3.1 TD Boresight Waveforms

14

A1.9-kV impulse was fed into the UWB-UWA transmitting antenna. At the
receiving antenna, a 12.5-V/m electric field was measured. After the im-
pulse passed through this antenna and the delay line, an amplitude of
250 mV was measured on the sampling scope.

Figure 11 shows the basic copolarized and cross-polarized waveforms for
the UWB-UWA LP. The antennas were copolarized if they were both set to
transmit and receive the same orientation E-field or H-field. This is indi-
cated by “HH” and “EE” in the caption. The cross-polarized plots represent
a situation in which the transmitting antenna transmitted one field and the
receiving antenna was polarized at 90° to that field. Cross polarization is
indicated by “HE” and “EH” in the caption. The first letter in these desig-
nations refers to the polarization of the transmitting antenna, and the sec-
ond letter refers to the polarization of the receiving antenna. For example, a
designation of “HE” indicates the transmitting antenna is polarized in the
H-plane while the receiving antenna is polarized in the E-plane. These plots
show that the signal strength measured at the receiving antenna in the cross-
polarized mode is 1/2 of that measured in a copolarized mode. This corre-
sponds to a 10-dB cross-polarization isolation.

Figure 13 represents the results from a configuration in which the antennas
are pointing directly at each other, boresight to boresight. The four plots
were taken with two identical commercial LP antennas. Multipath reflec-
tions were superimposed on the original signal. Reflections off the sandbox
lagged the start of the signal by ~22 ns and reflections off the garage door
lagged by ~70 ns.

Figure 12 shows the waveforms recorded by the Tektronix 11801 for both
polarizations of the UWB-UWA LP antennas. This figure shows the raw
data taken on boresight for antenna 2 of the inductively loaded LP. These
results are almost identical to the waveforms of antenna 1 (see fig. 11)
colocated in the cube (see fig. 1(a)).

Figure 10 illustrates how increasing the number of waveforms underlying
an average reduces the noise level. The plot on the left is an FFT of a signal
average of three waveforms. Plainly, the ambient noise floor becomes sig-
nificant above 2 GHz. This plot is in contrast to the plot on the right that
shows the Fourier transform of a signal averaged with 3000 waveforms and
the Fourier transform of the original Kentec output pulse (blue). The re-
ceived signal average appears in green. In this plot, the noise floor is signifi-
cant above 4 GHz. Hence, increasing the number of signals underlying an
average reduces ambient noise and permits us to observe higher frequency
components in the signal.



Gain (48)

so ' 10
60
| 0
_ 40
@ -20° -
= ; fi eo 10
= 10 | | § aol
= - 3
S 20 | |-20
5 i < 40: idl so
: | 60:
-20 i i
80 1 : 40
os . | i . . :
1 2 3 4 65
o 1 2 3 4 5 @ 7 Frequency (GHz)
Frequency (GHz)
Gain (dB)
(b) 8 7.
10 |
‘ | I | o
5 | i |
ty | > 4 ‘
‘ whoo \ 10
0 ist 1; § .
a Pye! 5 {* j-20
2 a, s
= vp z | |-30
oO 4 N 3 1
-10 < i 40
E ca a
-15 50
~20
ap’ 60
_ ee a ane YOO
%o 102 3 4 5 6 7 + 2 3 4 5 6
Frequency (GHz) Frequency (GHz)

Figure 6. Comparison of (a) HH and (b) EE in FD. Left-hand plots are at azimuth angle of 0° and right-
hand plots cover full azimuthal range.

2.3. Antennas

The three photographs in figure 7 show the UWB-UWA antenna close-up.
It consists of two LP antennas that are perpendicular to each other and that
are interlocking. Each antenna consists of a sequence of side-by-side paral-
lel linear dipoles that form a coplanar array. The tapered envelope shown
in figure 7(a) reflects the fact that the length of the individual dipoles in this
array increases logarithmically as you travel down the center of the an-
tenna. A crisscross connection links the dipoles. The inductive loading shown
in figure 7(b) allows the antenna to radiate and receive longer wavelengths
than its physical length by itself would permit. The dual polarization shown
in figure 7(c) permits cross-polarization experiments to be performed.



