber-optic system [10 and 11] uses an "optical transmitter" consisting of a single 48 cm dipole, rf amplifier, and light emitting

diode (LED) . The glass fiber "waveguide" is coupled to an "optical re- ceiver," using a semiconductor photodiode or photomultiplier tube to detect

the modulated infrared signal. A similar approach is used for the probe

developed at the National Bureau of Standards (NBS) . However, the NBS

antenna uses three orthogonal dipoles to achieve an isotropic response.

Also, the electronic package of each active dipole (rf amplifier, LED,

battery, etc.) is contained entirely within the dipole.

2
Isotropic probes have been employed in the past, but only for measuring

intense fields. The rf sensors consist of 3 mutually-orthogonal dipoles and

their associated detectors. The sensitive antenna described in this paper

represents a different design philosophy in which both the amplitude and

frequency information are preserved, and phase information could be re- covered if desired. This contrasts with the usual type of "hazard" probe in

which only the amplitude (energy density) is measured.

1.2 Objectives of the ECOM/NBS Contract

The overall objective of this project sponsored by the U.S. Army

Electronics Command (ECOM) is to develop a broadband, isotropic receiving

antenna with high sensitivity for measuring weak rf emanations from electronic equipment in a screen-room environment. The frequency range of

interest is 15 kHz to 200 MHz. In order to avoid the large ambient fields

of radio and TV stations at a field site, it is often necessary to make

these tests inside a shielded enclosure. The antenna should thus permit

measurement of the absolute magnitude of weak "leakage" fields having arbitrary polarization. Also, it is desired that the antenna system be com- patible for use with a conventional field intensity meter (FIM) so as to

measure level vs. frequency.

The limits for RE02 narrowband electric-field (E) emissions are specified in MIL-STD-461 [12 and 13] . The desired measurement sensitivity for

the receiver system is given by figure 21 of MIL-STD-461A or figure 10 of

MIL-STD-416B, reproduced here as figure 2. As shown, the specified limit of

radiated field strength varies with frequency, being most stringent at 25 MHz

where it is 10 yV/m or 20 dB above 1 yV/m (20 dBuV/m) . The limit at 15 kHz

(low frequency end of spectrum) is 35 dByV/m and at 200 MHz (high frequency

end) is 34 dByV/m. Thus, one important objective is to design an isolated

antenna with sufficient sensitivity to measure these low field levels.

A typical test setup for measuring radiated emission according to MILSTD-461 uses a vertical monopole antenna above a small horizontal ground

plane. The vertical whip is placed one meter from the equipment under test

(EUT) . This specified distance is in the near zone of the EUT source

(< IX) for frequencies below 200 MHz. The resulting wave impedance is thus

not necessarily 377 ft, the field polarization is not necessarily vertical,

and the perturbing effects of the antenna or coax cables are unknown. The

pickup antenna developed at NBS is an attempt to reduce these problems. It uses three mutually-orthogonal balanced dipoles as an isotropic field sensor. Each dipole has a length of 31 cm and a diameter of 3.5 cm.

!

3
1.3 Characteristics of an Ideal rf Probe

Some characteristics of an ideal antenna for surveying near-zone electric

fields can be listed as follows:

(1) Isotropic sensor — The E field response should be independent of

angular orientation and field polarization.

(2) Isolated sensor — The antenna should not require a conducting transmission line. If a transmission line is used, it should be "invisible" to

avoid perturbation of the rf field being measured. (The NBS probe uses

a glass fiber line to convey information between the antenna and re- ceiver. ) (3) Broad bandwidth — The sensor should cover a large frequency range with

no required tuning.

(4) Flat response — The antenna factor should be independent of the signal

frequency to be monitored.

(5) Small sensor — The probe sensor should be small in order to minimize

scattering. If dipoles are used, they should be electrically short

rather than resonant, thus not requiring adjustment of length. (A

31 cm dipole is "short" up to about 200 MHz.)

(6) High sensitivity — The antenna response should be adequate to measure

the very weak radiated fields specified in MIL-STD-461, that is, field

strength as low as 10 yV/m.

(7) Low noise response — The internal noise of the active antenna should be

low because the ultimate sensitivity is controlled by signal-to-noise

ratio rather than by the amplifier gain.

(8) Large dynamic range — The active antenna should have a wide range

between the weakest measurable field and the strongest field with

acceptable overloading.

(9) Fast response time — The overall response time of the receiver system

should be short, permitting rapid spatial probing and rapid frequency

scanning. (The NBS-designed system is quite slow due to the long

response time of the EMI receiver used.)

(10) Good stability — The measurement system should be stable with respect

to both time and environmental conditions.

In addition to the characteristics listed above, it is desirable that

the sensor measure a scalar rather than a vector quantity. This is inherent

in the concept of an isotropic probe. The meter indication of the measurement system designed at NBS is a quantity known as the Hermitian magnitude

of the electric field, defined in section 2.

4
»

2. TECHNICAL APPROACH

The approach used for the sensitive isotropic probe is shown in the

block diagram of figure 3. The frequency range is 15 kHz to about 200 MHz.

Each separate dipole of the isotropic antenna is electrically short (rather

than self resonant) in order to achieve a large bandwidth. A field effect

transistor (FET) is used for the input stage in each highly-reactive dipole.

All signal frequencies induced in each dipole are amplified by a broadband

amplifier and applied to a forward-biased LED. The resulting rf-modulated

infrared emissions from the three LED's are transmitted via glass fiber

lines to photodetectors in an electronic instrumentation package located

near the antenna-input terminal of a conventional EMI receiver. The small

antenna package and fiber-optic guides do not significantly perturb the

field being measured.

The equivalent circuit of each pickup dipole is essentially a voltage

source in series with a capacitance. The induced open-circuit voltage (VQ

) is approximately equal to the product of the field strength E and the halfdipole length. In other words, at frequencies up to 200 MHz:

V

oc " E Leff = E 2T (1)

where: L

e ff = Effective length of each dipole

L = Physical length of each dipole.

The actual input level (V\ ) to the FET rf amplifier is reduced by a capacitive voltage-divider effect., as given by the equation:

C. V. = V

in oc

a C + c + c. a g l

• = V \ °

a oc K + csJ

(2)

where: C

&

= Effective antenna source capacitance = 3 pF

C

g

= Stray capacitance at the center gap of the dipole = 1 pF

C^ = Input capacitance of the FET stage = 2 pF

C = Total shunt capacitance = C + C. = 3 pF.

s ^ g l

The isotropic field sensor consists of three mutually-orthogonal dipoles,

each 31 cm long, with approximately a common center. Because the induced

signal currents are at right angles to each other, the dipoles respond

independently to the impressed field. That is, the current on one dipole

does not induce an unbalanced potential across the gaps of the other two

dipoles. The induced voltages developed across the center gaps of the

individual dipoles are given by:

V = E L V = E L cc and V" = E„ L (3) x x eff y y eff z z eff

5
where E

x , E^, and E

z are the amplitudes of the E-field components along the

three orthogonal dipoles. An effective field strength magnitude |e| can

be defined as the root-sum-of -squares (RSS) value of electric field according

to the equation

|e| = A2 + E

2 + E2 11 x y z (4)

The scalar quantity |e| is known as the Hermitian magnitude of the electric

field. It is proportional to the square root of the electric-field energy

density at the measurement point. Each dipole of the isotropic probe must

be calibrated in a known "standard" field. The induced voltages from the

three dipoles are processed electronically according to eq. (4)

.

Since a good E-field sensor must be able to measure close to a source

or scattering surface without perturbing the field, it is necessary that the

scattering from the probe be small. This criterion is satisfied by an antenna made of electrically-short dipoles. It has been shown experimentally

[14] and theoretically [15] that the scattering cross section of a dipole

decreases rapidly as the length decreases below 1/2 wavelength. A 31 cm

dipole, which is self resonant at about 500 MHz, can therefore be used at

all frequencies of interest in this project.

A sketch of each untuned, broadband, receiving dipole is shown as follows

i r

i RF AMP

!) MOSFET AND LED LED BATTERIES

DRIVER 1

1

! I

Glass fiber line

Figure 3 on page 43 is a block diagram of the isotropic-antenna re- ceiving system. The received signal for each of the three channels is timemultiplexed by an rf switch at the antenna input of the conventional EMI

receiver. The dc outputs from the receiver (synchronized with the rf input

switch) are processed to give a dc voltage proportional to the Hermitian

magnitude of the field. This voltage is used to drive a field-strength

readout meter which is separate from the regular meter of the EMI receiver.

The sensitivity requirements for measuring field strength specified in

MIL-STD-461/462 are shown in figure 2. These field limits, plus the re- sulting requirements for measuring antenna voltage, are listed in table 1 for several frequencies.

6
3. OPTICAL TRANSMISSION LINKS

3 . 1 Discussion of Infrared Fiber Optics

The type of electrically-isolated isotropic antenna described in this

report depends for its success on using a fiber-optic link between the

pickup antenna and receiver. This approach is relatively new for EMI

antennas. A discussion of infrared fiber optics is therefore included early

in this report.

3.1.1 System Considerations [16 to 22]

The building blocks for one type of active antenna system with fiberoptic link are shown in figure 3. The basic components are: pickup antenna,

rf amplifier, optical source and means to modulate this source with the rf

signal, fiber transmission line, and a photodetector at the receiver to re- cover the rf modulation. Table 2 lists some of the advantages of using an optical link between the antenna and receiver. At the present state-of-theart there are some disadvantages to fiber-optic communications, and these

are also listed in table 2. However, opto-electronic systems are receiving

considerable attention for various uses and a few of these proposed applications are listed in table 2, including isolated links to EMI antennas.

It could be mentioned that as the "carrier" frequency increases, the

usable modulation band also increases. An infrared (IR) carrier is thus

theoretically capable of transmitting a huge signal bandwidth. Also, when

using fiber-optic lines, the transmission loss is independent of the rf

modulation frequency. This is because the modulation frequency is such a small percentage of the carrier (IR) frequency. By contrast, the attenuation in coaxial cable increases rapidly as the frequency increases. For

example, the attenuation of rf signals in a 10 meter length of RG-8A/U

coaxial cable is about 0.05 dB at 1 MHz and 2.5 dB at 1 GHz.

3.1.2 Infrared Sources [23]

A good IR source for use in an isolated antenna system is a semiconductor light emitting diode. An LED emits radiation through the mechanism of

electron-hole recombination in the PN junction region. Figure 4 shows

typical emission spectra of IR LED and laser sources, comparing them with

the response curves of silicon photodetectors and the human eye. Note that

1 micron = 1 micrometer = 1000 nanometers = 10,000 angstrom units.

7
Gaseous lasers are also being used for optical communications, but they

generally have high cost, large size, short life, and large voltage and

power requirements. In addition, they cannot be modulated easily or directly.

Therefore, an LED is considered the optimum source for a fiber-optic system,

especially if it must be battery operated and contained inside a small rf

probe. Gallium Arsenide (GaAs) is the oldest and most highly developed

semiconductor material used for LED's.

At the beginning of the NBS ECOM contract in mid 1974, two types of

solid-state laser diodes had been described in the literature, but were not

available commercially. One type (neodymium-doped yttrium aluminum garnet)

required an intricate external modulator, which precluded its use in an isolated antenna. The other type (single heterostructure gallium arsenide)

could not operate continuously at room temperatures. In the past, semiconductor laser diodes were not able to produce coherent continuous -wave

(CW) radiation except at liquid nitrogen temperatures. At room temperature

they were limited to pulse-type operation because of internal heating. Very

recently, however, CW injection laser diodes for room temperature operation

have been announced (double heterostructure gallium aluminum arsenide)

.

Some of these have been received for testing at NBS. However, they were not

available in time for use in this project. Also, the battery current re- quirement for laser operation is high, and the lifetime is unpredictable and

relatively short.

Above the lasing threshold of a GaAs diode, there is a steep rise in

the IR output and thus much higher efficiency. In the remainder of this

report a diode operated above this threshold will be referred to as a "laser

diode," and a sub-laser diode will simply be called an LED.* The term

efficiency as applied to an optoelectronic emitter is defined as follows:

j-r- Optical power output /r . Power conversion efficiency1 = r , —: ? —- . (5) Electrical power input

An ordinary LED has low efficiency according to the above definition,

typically about 1%. Also, as indicated in figure 4, the spectral line of

an LED is relatively broad, typically about 35 nm for an LED and about 2 nm

for a laser diode. This causes velocity spread in a long transmission line

and thus limits the maximum usable modulation frequency. In addition, an LED radiates into a larger solid angle than a laser diode. The radiation

angle is typically about 30°, making it difficult to focus or couple the IR

radiation into a propagating mode of the glass filament. The coupling

loss into a fiber is about 15 dB greater for an LED than for a laser diode.

Also, laser diodes are inherently much faster than LED's, permitting higher

modulation frequencies. The maximum theoretical overall efficiency of an

(LED/glass f iber/photodiode) system is about 0.0001, or a reduction in carrier power of about 40 dB.
One major advantage of the LED source is that it can be modulated

directly and easily by varying the forward current through the diode. No

external modulator is required; the rf signal is simply superimposed on the

dc "bias" current. As indicated in figure 5, the intensity of the IR radiation is proportional to the LED drive current. When the rf signal is larger

than the dc bias, severe clipping results. An increase in drive current

then produces only a small increase in radiation intensity. This point of

non-linearity is the upper limit of the dynamic range for the transmission

system.

Table 4 gives the characteristics of a few commercially-available highspeed LED's. These values are the CW specifications according to the

manufacturers' literature.

3.1.3 Fiber Transmission Lines [24, 25 and 26]

The transmission of light inside a fiber is by a process of multiple

internal reflections. The maximum angle for total internal reflection is

given by Snell's law of refraction. The numerical aperture (NA) of a fiber guide is a measure of the acceptance cone of radiation into the end

of the guide from an external source. The larger it is, the greater is the

fraction of light trapped inside the fiber guide. (NA) is defined as the

sine of the critical angle 9 , the maximum acceptance angle between the ray

and the fiber axis. It is given by:

(NA) = sin 9 c = /(n

2

) 2 - (n^ 2 ' (6)

where: = Index of refraction of the core material

n^ = Index of refraction of the cladding.

Similar to rf waveguides, an optical fiber can propagate either in a single mode (if the diameter is small enough) or in many modes. A typical

single-mode fiber for IR wavelengths has a core diameter of about 2 ym.

As indicated in [17-26] , this type of fiber has the lowest loss. However,

the small core size makes it difficult to couple energy into the fiber and

only coherent laser sources are coupled efficiently.

Multimode fibers have a core diameter of about 100 ym. This permits

easier coupling to less expensive sources such as LED's. However, the

number of propagating modes increases as the (NA) and fiber diameter increase.

The different modes propagate with different velocities, causing dispersion

of the signal and a lower possible modulation frequency. In spite of this

limitation, large multimode fibers appear to be the optimum guide for use with an LED source, especially for short transmission lines such as those

used between an antenna and EMI receiver. The typical coupling loss between

an LED and a fiber bundle is 20 to 25 dB.

9
3.1.4 Infrared Photodetectors

Demodulation of an IR signal carried in a fiber waveguide can be ac- complished with either a photomultiplier tube (PMT) or a semiconductor

photodetector . Both types were evaluated in this project. The detector

chosen is a silicon avalanche photodiode (APD) . This type of semiconductor

photodiode is operated with a high reverse bias voltage. As the bias

approaches the breakdown voltage, hole-electron pairs created by the absorbed IR photons generate additional pairs by avalanche multiplication.

This process is similar to electron multiplication in a photomultiplier

tube; that is, an APD is the solid-state equivalent of a PMT. The avalanche

effect produces a multiplication factor (current gain) up to about 100. For

linear operation the output current must be proportional to the input IR

power .The APD multiplication process also creates additional noise, mainly

shot noise. There is an optimum reverse bias voltage (or APD current) which

results in sufficient gain and highest signal-to-noise ratio. This optimum

value must be determined experimentally for each length and type of fiber

guide, and for each set of operating conditions. Table 5 gives the characteristics of a few commercially available photodetectors, according to the

manufacturers' specifications.

3 . 2 Tests of Fiber-Optic Components

Initial experimental work dealt with evaluation of the opto-electronic

components required in the infrared link between the antenna package and EMI

receiver. One commercially available LED appeared to be the most promising

on the basis of a published risetime of 1 ns, which is equivalent to a 350 MHz bandwidth. However, the fastest measured risetime was 12 ns

(^ 3 0 MHz) for this LED. The test setup consisted of a 1 ns risetime pulse

generator, a 150 ps avalanche photodiode, a 420 ps rf amplifier, and a 700 ps oscilloscope. The measurement system risetime was 1.3 ns . The

relationship between the 10-90% risetime (T ) and the -3 dB bandwidth (BW)

is given by the expression:

(T r

) (BW) = 0.35. (7)

The optical output of an LED is quite weak, typically about 500 yW

at a bias current of 50 mA. Considerable rf amplification is required after

the photodiode to observe the rf signal on an oscilloscope. The efficiency

of an LED is quite low, typically 1% (mW optical output/mW electrical input)

.

Also, the output beam from an LED is broad, at least when compared to a laser.

The -3 dB beamwidth is typically 30°. A lens system was used in our early

tests to focus the LED output onto the photodiode.

10
Photodetectors from several manufacturers were evaluated for transient

response using a mode-locked dye laser (30 ps risetime) or a pulsed GaAs

laser diode (150 ps risetime) . Several PIN photodiodes and avalanche photodiodes were tested. Both types produced good linearity for analog applications /but (in our system) the PIN diodes did not produce as good an overall

noise figure as the APD's.

Also, a photomultiplier tube was compared with the solid-state avalanche

photodiode. It had been reported in the literature that a PMT is capable of

higher speed and sensitivity than an APD. For this reason a series of tests

was made using a phototube which has maximum sensitivity near the desired

wavelength of 0.9 nm. We determined an optimum value of PMT anode current,

which is a function of the PMT anode voltage and luminous flux. A current

of 10 uA, produced by an anode voltage of 700 V, gave the highest signal-tonoise ratio in our system. The PMT was able to sense a weaker infrared beam

than the APD, but the speed and rf signal sensitivity were greater (in our system and for our application) when using an APD. The experimental data

are given in section 5.4.

Silicon avalanche detectors are commercially available with gainbandwidth products up to 100 GHz. For an avalanche gain of 100, the

attainable signal bandwidth is thus 1 GHz. It can be seen that photodiodes

are not the limiting factor in achieving large bandwidth in a fiber-optic

system. The upper usable frequency in our system appeared to be limited by

the carrier lifetime of the LED. Heterodyne conversions employing an rf

local oscillator inside the active dipole could perhaps be used to achieve

higher effective signal frequencies; however, an experimental investigation

of this approach was not pursued.

Measurements were made of the frequency response of 17 LED's and 1

laser diode operated as an incoherent LED. These were all different types

from various manufacturers. A pulsed laser was used to evaluate the response

time of 8 regular photodiodes, 6 PIN photodiodes and 4 avalanche photodiodes. The emitter/detector pair chosen for the NBS fiber-optic system is a spectrally matched LED/APD pair optimized by the manufacturer for a wavelength

of 800 to 900 nm. The type of fiber bundle chosen consists of 19 glass filaments packaged

in a plastic jacket with a connector at each end, similar in appearance to

a coaxial cable. The outer diameter of each coated filament is 135 ym,

with a core diameter of 85 ym. The numerical aperture of the guide is

approximately 0.14, which means that the maximum acceptance angle is ± 8° to

the fiber axis.

11
One problem in fiber-optics is the high attenuation of the optoelectronic portion of the system. Also, the risetime of all LED's tested was

> 10 ns, which limits the -3 dB bandwidth to 35 MHz. Thus, considerable

frequency compensation was required in the rf amplifier in order to achieve

a system response up to 200 MHz. Considerable difficulty was also experienced in capturing the limited power radiated by the LED's. Using typical

published values for GaAs LED's and silicon photodiodes , the theoretical

minimum coupling loss between an LED and an APD is 4 0 dB. For example,

assume an LED efficiency of 1 mW/50 mA current and a photodiode responsivity

of 0.5 mA/mW. The maximum current transfer ratio for the LED/APD pair is

Iout//I in

= LED e f f i- c;i- ency x APD responsivity = 0.01, or -40 dB.

We were not able to achieve the above overall efficiency in practice.

With both butt-joints between the LED and APD touching, or when using lenses

on X-Y-Z micrometers to focus the LED output on the APD, we experienced

typical losses of 70 to 75 dB. An optical power meter was assembled and

calibrated at NBS in order to measure the power output coefficients during

this development work.

Time-domain evaluations were made of several promising types of Si

avalanche photodiodes. The first type of APD tested produced a small-signal

avalanche gain of 16 dB with 330 volts of reverse bias. Its measured 10-90%

risetime was 2.5 ns, compared with a specified risetime of 2 ns . This

corresponds to a -3 dB bandwidth of 140 MHz. It was found that infrared

APD's are sensitive to stray radiation and must be kept shielded from

ambient light during measurements on an optical bench.

The second type of APD evaluated was tested for photocurrent gain and

internal random noise as a function of bias voltage and temperature, from

+10°C to +50 °C. Figure 6 is a plot of the measured results at an ambient

temperature of 28 °C. The gain and noise are both strongly dependent on temperature. With a regulated-voltage APD bias of 160 volts, the photocurrent gain (also called avalanche multiplication) ranged from 42 dB at

+10°C to 30 dB at +50°C. The optimum operating point for this particular

APD (at room temperature) was at 160 volts. As shown in figure 6, the

output signal-to-noise ratio (S/N) for this condition is about 45 dB.

3 . 3 Measurements of Two Infrared Optoelectronic Links

3.3.1 Performance of a Radiated System

An experimental infrared broadband transmission link was assembled and

tested. Figure 7 is a block diagram of the system. The arrangement is

similar to that of the final system, except that this first test did not use

a fiber guide. The modulator for the LED is a low-current drain (10 mA) rf

12
amplifier. The bandwidth of this first design was 60 MHz. The IR output

from the LED was focused by lenses onto the photodiode. The detected signal

was amplified by two wideband amplifiers and applied to the input terminal

of a spectrum analyzer used as a sensitive rf voltmeter.

Figure 8 shows the input/output performance characteristics of the IR

link for a signal frequency of 25 MHz. With a 100 Hz bandwidth the spectrum

analyzer noise level was -110 dBm (0.7 yV) for a 10 dB(S+N)/N ratio. The

upper limit for linear operation was about -35 dBm input. The overall attenuation of the rf signal in the IR link was 7 dB in this test. As seen

in figure 8, the dynamic range achieved in this first test was 65 dB, which

was greater than anticipated. It should be noted, however, that a typical

EMI receiver has a larger bandwidth than the 100 Hz used in this test. A

bandwidth would reduce both the sensitivity and the dynamic range.

3.3.2 Performance of a Fiber-Optic System

The optimum wavelength to use for a fiber-optic system must be chosen

on the basis of three considerations: (a) high source radiation, (b) low

fiber absorption, and (c) high photodetector efficiency. The near IR portion of the electromagnetic spectrum (800 to 900 nm) is a good choice for

transmission in glass fibers. GaAs LED's have a peak output near 900 nm so this wavelength was chosen for our system.

The fiber waveguide chosen consists of a bundle of 19 filaments. The

specified maximum attenuation in the guide is 0.03 dB/m, at a wavelength of

820 nm. However, this low value of attenuation was not achieved in practice.

Initial measurement of the bundle attenuation was 3.3 dB for a guide 10 m

long, using a focused He-Ne laser beam. However, when using an LED the

measured system loss was 28 dB, for either an (LED/fiber) butt joint or with

a set of focusing lenses. These measurements were made using an optical

power meter. It should be emphasized that the major loss was not the attenuation within the fibers but was due to the difficulty of injecting the

LED radiation into the fiber.

When passing an rf signal through an (LED/f iber/APD) link, the additional

loss due to the waveguide was typically 45 to 50 dB. The apparent discrepancy

between the rf attenuation and the optical power measurements (28 dB above)

is caused by the linear optical-power to electrical-current conversion by the

APD. If the modulation current (rf signal) is passed through a load resistor,

the power delivered to the load is proportional to the current squared.

Thus a given dB infrared power loss causes an output rf signal power loss of

double that number of dB.

13
The instrumentation used to measure the rf signal loss in a fiber-optic

system is given in figure 9. The LED driver circuit was an emitter follower

in series with a matching network. As shown in the simplified schematic,

the matching network was a 40 ft resistor in parallel with a variable capacitor,

which was tuned for maximum output. The maximum usable modulation frequency

for an LED depends partially on the driver circuit's ability to supply

charging current to the LED's capacitance. The input capacitance of highfrequency LED's is typically 100 to 1000 pF. Also, this capacitance is

shunted by a low dynamic resistance of only a few ohms. One method to

drive such a source would be by brute force, using a high-current, lowimpedance rf source. Another approach, a constant-current LED driver, was used in our system because it avoids the large dl/dt requirement for high

frequency modulation.

Figure 10 gives the measured values of overall rf signal attenuation

as a function of frequency. The minimum insertion loss, at low frequencies,

was about 60 dB for the APD temperature and voltage used. As seen, the

insertion loss increased rapidly for signal frequencies above 100 MHz.

Accurate measurements above this frequency could not be made (in this

system) because of signal leakage between the rf source and receiver.

4. FIELD STRENGTH AND ANTENNA CALIBRATIONS

4 . 1 Discussion of Field Strength Meters

A receiver used as a frequency-selective voltmeter is the keystone of

any system for measuring field strength and interference. Figure 11 is a block diagram of a typical FIM. It consists basically of a superheterodyne

communications receiver with certain added features. The model EMC-25

receiver 2 used for this project is a solid-state "interference analyzer"

covering 14 kHz to 1 GHz in 15 tuning bands. There are three intermediate

frequencies, depending on the tuning band in use. A multistage detection

system with logarithmic response is incorporated in the IF amplifier section. This is done to achieve a readout scale which is linearly proportional to dB above 1 yV. The detector function used for sine-wave signals

2 The overall fiber-optic antenna system described in this report was designed

to be compatible with an EMC-25 type of field intensity meter. This type of

receiver was chosen because it is a typical FIM and because it was available

both at NBS and at the sponsoring agency. The use and identification of

the receiver type number does not imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the equipment identified

is necessarily the best available for the purpose. Identification of the

FIM type will, however, enable another experimenter to reproduce the equipment and results described in this report.

14
has a nominal time constant of 600 ms for both. charge and discharge. A sum- mary of the characteristics of the receiver for signal frequencies up to

200 MHz is given in table 6. The term "tangential sensitivity" used here is

defined as the input signal level required to increase the meter reading by

3 dB above the receiver internal noise. Table 7 gives data on the measured

values of receiver bandwidth.

As mentioned previously, a short dipole antenna acts as a voltage

generator of high (capacitive) impedance having a pickup which is essentially independent of signal frequency. By contrast, the induced voltage

of a self-resonant dipole is inversely proportional to frequency. ' In order

to measure absolute field strength with a receiver (rf voltmeter) , it is

necessary to apply a multiplying factor to the dial indication. This con- version factor between the antenna pickup (volts) and the electric field

strength (volts/meter) is known as the "antenna factor." Figure 12 shows

the theoretically-anticipated value of antenna factor vs. frequency for a passive A/2 dipole. The factor for an active dipole must be determined

experimentally by immersing it in a known (standard) field [27 and 28]

.

4 . 2 Auxiliary Calibration of the Receiver as a Tunable rf Voltmeter

The first phase in calibrating a receiver to measure field strength is

to adjust the gain control setting, at the frequency of interest, in terms

of a known sine-wave voltage. This is commonly referred to as "calibration"

of the FIM as a tunable or frequency-selective rf voltmeter. Figure 13 is a block diagram of the instrumentation used for this project. The technique

shown employs a special L-pad with 50 ohms input resistance and a very low

output resistance. This attenuator pad has 57 dB insertion loss and consists of a selected 49.95 ft series resistor and a 0.071 ft annular-ring

resistor in shunt with the output terminal. It is housed in a coaxial chamber

1 inch long, with 50 ft characteristic impedance, and has a type N connector

at each end. For each voltage calibration the output level of the signal

generator is adjusted to obtain a power meter reading (accuracy of ± 1%) of

100 uW. This produces a 100 uV signal at the receiver input.

The 0.071 ft resistor in figure 13 supplies a known voltage to the

receiver which is essentially independent of variations in receiver input

impedance. For example, assume the use of a receiver with nominal 50 ft input but having an actual input VSWR of 2:1. The maximum possible variation

of receiver input voltage under these conditions would be 0.017 dB or 0.2%.

By comparison, the receiver input voltage could be in error by as much as 3.52 dB, or 50%, if a 50 H generator is used directly as the receiver

"voltage" source.

15
Figure 14 is a graph showing the measured non-linearity of the particular receiver we used. It can be seen that the dial indication has a variation from true voltage level which exceeds 2 dB at some points. These

corrections to the indicated values must be made manually in order to obtain

more accurate measurements of CW voltage and field strength.

4 . 3 Standard Field Setup Used to Calibrate the Active Antenna

Initial tests of the 31 cm active dipole were made at a frequency of

25 MHz. Figure 15 shows the instrumentation used to establish a known

electric field for calibrating the active dipole. The antenna factor and

system sensitivity for the antenna under test were determined by immersing

the dipole in the standard field of a transverse electromagnetic (TEM) cell

[29] . This cell consists of a large "coaxial" 50 ft transmission line in

which the center conductor is a flat metal strip and the outer (grounded)

conductor has a rectangular cross section. The TEM line we used (see figure

16) is about 3 m long and has a square 1.2 m x 1.2 m cross section. At

frequencies sufficiently low so that only the principal wave (TEM mode) will

propagate through the cell, a fairly uniform and calculable E field can be

established. The electric field at these frequencies, at the location shown

in figure 15, is given by:

= Voltage . /50~P , ft

. Distance D

(a '

where: E = Field strength, volts/meter

50 = Characteristic impedance of the cell

50 = Resistance of the cell termination, ohms

P = Measured power through the cell, watts

D = Distance between the center conductor and top of the cell,

meters

.

The 1.2 m x 1.2 m TEM cell exhibited cavity-mode resonances at frequencies above 140 MHz. This is similar to the well-known resonances

occurring in shielded rooms. In order to calibrate the active dipole at

frequencies above 140 MHz it was necessary to measure the field strength

inside the cell with a small (10 cm) transfer probe, which in turn had been

calibrated previously in a smaller TEM cell. The signal frequency vs. cell

power required to produce a field level of 0.1 V/m was measured for a larger

test probe having the same dimensions as the 31 cm active dipole. Corrections were thus made for field perturbations caused by the larger probe.

The high Q of the TEM cell for higher order modes was reduced by placing

several pieces of rf absorber in the bottom half of the cell. All subsequent

16
tests of the active dipole sensitivity were made in this improved test

chamber. It is felt that standard fields for calibrating the 31 cm active

dipole were produced with an uncertainty less than ± 1 dB at any frequency

up to 100 MHz and + 2 dB from 100 to 200 MHz.

5. DESIGN OF A SINGLE-DIPOLE FIBER-OPTIC ACTIVE ANTENNA

5 . 1 Description of the Dipole Unit and APD Circuit

Physically each dipole antenna consists of two identical brass tubes

15.25 cm (6 inches) long and 3.5 cm in diameter, with a 0.5 cm gap at the

center of the dipole. The overall dipole length is thus 31 cm. The rf

amplifier, LED, fiber waveguide connector and battery all fit within one of

the brass tubes. The second tube serves as the opposing half of the dipole

antenna. The amplifier construction features micros trip and ground plane

techniques to achieve wideband response in the rf amplifier and LED driver

circuitry.

The rf amplifier and FET-input stage, as originally designed, had one major problem. The unit was small enough, but the circuit required 15 volts

at 60 mA, an excessive battery consumption. The antenna circuit was then

redesigned and now requires 30 mA at 9 volts, supplied by a NiCd rechargeable battery. Figure 17 gives a schematic diagram of the final circuit.

The rf amplifier features a diode-protected MOS FET front end, Q 1. The

input impedance presented to the antenna is 10 megohms resistance shunted by

3 pF capacitance. The amplifier has a -3 dB bandwidth of 170 MHz and

nominal gain of 60 dB.

Figure 18 shows an equivalent circuit of the active dipole antenna.

It is similar to that discussed in section 2, but includes an effective noise

generator at the amplifier input. At frequencies below 200 MHz the source

impedance Z = is given approximately by the reactance of a 3 pF capacitor.

3, This capacitance value was determined experimentally by mounting half of

the dipole unit as a monopole above an extended ground plane. The measured

capacitance was 5.4 pF, corresponding to a dipole source capacitance of

half this value, or 2.7 pF. The nominal 3 pF value was also verified approximately by several theoretical calculations of dipole input imepedance

.

The shunt capacitance C

g of about 3 pF was determined by adding capacitance

at the antenna gap until the pickup voltage in an rf field was reduced by a factor of two.

The APD required specially designed auxiliary circuits and power supply.

Figure 19 gives a simplified schematic diagram. The circuitry provides

temperature compensation of the APD avalanche voltage, which helps stabilize

the APD gain. As shown in the figure, the temperature reference diode is

17
thermally connected to the APD by physically mounting the two components on

a common heat sink. The APD bias voltage (and hence its avalanche gain) are adjustable. A crowbar circuit is included in the power supply to- protect

the APD, since if too much light is applied to the APD and/or the bias

voltage is too high, the APD current will exceed its maximum permissible

value. The APD current is sensed across the 1 kft output resistor and applied

to the 741 operational amplifier (op. amp.) connected as a voltage com- parator. If the current exceeds a preset value, the crow-bar transistor Q 3 turns off the high voltage bias supply. The circuit automatically resets

when the overload is removed.

5 . 2 Preliminary Tests of the Active Dipole System

Figure 20 gives a block diagram of the prototype antenna system including the active dipole circuitry, LED, fiber guide and APD. An oscilloscope was used to observe the step response and operation of the system.

The test signal (CW or step) was applied across a 50 U termination at the

dipole gap. Table 8 lists the system gain and signal-to-noise (S/N) ratio

for several frequencies as measured with a spectrum analyzer.

The 31 cm active dipole antenna was then used to pick up rf fields in

the laboratory. Figure 21 shows the spectrum from dc to 200 MHz as received

by the active antenna system. The lower white band is the system noise.

The antenna system has also been demonstrated to receive TV signals from

broadcast stations, as shown by the photographs of a TV screen in figure 22.

5.3 Effects of Noise and Receiver Bandwidth on Sensitivity [30 and 31]

Adequate performance of the infrared (LED/fiber guide/APD) system

requires a combination of high sensitivity, low noise, and large antenna

bandwidth. One of the most important requirements of any active antenna

system is that the S/N ratio should not be degraded excessively between the

rf pickup and the FIM. The FET input stage, rf amplifier and the process of

converting to IR and back to rf should all maintain a high S/N ratio. In

other words, the sensitivity and reception characteristics depend on the

noise figure of the overall system. The S/N ratio must also be considered

as a major criterion when determining the FIM receiver bandwidth that can be

used. This is due to the fact that the random noise power received by an FIM is proportional to the receiver bandwidth.

The measured value of sensitivity for any receiving system depends on

the definition or criterion used. One common term for receiver sensitivity

is "minimum discernible signal" level (MDS) , generally defined as the signal

required to produce a receiver indication 0.5 dB above the background noise.

18
In this contract we chose to use a more conservative and more precisely

measurable criterion known as "tangential sensitivity." It is defined as the field intensity required to increase the receiver indication 3 dB above

the background noise level, or (S+N) /N = 3 dB.

In general, noise is defined as any unwanted disturbance which is

superimposed on the desired signal. In this report the term noise is used

for voltages which vary randomly in amplitude and phase, and which are present over a wider frequency band than the resolution bandwidth of the

receiver. Therefore, the response of a receiver (or spectrum analyzer)

depends on the resolution bandwidth used. Doubling the measurement bandwidth doubles the noise power accepted, which increases the receiver voltage

indication by a factor of /2~.

The maximum attainable sensitivity of any receiving system for narrowband signals is controlled by the signal-to-noise ratio. Since the noise

contribution of an (LED/fiber guide/APD) system is greater than that of the

rf amplifier, our approach was to minimize the noise in the optoelectronics

and use the lowest possible receiver bandwidth. The noise problem in a fiber-optic antenna system is severe compared with a conventional antenna

and receiver. The pickup antenna is intentionally broadband, to avoid

tuning or adjustment, and has a large mismatch to the rf amplifier. This

produces both a large noise pickup and a low signal level. In addition,

the optoelectronic link causes a large attenuation (requiring further rf

amplification) and a major addition to the random noise. Since the overall sensitivity is ultimately limited by noise, no amount of receiver gain

can improve the sensitivity.

One method to obtain additional receiver selectivity at signal frequencies above 25 MHz, and thus higher tangential sensitivity, would be to use the "Q-fiver" technique familiar to radio amateurs. This involves

loose coupling of the IF output of the conventional receiver to the antenna

input terminal of another receiver with low intermediate frequency and

narrow bandwidth. This technique of using narrow bandwidth to achieve high

receiver sensitivity causes tuning inconvenience and does not apply to the

measurement of broadband signals. For example, the impulse response voltage

of a receiver as measured with a peak detector is directly proportional to

the receiver bandwidth. Thus high sensitivity for impulsive signals (and ease of tuning for CW signals) is achieved by using a wide receiver bandwidth. The EMC-25 receiver is furnished with two selectable bandwidths

labeled on the front panel as "narrow" and "wide." The former may be used

for measuring CW signals and the latter for measuring noise and interference.

19
5 . 4 Measurements of the System Sensitivity

5.4.1 Sensitivity of the Active Dipole

Table 9 gives a summary of the measured sensitivity for the (dipole/

receiver) system at 25 MHz. It can be seen that CW sensitivity is a function of receiver bandwidth, dipole gap width, level of APD current, and the

criterion used to define the term "sensitivity." On the basis of tangential

sensitivity, using the EMC-25 receiver on narrowband selectivity, the active

dipole developed at NBS meets the desired MIL-STD-461 specifications at

25 MHz if the receiver is on Band 10. The measured tangential sensitivity

for a 0.5 cm dipole gap was 9 yV/m and the required sensitivity is 10 yV/m.

However, if the receiver is tuned to 25 MHz on Band 11 instead of Band 10,

the tangential sensitivity is 23 yV/m which fails to meet the specification

by 7 dB.

It appears from the data of table 9 that 50 yA is about the optimum

value of APD current when using the 2 m fiber guide. Also, it can be seen

by comparing the last of two conditions in table 9 that it is not necessary N

(or helpful) to use an extra rf amplifier between the APD and the FIM. As

noted previously, the MDS level could not be measured as precisely as the

other three criteria and was therefore not used. The last column of table 9 gives data on the measured threshold noise level of the receiver. It shows

the increase in noise floor caused by the fiber-optic system.

All of the sensitivity measurements in the remainder of this report are for a dipole gap width of 0.5 cm. Also, the remainder of the data were

taken with the receiver switched to the narrowband position.

A series of measurements was made with the 31 cm active dipole to

determine the system sensitivity as a function of signal frequency.

Several criteria of sensitivity were used, varying from the MDS level to

a signal 20 dB above the noise level. A summary of the data is shown

in figure 23. For comparison purposes, similar measurements of system

sensitivity were taken using a photomultiplier tube (PMT) instead of the APD.

Figure 24 is a graph of the data (PMT vs. APD) for the case of tangential

sensitivity

.

The electronic circuitry of the rf amplifier in the antenna was optimized in an attempt to increase the sensitivity at frequencies above 25 MHz,

while maintaining good performance at frequencies below 25 MHz. The circuit

design includes frequency compensation to overcome some of the amplitude

rolloff at frequencies above 100 MHz. This rolloff is caused mainly by

frequency limitations of the LED. Figure 25 is a graph summarizing the

measured sensitivity of the system after optimizing the rf amplifier. As can be seen, the sensitivity was improved significantly at frequencies between 25 and 150 MHz. The abrupt changes in sensitivity at signal frequencies of 2.3 MHz and 25 MHz are readily apparent; they are due to step

changes in the receiver bandwidth. 9f)
In the remainder of this report the only criterion used for system

sensitivity is "tangential sensitivity," the E field required to produce a receiver indication 3 dB above the noise. Using this criterion, the antenna

system developed at NBS (with an EMC-25 receiver) meets the specifications

of MIL-STD-461 for all frequencies from 20 kHz up to 25 MHz. As seen in

figure 25, the sensitivity is slightly low at frequencies between 25 and

50 MHz and at frequencies above 100 MHz.

The antenna data in the remainder of this report are for an improved

type of fiber guide 10 meters long, whereas a 2 m guide of less expensive

material was used for all of the previous data. The loss of optical energy

in the longer guide changed the antenna factor and caused a slight deterioration in the system tangential sensitivity. The relative loss was greater at

the lower signal frequencies. Before taking quantitative data, the con- nectors at both ends of the fiber guides were adjusted until maximum IR

transmission was achieved. This condition of maximum transmission is easily

determined experimentally because it corresponds with maximum indication of

APD current on the panel meter.

5.4.2 Effect of Using an Antenna Resonating Inductor

Antenna resonance can be used to produce a significant improvement in

the system sensitivity at frequencies near resonance. A broadly resonant

dipole can be obtained by connecting a low-Q coil across the center gap of

the dipole. Theory indicated that a parallel inductor would boost the sensitivity over a frequency band of about one octave. No inductor would be used

at frequencies below 25 MHz, or whenever the added sensitivity is not re- quired. The following sketch shows the equivalent circuit for an electricallyshort dipole with an inductor connected across the center gap.

C = 3 pF C =C + C. = 3 pF

s g l ^ ©Voc

V.in 1 pF —1— L 3 111 R £ 2 PF =b V

Qut ill FET

21
where

:

C C = C = 3 pF (see eq. 2) a s L

R

Inductor connected across the antenna gap

Resistor connected across the antenna gap.

Figure 26 is a plot of the calculated gain for a 7 yH coil, for four

values of coil Q. Figure 27 gives actual measurements of system sensitivity

vs. frequency for the three inductors chosen. As can be seen, the 23 turn

toroid produces sufficient improvement near 25 MHz so that MIL-STD-461 is

essentially satisfied from 25 to 100 MHz. In like manner, the 6 turn coil

produces the required sensitivity in the frequency range of 75 to 150 MHz.

The 4 turn coil can be used for improved sensitivity between 100 and 200 MHz,

but we were not able to achieve sufficient improvement at frequencies above

150 MHz. At these higher frequencies it might be advantageous to use a series inductor, but this approach was not explored. A description of the 3 types of coils used in the final design is given in table 10. The coils

were wound on ferrite toroid cores having a diameter of 0.64 cm and a thickness

of 0.19 cm.

The type of waveguide generally used for fiber-optic communications

consists of a bundle of several glass filaments. Multifilament bundles are used to achieve flexibility of the guide together with a large numerical

aperture. In a "coherent" bundle the relative positions of the fibers are maintained the same at both ends of the guide. This type of bundle must be

used for imaging through a guide but is not required simply for transmission

of energy. All of the "low loss" guides we tested use incoherent bundles.

However, an LED chip is much smaller in area than the end of a fiber guide,

so only a few of the filament ends are illuminated. This causes random

spreading of the light beam along the length of the guide, so that only a fraction of the incident beam is captured by the small APD chip at the far

end of the line. The longer the guide, the greater is the incoherence of

the filaments in the emergent beam. As a result, the total transmission

loss was found to be greater than that expected on the basis of path-length

attenuation alone.

5.4.3 Effect of Non-coherence in the Fiber Bundle

22
5 . 5 Overall Antenna Performance vs. Frequency;

Tangential Sensitivity, Linearity, Dynamic Range and Antenna Factor

The design of the active dipole and fiber-optic link was optimized on

the basis of three considerations, namely: (1) high overall sensitivity,

(2) large dynamic range so as to receive weak signals/ but keep spurious

responses caused by strong signals to an acceptable level, and (3) wide

bandwidth of the antenna so only one (isotropic) antenna unit is required

over the total frequency range of 15 kHz to 200 MHz.

In the NBS antenna system (active dipole/f iber-optic link/FIM receiver)

the dynamic range is controlled by, and defined as, the difference between

the weakest measurable signal and the signal level strong enough to drive

the system into non-linear operation [32] . More precisely, the dynamic

range was defined as the difference between the tangential sensitivity

[ (S+N/N = 3 dB] and the point of 3 dB compression in gain. The latter point

is where the receiver response falls 3 dB below a linear extrapolation of

the characteristic curve plotted to give receiver response vs. field intensity. The overall design goal was to achieve high sensitivity and good

linearity over a wide range of field intensity.

Tables 11 and 12 give a summary of the measured performance characteristics of the fiber-optic antenna system. Figure 28 is a graph of the raw data at four different receiver frequencies. On this graph an ideal

curve is a straight line with a slope of 45°. The bottom (left) end of each

line is a measure of the antenna system sensitivity. In fact, the bottom

point is a plot of the tangential sensitivity. The curve becomes non-linear

at the upper end of the line and tends toward a horizontal slope. This is

where the rf amplifier or LED becomes overloaded. The length of the straight

line portion is a measure of the system dynamic range.

The "antenna factor" is defined as the difference between the E field

i level (dB yV/m) and the receiver indication (dByV) . The lateral position of

' the characteristic curve is thus a measure of the antenna factor. An unknown field strength may be measured with the calibrated active antenna by

using the expression:

I! E = M + K (11)

( where: E = field strength, dB above 1 yV/m

M = meter indication of the receiver, dB above 1 yV, on narrowband

selectivity, after calibration as a tunable voltmeter

k = antenna factor, dB.

23
Figure 29 is a graph of the tangential sensitivity vs. frequency for

the fiber-optic antenna system, from 15 kHz to 200 MHz. The figure includes

the measured sensitivity when a parallel resonating coil is plugged into the

antenna across the dipole center gap. Figure 30 is a graph of the dynamic

range as a function of frequency. It also shows the calibrated value of

antenna factor to be used when measuring field intensity.

The dynamic range of the fiber-optic antenna system generally decreases

as the frequency increases. The measured dynamic range is about 70 dB at

low frequencies, between 25 kHz and 2 MHz. It is about 65 dB for frequencies

between 2 and 25 MHz, and 45 to 55 dB at higher frequencies up to 200 MHz.

The use of a parallel antenna coil increases the sensitivity but appears to

have little effect on the dynamic range. The dynamic range is fairly

constant as a function of frequency when compared with either the tangential

sensitivity or the overload level.

5 . 6 Overloading of the Active Antenna System by a Strong

Interfering Signal

The (active-antenna/fiber-optic) system has a limitation regarding the

measurement of absolute field strength. The antenna becomes desensitized by

any strong signal within its total frequency range of 15 kHz to 200 MHz. A

strong out-of-band signal may thus cause an error in the measured field

strength at another frequency, or it may produce cross modulation products

at various frequencies. This is a common limitation when using active

antennas. In this case the same signal may be picked up at more than one receiver frequency. For example, a local FM station near 100 MHz could also

be heard when the receiver was tuned near 200 MHz. This was due to second

harmonic generation, either in the rf amplifier of the active dipole or in

the optoelectronic portion of the system. It is, therefore, advisable to

check that the signals being measured are true signals and not spurious

responses of the receiver system.

The use of fiber optics, with its large signal loss, demands a large

amount of rf amplification. If a strong signal voltage is present, this

leads to the possibility of overloading in the latter stages of the amplifier. A switchable attenuator could be incorporated in the front end

of the active dipole to test for antenna overloading. Alternatively, a plug-in capacitor producing 20 dB attenuation could be fabricated to con- nect across each dipole gap, similar to the "outboard" resonating inductors.

A design trend in broadband communications receivers for operation in strong

signal environments is to eliminate the rf amplifier stage and make up the

24
gain in the IF amplifier. However, our active antenna system is intentionally designed for high gain and broadband, untuned operation. One

apparent alternative is to use a tunable, tracking preselector, which would

be a major design problem for a three-dipole isolated antenna.

As mentioned previously, the upper end of the dynamic range was defined

as the point of 3 dB compression in gain. This type of non-linearity leads

to desensitization by any strong signal [33] . Therefore, tests were made to

determine the error introduced when a weak signal at frequency Fl must be

measured in the presence of a strong signal at frequency F2 . Figure 31 is a block diagram of the instrumentation used for the test and table 13 gives a summary of the data. The level of the "interfering" field at 100 MHz was adjusted in steps from 0 to 31.6 mV/m. Measurements of the "desired" signal

at 75 MHz were then made to determine the effect on indicated field level,

overall linearity, etc. It can be seen from the experimental data of table

13 that the tangential sensitivity and dynamic range for the weak signal at

75 MHz were not altered by the interfering signal at 100 MHz unless the

interference level was greater than 10 mV/m. However, the antenna factor

was changed when the interfering field was greater than 1 mV/m. When the

out-of-band signal at 100 MHz was large, the receiver indication was less,

causing error when using the antenna system to measure an unknown field

strength.

6. DESIGN AND FABRICATION OF THE COMPLETE ISOTROPIC ANTENNA AND CONTROL UNIT

6 . 1 Description of the System

The three active dipoles and associated fiber-optic circuitry have been

assembled and tested. Figure 32 is a photograph showing how the dipoles are mounted orthogonally with essentially a common center. The total frequency

range of each induced field component is amplified and used to modulate a high-speed LED. The isotropic antenna unit is connected to three glass

fiber waveguides, each 10 meters long. The auxiliary circuitry for the

avalanche photodiodes, time sequencing switches, and the analog "computer"

required for the RSS processing of the three dipole signals have also been

designed and tested. The latter circuitry is mounted on a separate "control

unit" chassis.

Figure 33 is a block diagram of the antenna control unit. Figure 34 is an interior (rear) view of the chassis showing the circuit cards and power

supply. The control unit performs two major functions, namely, to (1) demodulate the rf signal from the optical carrier, and (2) calculate and display a voltage proportional to the Hermitian magnitude of the electric field. The

25
first function is performed by three avalanche photodiodes (APD's) that

receive the infrared signals from the three antennas via glass fiber waveguides. The outputs from the three APD's are sequentially switched into the

antenna input of an FIM receiver.

Figure 35 is a view of the entire field strength measuring system. It

shows the isotropic antenna on a tripod, the control unit placed on top of

the conventional FIM, and the three fiber guides used to convey the infrared

signal between the antenna unit and the rear of the control unit.

A dc voltage from the FIM proportional to the signal-strength meter

indication is returned to the antenna control unit. This voltage is processed by an analog computer to give the RSS magnitude |V^ n

|, where

|Vin i - ^(X) +vJn

(Y) + V

2in (Z)] 1/2

. (12)

V^

n

(X, Y, or Z) is the rf voltage present at the FIM antenna input port when

either the X, Y, or Z APD and hence X, Y, or Z dipole is selected. The re- sulting voltage magnitude is displayed on a panel meter identical to the FIM

panel meter, but located on the control unit chassis.

A front-panel switch is provided to select and display the output from

any individual X, Y, or Z channel or the RSS magnitude. Other controls

include separate photocurrent meters, gain controls, and overload indicators

for each APD. Also on the front panel is an rf calibrator output supplying

a -20 dBm, 5 MHz square wave signal. All interconnections are made on the

back panel of the instrument. These include the three APD optical fiber

inputs, the rf signal output, the FIM interface cable, three battery charger

outputs, DVM outputs, and an IR source calibrator.

The following sections discuss the separate blocks in more detail.

6 . 2 Avalanche Photodiode Circuitry

The modulated IR signal from each LED is received and detected by a silicon APD. The circuits for the APD and bias voltage regulators are shown

in figures 36 and 37, which are similar to the simplified schematic of

figure 19. The APD is reverse-biased close to its breakdown voltage, specified by the manufacturer to lie between 140 and 200 volts. The APD is

capable of providing internal photocurrent gain in excess of 40 dB. The

gain is adjusted by varying the bias voltage. The demodulated rf signal

from the APD is coupled directly to the FIM 50 ft antenna input through the

dc blocking capacitors at the output terminal of figure 36.

26
The APD bias regulator, figure 37, consists of three major subsections,

(1) a 200 volt pre-reguiator , (2) adjustable APD bias, and (3) excess photocurrent clamp. The pre-regulator drops the raw dc (280 V nominal) from the

high voltage rectifier card to a regulated +200 V. The 200 V zener diode D2

provides a reference voltage to the Darlington pair emitter follower, Q1-Q2.

The adjustable APD bias current supply in figure 37 consists of" transistors Q4 through Q9 and their associated components. All of the transistors are connected in Darlington pairs, thus functioning as very high

beta transistors. Q4-Q5 and Q6-Q7 are connected as a "long tailed," emitter

coupled differential amplifier. The 43 kft "long tail" resistor functions as

a 3 mA current source. The 150 V Zener diode (Dl of figure 36) provides a 150 V reference voltage which is dropped by the 20 kft resistor (figure 37)

to provide a stable +130 V to the plus input (Q4 base) of the differential

amplifier. The APD bias voltage is applied to the minus input (Q7 base)

through the adjustable voltage divider labeled "APD bias adjust." Q8-Q9 is

a low output-impedance, emitter-follower with its base (Q8) connected to the

output (Q6-Q7 collector) of the differential amplifier. The APD breakdown

voltage, and hence its current gain at a fixed bias, is temperature sensitive. To compensate for this the APD bias is adjusted automatically. The

temperature reference diode Dl is selected to have a breakdown voltage and

temperature coefficient similar to the APD. Dl and the APD are mounted on

the same thermal heat sink.

The remaining components are for the APD excess photocurrent protection

circuit. The photocurrent is sensed by developing a small voltage across

the 1 kft current-sensing resistor of figure 36. This voltage is applied to

a 741 op. amp. (figure 37) operated open loop as a comparator circuit. The

other input to the 741 is fixed at 300 mV. When the photocurrent exceeds

300 uA, the voltage across the 1 kfi sensing resistor exceeds 300 mV and the

741 output goes positive, saturating near +15 V. This in turn causes Q10,

Qll, and Q3 to conduct. When Q3 conducts it drops the pre-regulator voltage

to +35 V. If the photocurrent then drops below 300 yA again, Q3 shuts off

automatically, allowing the pre-regulator to reset to +200 V. An LED is

provided on the front panel to indicate when an overload condition exists.

6 . 3 rf/IR Calibrator Circuit

As an aid in the initial setup and adjustment of the isotropic antenna

system, a calibrator is provided with an rf output and also a modulated IR

output for checking the APD's. The rf output is a 5 MHz square wave with

strong harmonic content. The output impedance is 50 ohms. The output level

across a 50 ohm load is a nominal -20 dBm at 5 MHz. The IR calibrator is a

27
high-speed LED identical to those used in the active antennas. It is

biased and also modulated with a 5 MHz square wave. The average LED current

is approximately 16 mA, the same as the LED current in the final active

antenna unit.

Figure 38 is a schematic diagram of the calibrator circuit. Ql is a

10 MHz crystal-controlled oscillator. Q2 is a buffer and Q3 is a TTL logic

driver. A 7474 flip-flop is used to obtain a 50% duty cycle, 5 MHz square

wave. Q4-Q5 from the discrete emitter-coupled-logic, high-speed output

stage. Q6 is the LED modulator.

6 . 4 Interface with the EMC-25 Receiver

Figure 39 shows the interface card used to mate the antenna control

unit to the receiver. The rf signals from the APD's are sequentially

switched into the single antenna input terminal of the receiver. Each

signal is applied to the FIM one second. The complete measurement cycle

takes 3.3 seconds. Switching is done by small reed relays K3 , K4 , and K5

.

The EMC-25 provides a dc output voltage V

Q across a 1.5 kfi, 1% resistor

in the inter-tie cable that is proportional to the signal strength meter

indication (1.5 volts full scale). A separate cable is used to connect this

"output meter" voltage and the EMC-25 "dump" control signal to the isotropic

antenna control unit. The meter dial indication is in terms of rf microvolts at the FIM antenna input cable. A second meter scale is linear in dB

above 1 yV.

The basic measurement cycle time of one second for each field component

(X, Y, or Z) is fixed by the slow response time of the EMC-25 receiver.

Figure 40 shows examples of the FIM response, V , to various-length rf

pulses for» respectively , the "carrier," "quasi-peak , " and "peak" modes of

detection. In the carrier mode the 0%-98% rise time is 0.6 s while the

100%-2% fall time is 1 s. For quasi-peak detection the risetime is 6 ms while the fall time remains at 1 s . For peak detection the risetime is only

a few microseconds. The discharge characteristics are determined by the

DUMP control. With the DUMP control fully ccw and into the switch detent

the circuit is in a hold mode with a fall time in excess of 10 s. With the

DUMP control ccw the meter reading is held for a dump delay of 300 ms and is

then discharged rapidly. The minimum dump delay is 20 ms when the control

is cw.The two op. amps, of figure 39 are used as a peak detector to hold the

maximum meter deflection V during each measurement time interval (X, Y, or

Z) . The peak detector is reset to zero at the end of each measurement

interval by relay Kl . At the same time relay K2 provides a contact opening

to "dump" the EMC-25 output meter circuit when the FIM is operated in the

PEAK mode.

28
6 . 5 Vector Magnitude Analog Computer

The vector magnitude of the X, Y , and Z inputs to the FIM is determined

by an analog computer that consists of three circuit boards, figures 41, 42,

and 43. Operational amplifiers, an antilog integrated circuit (IC) , sample

and hold circuits, an analog multiplier IC, and a logarithmic amplifier IC are used to calculate a voltage according to eq. (12) . The following discussion presents in detail the various mathematical operations performed.

The FIM output meter voltage V

Q follows the equation

V

Q

= 1.05 + 0.45 log

1() [Vin (yV)/l yV] volts. (13)

Thus for V

in = 10 uV, V

Q

= 1.5 V; for V

in

1' yV# V 1.05 V; for V

in = 0.1 yV, V

Q

= 0.6 V; and for V

in =0.01 yV, V

Q

= 0.15 V. The maximum meter

voltage during each sample time is detected and held in the. EMC- 2 5 interface,

where

V

om

- V

c (max). (14)

The first op. amp. of figure 41, IC-1, is simply a high input impedance

buffer such that

V

l = V

om' (15 >

IC-2 of figure 41 provides signal inversion, amplification and level shifting

to obtain

V

2 = -2.22 V

x + 3.33 (16)

= 1 - log10 [Vin (uV)/l yV] volts. (17)

IC-3 is an antilog IC that is used to extract a voltage proportional

to the FIM input voltage (in yV) squared. That is,

V

3 = V

ref 10 _(V2/K)

. (18)

The circuit constants are chosen to give

V

ref = 10 V, (19)

K = 0.5 (20)

thus producing a voltage given by

V

3 = TQWin (W)/l yV] 2 volts. (21)

2 Thus at this point we effectively have V

in

29
2 2 2 The next step is to obtain the summation of v

^n

( x )' V

in ^ ' and Vin

This is accomplished by first storing V" 3

(X) , V"3

(Y) , and V~ 3 (Z) in three

separate sample-and-hold circuits, IC-4, 5, 6 in figure 41. The - output

voltages are

v

4 = V3

(X) « V

^n

(X)

V

5 = V3^Y) a Vin (Y) and

v

6 = V

3 (Z) « v. n

(z).

(22)

(23)

(24)

These voltages are algebraically added in the op. amp. adder IC-7 of figure

42 to obtain , given by

V

7 " " J

(V

4 + V

5 + V (25)

2l

30-

VinWi

l l uv

IC-8 is simply an inverter, so

V

in (Y >-

1 yV

V. (Z)! in

l yv J

, (26)

V = -V . (27)

IC-9 is an analog multiplier connected to perform the square root operation,

V

9 = -/10 V

g (28)

[f

V

in (X ^

2 +

rV. n

(Y), 2 +

f Vin (Z)l 2

1 1/2

} l uv. - 1 1 yV J L 1 yV •

IC-10 is an inverter, so

V

10 = "V

(29)

(30)

V^q is sampled by K4 each second and held by the 0.1 yF capacitor and IC-11

That is

= V^q (sampled) (31)

is the desired answer and is linearly proportional to |v\ n(yV)| in equivalent microvolts. The EMC-25 meter scale is linear in dByV, thus must

be processed by a log circuit, IC-12 of figure 43, before driving the meter.

Thus

V

12 = -log10 (V1:L /5.77 V) (32)

30
The meter on the control unit panel is driven by IC-13 according to the

equation

I (meter) = -0.3 V

12 (volts) + 1 mA.

In addition to the front panel meter shown in figure 43, two outputs

are available on the back panel for connecting an external digital voltmeter

(DVM) . IC-14 provides an output proportional to dByV while IC-15*s output

is proportional to uV. These voltages are given by

V14 = "2 V

12 + 2, (34)

V

15 = 0.173 Vu. (35)

Table 14 lists the voltages throughout the analog computer for various

rf input voltages. As can be seen from the table, for antenna input voltage

below 0.1 uV, the squaring process yields values for V" 7 in the microvolt

region. Due to noise, drifts, offsets, etc., in the analog computer these

low level signals are useless. To obtain accurate field strength measurements,

the rf signals should be greater than 1 yV.

6 . 6 X-Y-Z Measurement Sequencer

The measurement sequencer board, figure 44, provides the timing and

control signals for the interface circuits and the analog computer. As

discussed previously, the 3.43 s measurement cycle time is fixed by the slow

response time of the FIM. Figure 45 shows the important control signals and

their timing relationship. Figure 46 shows the internal signals within the

measurement sequencer. CMOS integrated digital logic is used.

An internal clock is provided which consists of a 555 timer, IC-1, and

two flip-flops, IC-2 and 3. The 555 timer produces a pulse train at 28 Hz.

The flip-flops produce square waves at 14 Hz and 7 Hz.

The X, Y, Z pulses are the antenna channel selectors. Each pulse is of

1 s duration. These pulses are generated by three 8-bit shift registers,

IC-6 , 7 and 8. A measurement cycle is started by a LOAD pulse which loads

the data words X = 11111110, Y = 00000000, and Z = 00000000 into the shift

registers. The 7 Hz square wave, CLK, then sequentially shifts these words

out of the shift registers. As CLK repeats, a "0" on the serial input to

IC-6 is shifted in until the first register is full of all "0"s. The output

of the first register, X, is the serial input to the second register, IC-7.

Thus 8 CLK pulses later in time the X word appears as the Y pulse. The same thing occurs for the last register, IC-8, to generate the Z pulse.

31
With the particular data word selected for X, a dead time of one CLK

period (1/7 second) is provided between the X, Y, and Z pulses. During this

time interval the peak detectors on the interface board of figure 39, and in

the EMC-25 receiver, are discharged. This is accomplished through the DUMP

pulse. A NOR gate IC-18 determines when X, Y, and Z are all "0"s. In the

middle of the DUMP pulse a SAMPLE (IS) pulse is produced to activate the

sample and hold relay in the analog computer, K4 in figure 42. This is a 1/28 second pulse selected by the NOR gate IC-17 when DUMP, CLK, and CLKX2

are all "0"s. The DUMP and SAMPLE (IS) pulses occur three times during each

measurement cycle.

At the end of the measurement cycle the shift registers must be re- loaded. Figure 46 shows the various signals required to generate a LOAD

pulse. The negative-going transition of Z is inverted by IC-9 of figure 44

and used to clock the D type flip-flop IC-4 . The D input was a "1," thus the

flip-flop Q± is toggled to a "0." This "0" applied to the NOR gate IC-14

allowed it to pass a 1/28 second pulse when flip-flop Q^, CLK, and CLKX2 are

all "0"s. This pulse is labeled SAMPLE (3S) and is used to set the next

flip-flop IC-5. SAMPLE (3S) can be used to control the analog computer

sample-and-hold if a new meter reading is desired only every 3-1/2 seconds.

When flip-flop Q2 is set, this enables the NOR gate IC-15 to generate a 1/28

second LOAD pulse when flip-flop Q2

, CLK, and CLKX2 are all "0"s. The

positive-going transition of LOAD is used to clock a "0" into the flip-flop

Q

2 (IC-5) , thus resetting it.

The above discussion describes all sequences in a normal measurement

cycle. However, at power turn-on time it is possible for either or both of

the flip-flops, IC-4 and IC-5, to latch up in the wrong state. If this

occurs, a LOAD pulse would not be generated, thus resulting in X, Y, and Z all remaining at "0." To alleviate this problem a train of several LOAD

pulses is generated for one second after the power is turned on. This is

accomplished by interrupting the connection of flip-flop IC-5 into the LOAD

NOR gate IC-15 with an alternate gating pulse, START. An RC delay circuit

(1 Mft, 1 yF) is connected to the +5 V line. This RC circuit delays applying

a "1" to the input of inverter IC-11 for approximately one second. Thus the

output of the inverter is the 1 second START pulse. The NOR gate IC-16 is

used to route either the START pulse or the Q2 output of IC-5 into the LOAD

NOR gate, thus always assuring several LOAD pulses at turn-on time.

6 . 7 Auxiliary Equipment

Figure 47 is a schematic diagram of the battery charger for recharging

the 9.6 V Ni-Cd batteries in the active antennas. The circuit consists of

three 15 mA constant-current sources with over-voltage protection provided by

the 13 V Zener diode. The three diodes at the charger output terminals are
used to prevent the batteries from discharging back into the charger when the

ac power is turned off.

The high voltage (2 80 V) supply for the APD bias regulator is shown in

figure 48. The + 15 V supply is shown in figure 49. It is an ordinary regulated power supply. The only special requirement is that the voltages must

be precisely set to +15.00 V and -15.00 V because they are used as reference

voltages in the analog computer. Deviations in these voltages will affect

the computer accuracy.

Figure 50 is a diagram showing the interconnecting cables between the

antenna control unit, the rf/IR calibrator, and a conventional EMC-25

receiver.

A complete parts list and detailed instructions for proper alignment of

the electronic circuitry in the control unit have been furnished to ECOM in a separate manual.

7. SUMMARY AND CONCLUSIONS

A broadband isotropic antenna system was developed for measuring rf

emanations in a screen room environment. It uses a fiber-optic link between

the electrically-isolated antenna unit and a conventional EMI receiver. The

frequency range of interest is 15 kHz to 200 MHz. Above this frequency the

optoelectronic link suffers severe rolloff in response. However, at these

higher frequencies it is possible to use "hooded" antenna techniques and

anechoic chambers. The NBS active antenna consists of 3 mutually orthogonal

dipoles and responds to arbitrary polarization, which decreases some of the

difficulty encountered in screen room measurements. Each of the dipoles is

31 cm long x 3.5 cm diameter and contains an rf amplifier, electroluminescent

diode (LED) , and a battery. The dipole is electrically short at all frequencies, thus achieving a sensor with wide bandwidth. The antenna has high

sensitivity (up to 10 yV/m) and does not require tuning.

The signal picked up by each dipole is applied to a FET-input rf amplifier which modulates the infrared (IR) intensity of a solid-state LED. The

IR radiation, which is injected into the end of a glass fiber waveguide, is

thus modulated with all signal frequencies within the system bandwidth. The

far end of each guide is optically coupled to an avalanche photodiode (APD)

in the "control" unit, which recovers the rf modulation from the IR carrier.

Each of the 3 fiber guides is 10 m long, thus isolating the antenna electrically from the equipment under test and the enclosure walls. The 3 re- covered rf signals are applied to the EM receiver through a time sequencing

switch, sample-and-hold circuitry, and RSS processing. The one indication on

the field strength readout meter is proportional to the Hermitian magnitude

of the electric field.

33
The present antenna system uses an incoherent LED for the IR source, a silicon APD for the photodetector , and a multimode glass fiber bundle for the

transmission line. Compared with lasers, LED's are slow (5 to 10 ns risetime) , have relatively large spectral width (30 to 40 nm) and large beam

divergence (about 30°) . The slow response time limits the analog bandwidth

in the antenna to about 150 MHz. The large beam divergence makes it difficult to couple the LED output into the end of the fiber, causing coupling

losses of 20 to 25 dB. The APD used has a gain-bandwidth product of 100 GHz,

so it is not the limiting factor for high signal frequencies. The rolloff in

sensitivity at frequencies above 150 MHz is quite steep and is due mainly to

limitations of the LED source. An attempt was made to use current state-ofthe-art technology for the fiber-optic link and antenna system. The overall

system was designed to be compatible with an ECM-25 type of FIM.

New technologies employing laser diodes, single-mode fibers, and

integrated-circuit optical components may eventually provide the answer to

the search for an EMI antenna with huge bandwidth and high sensitivity.

However, the only battery-operated source available for our project, which

was compatible with fiber optics, was the incoherent LED. It is small,

light, rugged, reliable, inexpensive, and (most important) is easy to modulate with an rf signal. It can be battery operated with low voltage and low

current. The data presented in this report are believed to represent the

optimum performance for a non-lasing diode at the time of this project.

The initial tests for choosing the optimum components and improving the

circuit design were made by both time-domain and frequency-domain techniques.

In general, the testing of optoelectronic components was done by observing

pulses on a sampling oscilloscope. The preliminary design of the circuitry

was optimized by observing swept-frequency results on a spectrum analyzer.

Later refinements were then accomplished at discrete frequencies by immersing

the active dipole in a known field and checking the performance characteristics

The rf open-circuit pickup of a 31 cm dipole is quite low and the signal

is further reduced by the impedance mismatch between the short antenna and

the rf amplifier. The sensitivity of the antenna system developed at NBS is

adequate for signal frequencies below 25 MHz. At higher frequencies the

bandwidth of the receiver used is larger (50 kHz) and the sensitivity is

generally below that required for making MIL-STD-461 measurements. The

circuitry of the rf amplifier incorporates frequency compensation to ac- centuate the signals above 50 MHz, thus compensating for some of the LED

rolloff at these higher frequencies. Also, small toroid inductors were

designed to be connected across the center gap of each dipole. The resulting

antenna resonance increased the sensitivity enough to meet the specifications

up to a frequency of 150 MHz.

34
The antenna factor vs. frequency for the fiber-optic antenna was determined by calibrating the individual dipoles in the known field of a TEM transmission line. For our project the antenna sensitivity was defined as the

field intensity required to increase the receiver indication 3 dB above the

system noise level; that is, the tangential sensitivity. The ultimate sensitivity of any measurement system is set by the noise accompanying the signal,

and therefore by the system bandwidth. Using an EMC-25 receiver, the fiberoptic antenna system essentially satisfied the sensitivity requirements for

measuring narrowband fields according to MIL-STD-461 up to a frequency of

150 MHz. The linear dynamic range of the antenna system was defined as the

difference between the tangential sensitivity and the field level causing

3 dB compression. The dynamic range of the fiber-optic antenna system varied

from 70 dB at a signal frequency of 20 kHz to about 50 dB at a frequency of

150 MHz.

Acceptable performance of an infrared (LED/fiber guide/APD) antenna

system requires a combination of high sensitivity, low noise, large bandwidth

so no antenna tuning is required, and large dynamic range so the distortion

is low. Engineering tradeoffs were made in an attempt to achieve optimum

overall performance. The high-sensitivity system developed at NBS is some- what restricted by the low dynamic range of the system. The antenna is thus

most suitable for measurement of weak signals. When measuring a strong field

it is advisable to insert an attenuator in the rf amplifier. Also, the

present active antenna becomes desensitized by any one strong signal in the

total frequency range; this may cause error in the measured field strength of

a weak signal in the presence of a strong out-of-band signal.

It was not possible to demonstrate experimentally the degree to which

the antenna response pattern is truly isotropic. NBS does not have an anechoic chamber for measuring antenna patterns below 1 GHz, and calibration

at an outdoor field site was hampered by large ambient signals from local AM

and FM stations. However, from past experience in evaluating the response vs. orientation angle for microwave hazard probes, which also employ 3 orthogonal

dipoles in the rf sensor, it is known that an isotropic response is achieved

if the dipoles have separate responses which are equal and if accurate signal

processing is used to obtain the Hermitian magnitude. The three dipoles

in the engineering prototype antenna delivered to ECOM were adjusted for

equal response at a frequency of 25 MHz and a field level of 100 yV/m. The

required alignment procedure for the antenna, a parts list, and a set of

schematic diagrams were furnished to ECOM in a separate instruction manual.

35
It is not possible to measure a single transient pulse of interference

with the time sequencing approach used here because the metering unit does

not sample the output of all 3 dipoles simultaneously. The reception and

measurement of impulse interference caused by repetitive pulses can be

accomplished, however, even when time sequencing the X-Y-Z field components.

One procedure to insure pickup of an isolated pulse of unknown polarization

would be to use three separate EMI receievers, although this is not an economical approach.

In conclusion, we have developed a sensitive, isotropic, electricallyisolated antenna, using commercially-available fiber-optic technology. The

antenna can be used to measure weak steady-state emanations in a shieldedroom environment. It was found that an infrared system employing LED

sources, multimode glass fiber transmission lines, and avalanche photodiode

detectors can offer certain improvements over a more conventional antenna.

Some of the problems relating to fiber-optic technology have not yet been

solved, and should receive further development work. However, the basic

concept of an isotropic EMI antenna has been verified. Newer and better

components and techniques, such as integrated optics and solid-state laser

diodes, could lead to significant improvements in performance.

8. RECOMMENDATIONS AND FUTURE PLANS

The results achieved in this NBS/ECOM program have been very encouraging

in nearly all aspects. The concept of measuring field strength with an active isotropic antenna using a fiber-optic link to the EMI receiver has

been demonstrated. In view of certain advantages gained by this type of

isolated probe, it would be highly desirable to continue this research effort,

evaluating additional opto-electronic components and hardware for improving

the measurement system. Also, it might be possible to simplify some of the

circuitry, alignment procedure, and technique for measuring absolute field

strength; these now require a fairly high degree of skill. It would then be

possible to fabricate a general purpose EMI antenna with a more repeatable

antenna factor and a greater dynamic range.

In particular, it is suggested that a more thorough evaluation be made

of a new type of "high speed" LED which became available recently [34 and 35]

.

One of these LED's was checked briefly at signal frequencies from 25 to

200 MHz. The overall performance (tangential sensitivity, linearity and

dynamic range) was no better than the LED used in our project. In fact,

at the upper frequency end of the spectrum (150 to 200 MHz) the sensitivity

was worse by about 2 dB. This type of LED has a specified frequency rolloff

of only 3 dB at 200 MHz. Our measured rolloff was much greater. However,

the new LED was tested for only one value of bias current (16 mA) which may

have been too low to achieve the expected high frequency improvement.

36
NBS thus recommends a follow-on program in which the existing isotropic

antenna and control unit are more thoroughly evaluated in order to establish

the optimum procedure for measuring absolute field strength. Additional

development work could then be initiated if that is considered advisable.

Another possible investigation would be to evaluate the new GaAlAs laser

diodes which have just become available. These are solid-state replacements

for bulkier and less efficient gas lasers. They have a specified risetime

of only 0.1 ns, beam spread of only 5° and spectral width of only 2.5 nm,

with peak emission at 850 nm. However, the specified minimum forward

current is about 150 rnA, so they would require frequent battery replacement

or other means of supplying power to the isolated antenna.

9 . ACKNOWLEDGMENTS

This work was sponsored by the U.S. Army Electronics Command, Fort

Monmouth, New Jersey, under Contract No. 7 4-93095. Appreciation is expressed

to all those who have contributed time, interest, and suggestions for the success of this project, especially to the project monitor at ECOM,

Mr. Paul Major.

Several employees in the Electromagnetics Division of NBS have helped

and encouraged us in various aspects of this project. Recognition of the

contributions of some of these people can be listed as follows:

Dr. Ramon Baird — Helpful advice as Chief of the Antennas and Fields

Section at NBS,

Stanley Kilgore — Preliminary testing and field strength calibrations,

William Jessen — Assembly of the isotropic antenna unit and performance

testing of the field strength measurement system,

John Adams and Richard FitzGerrell — Useful suggestions and technical

readers for this report,

Sharon Foote — Efficient typing of the instruction manual and final

report, and

Dr. James Wittke of RCA — Conversations regarding the latest developments

in high speed LED's.

10. REFERENCES

[1] W.R. Free (Georgia Inst, of Tech.), "Radiated EMI Measurements in

Shielded Enclosures," IEEE EMC Symposium Record, Washington, D.C.,

pp. 43-53 (July 1967)

.

[2] W.C. Dolle and G.N. VanSteenberg (Southwest Research Inst.), "Effects

of Shielded Enclosure Resonances on Measurement Accuracy," IEEE Inter--

national Symposium on EMC, Anaheim, Calif., pp. 417-420 (July 1970).

37
[3] C.W. Stuckey, "The Hooded Antenna -- An Approach to Meaningful Field

Strength Measurements in Shielded Enclosures," IEEE Trans, on EMC,

pp. 360-367 (December 1965)

.

[4] W.R. Free and C.W. Stuckey, "Electromagnetic Interference Measurement

Methodology-Communication Equipment," Technical Report ECOM-0189F

(October 1969)

.

[5] C.W. Stuckey, W.R. Free and D.W. Robertson, "Preliminary Interpretation

of Near-Field Effects on Measurement Accuracy in Shielded Enclosures,"

IEEE EMC Symposium Record, Asbury Park, NJ, pp. 119-127 (June 1969)

.

[6] R.L. McKenzie, "Advanced Measurement Techniques Within Shielded Enclosures," Research and Development Technical Report ECOM-4226, U.S. Army

Electronics Command, Fort Monmouth, N.J. (June 1974)

.

[7] R.R. Bowman, E.B. Larsen and D.R. Belsher, "Electromagnetic Field Measuring Device," U.S. Patent No. 3,750,017 (July 31, 1973).

[8] F.M. Greene, G.R. Reeve, E.B. Larsen and W.E. Jessen, "Near-Zone E-M

Field Measurement Studies," DASA Order No. HD-1102-3171-0001 , NBS

report (unpublished), Chapter 2 (June 20, 1963).

[9] J.C. Ross, Jr. and S. Jauregui, "A Wideband RF Application of Fiber

Optics," (2 to 30 MHz), Naval Postgraduate School Thesis, Monterey,

Calif. (June 1974)

.

[10] G.N. VanSteenberg, E.L. Bronaugh, O.L. Jouffray and W.C. Dolle, "Radiation Field Measurement Study," USAF Contract F 41609-69-C-0008 , 40 pages

(October 1969)

.

[11] G.N. VanSteenberg, E.L. Bronaugh, G.T. Darilek and W.C. Dolle (Southwest

Research Inst.), "An Isolated Antenna System for Shielded Enclosure Measurements, 20-200 MHz," IEEE International Symposium Record, Philadelphia

Pa., pp. 198-202 (July 1971).

[12] MIL-STD-461A, "Electromagnetic Interference Characteristics, Requirements for Equipment," U.S. Department of Defense (August 1, 1968).

[13] MIL-STD-462, "Electromagnetic Interference Characteristics, Measurement

of," U.S. Department of Defense (July 31, 1967).

[14] C.L. Andrews, "Microwave Probes for Electric Fields Near Metal Surfaces,"

IEEE Trans. A&P, pp. 441-445 (July 1968).

[15] CD. Taylor, S.M. Lin and H.V. Adams, "Scattering From Crossed Wires,"

IEEE Trans. A&P, pp. 133-136 (January 1970).

[16] L.R. Bloom, "Optical Communications — In the 70' s and Beyond," Report

to the National Science Foundation under Contract H-1221, Panel on Telecommunications Research, pp. 118-129 (June 1973)

.

[17] S.E. Miller, E.A.J. Marcatili and T. Li, "Research Toward Optical

Fiber Transmission Systems. Part I: The Transmission Medium.

Part II: Devices and Systems Considerations," Proc. IEEE, Vol. 61,

pp. 1703-1751 (December 1973).

38
[18] R.L. Gallawa, "Optical Waveguide Technology for Modern Urban Communications," IEEE Trans. Commun. , No. 1, pp. 131-142 (January 1975).

[19] J.J. Pan, "High Frequency, Wideband Fiber-Optic Link," Digest of Technical Papers, Meeting on Optical Fiber Transmission, Williamsburg, VA

(January 7-9, 1975)

.

[20] J. P. Wittke, "Designer's Guide to: Optical-Fiber Communications

Systems," EDN, pp. 30-36 (May 20, 1975).

[21] F.M. Mims, III, "Light-Beam Communications," 160 pages (H.W. Sams & Co.,

Inc., First Edition, 1975).

[22] R.L. Gallewa, "A User's Manual for Optical Waveguide Communications,"

OT Report No. 76-83, U.S. Dept. of Commerce, Office of Telecommunications, 287 pages (March 1976).

[23] J.K. Gait, "Lasers and LED's for Optical Fiber Transmission," Digest

of Technical Papers, Meeting on Optical Fiber Transmission, Williamsburg,

VA (January 7-9, 1975).

[24] F.L. Thiel and W.B. Bielawski, "Optical Waveguides Look Brighter Than

Ever," Electronics, pp. 89-96 (March 21, 1974).

[25] J. P. Wittke, "Dispersion-limited Modulation Bandwidths of Optical

Fibers," RCA Review, Vol. 35, pp. 198-215 (June 1974).

[26] M. DiDomenico, Jr., "Wires of Glass," Industrial Research, pp. 50-54

(August 1974) . [27] F.M. Greene," NBS Field-Strength Standards and Measurements (30 Hz to

1000 MHz)," Proc. IEEE, Vol. 55, pp. 970-981 (June 1967).

[2 8] H.E. Taggart and J.L. Workman, "Calibration Principles and Procedures

for Field Strength Meters (30 Hz to 1 GHz)," Nat. Bur. Stand. (U.S.), Tech.

Note 370, 151 pages (March 1969).

[29] M.L. Crawford, "Generation of Standard EM Fields Using TEM Transmission Cells," IEEE Trans, on EMC, Vol. EMC-16, No. 4, pp. 189-195

(November 19 74)

.

[30] D.F. Wait, "Measurement of Amplifier Noise," Microwave Journal, Vol. 16,

No. 1 (January 1973)

.

[31] M.G. Arthur, "The Measurement of Noise Performance Factors: A Metrology

Guide," Nat. Bur. Stand (U.S.), Monog. 142, 188 pages (June 1974).

[32] W. Hayward, "Defining and Measuring Receiver Dynamic Range," QST,

pp. 15-21 (July 1975)

.

[33] "Nonlinear System Modeling and Analysis with Applications to Communications Receivers," Rome Air Development Center Report No. RADC-TR-73-178

,

Edited by J.W. Graham, L. Ehrman Consulting Scientist, Signatron, Inc.

(June 1973)

.

39
[34] H. Kressel and M. Ettenberg, "A New Edge-Emitting (AlGa) As Hetero junction

LED for Fiber-Optic Communications," Proc. IEEE, Vol. 63, pp. 1360-1361

(September 1975)

.

[35] J.C. Blackburn, "A 120-MHz Bandwidth Linear Signal Transmission System

Using Fiber Optics," IEEE Trans. Instr. & Meas., Vol. IM-24, pp. 230-232

(September 1975)

.

40
41
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43
Table 1. Sensitivity requirements for measuring narrowband

signals according to MIL-STD-461/462

.

Signal

Frequency

MIL-STD-461/642

E Field Limits Open-Circuit

Pickup of a 31 cm

Dipole = 0.15 E

Approximate

Receiver

dByV/m yV/m Input

15 kHz 35.0 56 8.4 yV 4.2 yV

100 kHz 31.2 36 5.4 2.7

1 MHz 26.5 21 3.2 1.6

10 MHz 21.9 12 1.9 0.9

25 MHz 20.0 10 1.5 0.8

100 MHz 29.1 29 4.3 2.1

20 0 MHz 33.7 48 7.3 3.6

1 GHz 44.5 168

10 GHz 60.0 1000

44
Table 2. Characteristics of fiber-optic and radiated infrared

communication links.

Advantages

:

Large information bandwidth

High security, low cross-talk and pickup

- Freedom from EMI, no E or H coupling

Electrical isolation, absence of ground loops

No short circuits, arcing or grounding problems

Components are small, lightweight, and low power

Transmission lines are light, flexible and

low loss

For radiated links: large antenna gain, no

license required

Will eventually be cheaper than coax or radiated

rf links.

Disadvantages

:

Shortage of off-the-shelf opto-electronic components

Glass fibers are difficult to splice and are not

rugged to handle

.

i- -1- WU\J O ^ \wi w O >— Q Outer space and aircraft applications (lightweight)

Military airplanes and ships (high security)

Inter and inner city trunk lines (large

bandwidth)

Electrically isolated links for EMI antennas and

rf hazard probes.

45
Forward current, mA

Figure 5. Intensity of radiation vs. forward current for a typical infrared LED.

46
Table 3. Specified CW characteristics of some typical LED's.

Type Number *471 31 3 930 4120 2430 6478

Forward bias current, mA 150 100 50 100 100 150 3000

Radiated power output, mW 1 6 0.8 0.65 0.2 0.3 300

Peak emission wavelength, nm 910 940 900 900 890 907 935

Spectral bandwidth at 50% of

peak intensity, nm

23 60 50 40 35 22

Beam spread at 50% of peak

intensity, degrees

130 10 56 30 90 32 180

LED emitting diameter, mils 18 24

10%-90% risetime, ns 15 600 1 100 20 700

Table 4. Specified characteristics of some typical photodetectors

.

Type Number 30807 2 4220 3421 30817 59

-*- 451

7102

(Tube)

Type of device Si PIN

diode

Si PIN

diode

Si PIN

diode

Si PIN

diode Si APD Si APD Si APD

Photomultiplier

Responsivity , uA/yW 0.65 0.7 0.5 0.55 80 24 400

Peak spectral response, nm

900 900 770 890 900 770 850 800

Spectral bandwidth at

10% points

400-

1100

310-

1140

400-

1040

420-

1100

420-

1100

550-

960

550-

1100

Active reception area, mm 2 0.8 0.58 0.2 1.3 0.5 0.45 0.45 780

Current amplification

factor

1 1 1 1 120 120 200 150,000

Noise equivalent power,

pW/Hz

0.1 0.01 0.2 0.2

10-90% risetime, ns 3 0.5 < 1 < 5 2 < 10 < 10 25

47
Reverse bias on APD, volts

Figure 6. Measured performance of avalanche photodiode No. 2

at an ambient temperature of 28°C.

48
SIGNAL

GENERATOR

(25 MHz)

RF AMF

1 TO 2 20 dE

LIFIER

00 MHz

GAIN

LED C (AM MOC

RIVER

ULATOR)

- HIGH

VOLTAGE

Q

RF AMPLIFIER

1 TO 200 MHz

40 dB GAIN

®

SPECTRUM

ANALYZER

(25 MHz)

Figure 7. Instrumentation for an experimental radiated

communications link.

49
Figure 8 . Performance of the experimental radiated link at a

signal frequency of 25 MHz.

50
rf signal attenuation at low frequencies:

LED alone 75 dB loss

APD alone • 60 dB gain

LED and APD 15 dB loss

Fiber coupling 45 dB loss

LED/f iber/APD system 60 dB loss

Figure 9. Simplified schematic of the instrumentation for

measuring rf attenuation of the fiber-optic link.

51
52
53
Table 5. Specified characteristics of the EMC-25 receiver.

Receiver

Band

Number

Frequency

Range

Intermediate

Frequency

Nominal -3 dB

Bandwidth

Tangential

Sensitivity

NB WB

NB (CW)

(yV) • V«!B (Impulses)

[dByV/MHz)

1 14.0-28.5 kHz 175 kHz 500 Hz 4 kHz 0.06 39

2 28.5-59 kHz 175 kHz 500 Hz 4 kHz 0.06 39

3 59-120 kHz 175 kHz 500 Hz 4 kHz 0.06 39

4 120-240 kHz 175 kHz 500 Hz 4 kHz 0.06 39

5 240-480 kHz 175 kHz 500 Hz 4 kHz 0.06 39

6 0.48-1.05 MHz 175 kHz 500 Hz 5 kHz 0.06 39

7 1.05-2.3 MHz 175 kHz 500 Hz 5 kHz 0.06 38

8 2.3-5 MHz 1 . D

1 ft T 7 rt MHZ cJ Knz D U Krl Z. U -L £ 22

9 5-11 MHz 1.6 MHz 5 kHz 50 kHz 0.12 22

10 11-25 MHz 1.6 MHz 5 kHz 50 kHz 0.12 22

11 24-49 MHz 8.7 MHz 50 kHz 500 kHz 0.80 20

12 49-98 MHz 8.7 MHz 50 kHz 500 kHz 0.80 20

98-210 MHz 8.7 MHz 50 kHz 500 kHz 0.80 23

Table 6. Bandwidth of the EMC-25 FIM receiver in narrowband

selectivity position.

(a) Bandwidths according to the manufacturer:

Band

Number

Frequency

Range

Nominal

Bandwidth

1 to 7

8 to 10

11 to 13

14 kHz to 2.3 MHz

2.3 to 25 MHz

24 to 210 MHz

500 Hz

5 kHz

50 kHz

(b) Measured bandwidths:

Band

Number

Receiver

Frequency

Signal Frequency for

-3 dB Indication

-3 dB

Bandwidth

1

37

8 10

11

13

15 kHz

100 kHz

2.3 MHz

2 . 3 MHz

25 MHz

25 MHz

200 MHz

14.676-15.350 kHz

99.742-100.312 kHz

2.29974-2.30029 MHz

2.29754-2.30256 MHz

24.99758-25.00280 MHz

24.96805-25.02701 MHz

199.9738-200.0268 MHz

0.67 kHz

0.57 kHz

0.55 kHz

5.0 kHz

5.2 kHz

59 kHz

53 kHz

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63
Table 7. Frequency response test of the prototype antenna

system. (Input signal = -80 dBm, Spectrum analyzer

bandwidth = 3 kHz .

)

Frequency, MHz Gain, dB vS+N)/N ratio, dB

lo lb

25 16 20

50 15 19

c0 lo

100 5 16

125 0 12

150 -3 10

! 175 -4 8

200

_

-6 6

Figure 21. Signals received in the laboratory by a prototype

active dipole. (Horizontal scan from DC to 200 MHz,

spectrum analyzer bandwidth = 3 kHz.)

64
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Q) • iH CO - O ^ a o i -H CO

<u

cd to >

•H

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CD "4-1 & c

-P -H

O g

>i

-p "ri H

>

-H

•P Q

•H CU

CO

-<C C

CD <4-i CO O

CM

CD td c g

rd O

Eh O

CN

CD H

fa

CD •H3

CP

>-l CD XIH

m

M-lO

HP e = N/(N+S) *oj 'ui/Artgp 'PI©!? a paJxnbeH

68
c

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<D

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tn <u 3

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i-i • < 0 r-~ ftiH - hODE<J H

(D 3 Lfi > tT*1

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33 -P CU 3 2 H U

<4-l 4-1 0 -H

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h id » 5 > g D1 •H 4-1 0) •PUN U •H

4-1 oj -a ii c a> iH CI N ll to 03 -H XI c g "H

CP

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r~

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C S-l <u o o w

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n -H O

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g

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n

CNH

Ed

HP 'SSOI UOTq.J3SUI

70
gp e = N/(N+S) Jogp 'ui/ArtaP 'PT9TJ a paaTnban
Table 9. Description of the parallel-resonating coils

used with the final active antenna.

Ton 1 Number of Core

Wire

Size

Number

of Turns

Series

Resistor

1 Two Q3 #36 23 so n

2 One Q3 #30 6 10 n

3 One Q3 #30 4 None

Table 10. Measured performance of the fiber-optic antenna system.

Signal Receiver Tangential Overload Dynamic Antenna

Frequency Band Sensitivity, Level

,

Range

,

Factor,

MHz Number dByV/m dByV/m dB dB

0.015 1 43.5 110.5 67 54.5

0.025 1 37.5 110.5 73 47

0.05 2 30 103 73 37.5

0.1 3 25 99 74 33.5

0.3 5 20 92 72 30.5

1 6 20 93.5 73.5 32.5

2.3 7 17 87 70 34.5

2.3 8 25 92 67 32.5

5 9 23 86 63 32.5

10 9 21.5 91.5 70 37

25 10 20 86 66 30.5

25 11 30 85 55 30.5

40 11 26.5 77.5 51 32.5

60 12 24.5 66.5 42 20

80 12 ! 25 64 39 1 17

100 13 27.5 73.5 46 27

150 13 43.5 92 48.5 44.5

! 200

i

13 58 102 44 60

72
Table 11. Measured performance of the fiber-optic antenna

system with an inductor across the dipole gap.

1 Type of Signal LCI VCI ' 1 1 T-j r~r f\ *-\ -i— i

"] langenciai Overload Dynamic Antenna

Parallel Frequency, Band Sensitivity

,

Level

,

Range

,

Factor

Inductor MHz Number dByV/m dByV/m dB dB

23 turn 10 9 42 110 68 55.1

toroid. 15 10 30 .5 99.5 69 43.4

50 ft series 25 10 12 81 69 23.7

resistor 25 11 22.5 79 .5 57 23.9

40 11 21 72 51 17.7

60 12 23 57 34 18.3

80 12 25 66 41 18 .2

100 13 28.5 75.5 47 28.4

120 13 33 78 45 34.1

6 turn 10 g by 76.5

toroid

,

15 10 57 121 64 67.2

10 ft series 25 10 46 110 64 55.9

resistor 25 11 56 .5 109.5 53 55.8

40 11 44.5 94.3 50 39.7

60 12 33.5 66.5 33 20.6

80 12 26 62 36 14.2

100 13 20 .5 68.5 48 20.3

120 13 20 66 46 19.9

140 13 28 77 49 35.6

160 13 38.5 86.5 48 41.6

180 13 47.5 92.5 45 51.4

200 13 55 .5 100 .5 45 58.2

4 turn 25 11 63.5 116.5 53 63.2

toroid 40 11 52 100 48 47.9

60 12 41.5 74 .5 33 27.3

80 12 35 71 36 24.7

100 13 32 .5 78.5 46 33.5

120 13 31 78 47 34

140 13 30.5 83.5 53 36.8

160 13 31 75.5 44.5 30

180 13 38.5 85.5 47 40.3

200 13 51.5 95.5 44 54.2

73
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76
SIGNAL

GENERATOR

F

] = 75 MHz

SIGNAL

GENERATOR

F 2

= 100 MHz

< LOW PASS

FILTER

76 MHz

LOW PASS

FILTER

135 MHz

HYBRID POWER

COMBINER

ISOLATION > 70 dB

>

TEM

CROSS

CELL

1 .22 m X 1 .22 m

SECTION

Figure 31. | Instrumentation for measuring strong-signal desensiItization of the active dipole system.

77
Table 12. Desensitization of the active dipole system; measurements

of a "desired" signal at 75 MHz in the presence of an "interfering" signal at 100 MHz.

( Inte_r__ ering ( Desired ' , 7

( Inter r ering ( Desired —_____

S i grid. 1

S ign_i XSignalS igns. 1

TP f i £i IlclU __ neia Receiver Antenna £j IlclU __. IlclU Receiver Antenna

ciX.

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16 .35 10 .

24 .

40 23 17 A A 40 16 24

45 28 17 45 21 24

50 32 .17 .

50 26 .23 .

55 38.2 16 .

55 30 25

60 41 19 60 34 .

25 .

65 42.2 22 .

65 40 25

70 43.5 26 .

70 42 .

27 .

75 45.5 29 .

75 44 .30 .

80 47.2 32 .

80 46 .33.8

85 47.8 37.2 85 47.2 37 .

90 A C 46 44 90 44.7 45.3

50 24.1 7 17.1 80 27.1 -3 30.1

27 9 18 30 -1 31

3 A J u ±1.0 11 Q it) . Z*3 c "3 Q J . O ol , Z

J-> 1 "7 "3 1 / . O 1 / . O

A n 4U O.J IT *7 A A Z Z . Z

1 "7 Q 4 0 IJ . Z J_L . o 4 0

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1 -7 1 / C A TO "7 10 . / J 1 . o

C A "5 1 O Jl . o lo . Z

c r 3D ZZ . 0 DO j / . Z 17.8 C A oU zy Jl

£ A 40.5 19 . 5 ^ c 03 "3 O C JJ.D 31.5

65 42 23 70 37.5 32.5

70 43.2 26 .75 41.5 33.5

75 45 30 80 44 . 2 35.8

80 46 .34.5 85 46 .

38.2

85 47.2 37 .90 45.5 44.5

90 44.5 45.5 95 43.7 51 .

60 24.7 6.8 17.9 90 36.1 -4.8 40 .9 27 8.7 18.3 40 -1.5 41.5

30 11.3 18.7 45 3.5 41.5

35 17.2 17.8 50 8 42

40 22 18 55 12.8 42.2

i 45 27.5 17 .5 60 18 42

50 31.5 18.5 65 23.5 41.5

55 36.7 18.3 70 29 41

60 40 20 75 34 41

65 41.8 23.2 80 39.5 40 .5 70 43 27 85 42 43

75 44.8 30.2 90 45 45

80 46.3 33.7 95 45.5 49.5

85 47.2 37.8 100 45.7 54.3

90 45.5 44.5 105 45.8 59.2

7 8
Figure 32. Photograph of the isotropic antenna.

79
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x o. «t a.

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<: " °-

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O <" -«

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(— «t

U

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(0 • •H

X) H

.* 4-1 O M

0 (0 H 0t m —

80
Bl
82
Figure 35. Photograph of the complete fiber-optic antenna

measurement system including the crossed dipoles,

fiber guides, control unit and conventional receiver.

83
TEMPERATURE

REFERENCE

DIODE

V

ZENER Jl 01

1N5276A*

*

(150V)

*CLOSEST ZENER TO

MATCH APD (lOyA

DARK)

COMMON

APD BIAS

(135 - 175V)

PHOTOCURRENT

(200yA,FS)

THERMAL

BOND

APD

lOyH

4.7k

"•-3

lk

C001

APD

CURRENT

SENSE

RF OUTPUT TO

50 n RECEIVER

BNC

Figure 36. Schematic of each avalanche photodiode circuitry.

84
85
86
O < J

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— *r — CO — CM

ooQZo

LU

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— o

COCD 0 T5 P 0 U

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ain

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en 9 0
HH

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(0c

(00

-O3

-U •HC

Di

id6uo

-uo

<u>

-pOo o

-Hme -a- u o

0)u3 > 5
Table 13. "Analog computer" voltages for various antenna input voltages.

unanne x a , x or ^ antenna voltage Vector

Magnitude

x / . -3«£ yv

0.01 yV 0.1 yV 1.0 yv 10 yV !i.e. , 10 yV on

1

i {, -4U aB yv ; * zu do yv; (0 dB yV) ! (+20 dB yV) each channel)

V V

om' V1 : 0 . lb V U . oU V 1.05 V !

1

I

1.50 V j 1 . 50 V

3 . 00 V 2 . 00 V 1.00 V

|

1 0.00 V 0 . 00 V

V .V ,v ,v 3' 4' 5' 6

10 yV 1 mV 100 mV 10.0 V 10.0 V „V

7 -3.33 yV -333 yV -33.3 mV -3.33 V -10 V

V

8

3.33 y

V

333 yV 33.3 mV 3.33 V 10 V

V

9

-5.77 mV -57.7 mV -577 mV -5.77 V -10 V

v

io' vn

5.77 mV 57.7 mV 577 mV 5.77 V 10 V

V

12

3.0 V 2.0 V 1.0 V 0.0 V -239 mV

V

13

1.0 V 4.0 V 7.0 V 10.0 V 10.7 V

I (meter) 0.1 mA 0.4 mA 0.7 mA 1.0 mA 1.07 mA

V

14

(dB out) -4.0 V -2.0 V 0.0 V 2.0 V 2.48 V

V

15

(yV out) 1 mV 10 mV 100 mV 1.0 V 1.73 V

Meter Reading 0.01 yV* 0.1 yV 1.0 yV 10 yV off scale

*Note: Actual analog computer voltages and outputs are invalid for antenna

input voltage less than 0.1 VV.

92

94
CLKX2

[*-l/14S-^

J L_T 14 Hz

CLK 1

7 Hz

DUMP

SAMPLE

(IS)

F/F Q 1

( IC-4 )

SAMPLE 1 (3S) 0

F/F Q 21

( IC-5 ) 0

F/F R 1

( IC-4 ) 0

_l—

L

LOAD

Figure 46. Timing diagrams of the measurement sequencer interval

signals

.

95
36V AC

CT 1N4004 (+18 TO 23V)

115V AC

1N4004 I

640

2N3638

1N5243A

(13V)

15mA 15mA 15mA

Figure 47. Schematic of the charger for the NiCd batteries.

96
97
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99
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 75-8192. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Sensitive Isotropic Antenna with Fiber-Optic Link

to a Conventional Receiver

5. Publication Date oeptemDer iy/o

6. Performing Organization Code

276

7. AUTHOR(S)

E.B. Larsen, J.R. Andrews, E.E. Baldwin

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. NBS No. 2767402

11. Contract/Grant No. ECOM No. 74-93095

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

U.S. Army Electronics Command, AMSEL-NL-C

Fort Monmouth, New Jersey 07703

13. Type of Report & Period Covered . Final

April 1973-Sept. 1974

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) A broadband, active, isotropic receiving antenna was developed at NBS for the

frequency range 15 kHz to 150 MHz. It was designed for use with a conventional re- ceiver to measure weak, near-zone electric fields of unknown polarization, such as leakage emanations from electronic equipment placed within a shielded enclosure. The

antenna system consists of three mutually-orthogonal active dipoles, each 31 cm long by

3.5 cm diameter. The entire frequency range of each of the three field components is

amplified and used to modulate a high-speed light emitting diode (LED) located inside

the dipole. The modulated infrared (IR) signals are guided through glass fibers 10

meters long which connect the "isolated" dipoles to avalanche photodiodes at the far

end of the fiber guides. These photodetectors recover the rf modulation from the IR

carrier for input to the receiver.

The fiber-optic antenna system described in this paper has high sensitivity (down

to 10 yV/m) and fast response time (rf modulations up to 150 MHz). The readout indication at each receiver frequency is proportional to the Hermitian magnitude of E, which

is the root-sum-square value of three orthogonal E field components at the measurement

point. The linear dynamic range of the antenna system depends on the receiver bandwidth and signal frequency. It is 70 to 75 dB at frequencies between 0.02 and 2 MHz

(for 0.5 kHz receiver bandwidth), 60 to 70 dB between 2 and 25 MHz (for 5 kHz receiver

bandwidth) and 40 to 55 dB between 25 and 200 MHz (for 50 kHz bandwidth)

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Electromagnetic compatibility; fiber optics; field strength measurement; isolated EMC

antenna; sensitive isotropic probe.

18. AVAILABILITY [j0 Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

[ For Official Distribution. Do Not Release to NTIS

UNCL ASSIFIED 107

1 Order From Sup. of Doc, U.S. Government Printing Washineton. D.C. 20402. SD Cat. No. C13

Office 20. SECURITY CLASS

(THIS PAGE)

22. Price

| X ! Order From National Technical Information Service

Springfield, Virginia 22151

(NTIS)

UNCLASSIFIED $5.50

USCOMM • ERL

i 6 4 3 <M


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