(a) (b) Gain (dB)

Gain (4B)
S
Azimuth angle (°)

0 2 4 6 8 10
Frequency (GHz) Frequency (GHz)

Figure 15. LP EE radiation pattern (a) at azimuth angle of 0° and (b) for full azimuthal range.

Gain (4B)

Gain (dB)

2 4 6 8 10 ‘5
Frequency (GHz) Frequency (GHz) —

Figure 16. LP HH radiation pattern (a) at azimuth angle of 0° and (b) for full azimuthal range.

18



Contents

1 Introduction

2 Experimental Configuration
21 TD Setup .....ceecccecseeceeeecsssseeeeeeeesseeeeeeeseensseeeeeesenenieeeee
2.2 FD Setup ........cccceccceeeeceeeeeeeeeeesseeeeeeeeeestaeeeeeeseeeniaeees
2.3 ANtenmas........cccccccceecesssceseeeeeceesseeeeeeeeeensaeetereeeenenneee®

2.4 Kentec Impulse Source .........c0ccccceeeeeeeceeeeeeseeeeeteeeeneeees

3 Experimental Results
3.1 TD Boresight Waveforms ............00:ccceceeseseeeeteeeeteeeeentes
3.2. TD Radiation Patterns ...........:ccccsseceeeeeeesssrseeeeceeenteeeees
3.3. Comparison of FD and TD Measurement Results ...............
3.4 High-Voltage Limit of UWB-UWA Antenna ............ eee
3.5 Noise Reduction ........cccccccececceeseeeeeeeeeeesseeeeeeeenenneeeege

4 Conclusions
Distribution

Report Documentation Page

9

12
14
15
16
17
17

19

21

23

iii



3.2 TD Radiation Patterns

Gan (a3)

,s

Azimuth angle (°)

Azimuth angle (-)

Frequency (GH2)

In figure 14, the E-plane patterns (top row) and the H-plane patterns (bot-
tom row) are compared for all three antennas, UWB-UWA 1, column (a);
UWB-UWA 2, column (b); and commercial LP, column (c), respectively. The
radiation pattern for the H-plane is spread out over the full azimuthal range
of -90° to +90°, whereas the E-field lies in a narrower angular band of
~ -60° to +60°. The commercial LP antenna pattern reaches a maximum
gain of 15 dB while the pattern for the other two inductively loaded UWB-
UWA antennas reaches a maximum of only 10 dB. The trade-off of gain
versus compact packaging and size should be expected. Also, the gain in
the H-plane is present down to 150 MHz for the commercial LP antenna
while the gain of the UWB-UWA antennas is present down to 300 MHz.

In general, the H-plane gain is about 10 dB greater than that of the E-plane
gain (not shown are plots of boresight gain). The HH gain pattern extends
out through the full azimuthal range, whereas the EE gain pattern falls
sharply at an azimuthal angle over 60°. The cross-polarized patterns for
UWB-UWA 1 exhibit a much lower gain than do the copolarized patterns,
as would be expected (figures not shown.) The EH gain pattern is about
10 dB down from either of the copolarized patterns. The HE gain pattern is
15 to 20 dB down. The results for the UWB-UWA 2 are very similar to those
for the UWB-UWA 1 and therefore are not repeated here.

(b) Gan (88) (c) Gan (08)

TEN *

ae! Syd
cre pes

Frequency (Gti

Azimuth angle (°)
Azimuth angle

Gan (8)

‘Azimuth angle (°)

Frequency (6 Frequency (SH)

Figure 14. Radiation patterns for two copackaged, inductively loaded UWB-UWA antennas and
commercial LP. Top row shows radiation pattern in E-plane of each antenna while bottom row shows
H-plane radiation patterns. Patterns are generated by (a) UWB-UWA 1, (b) UWB-UWA 2, and

(c) commercial LP antennas.

15

15

5



Hicks & Associates Inc
Attn G Singley III

1710 Goodrich Dr Ste 1300
McLean VA 22102

Mission Rsrch Corp
Attn G Salo

3975 Research Blvd
Dayton OH 45430-2108

Palisades Inst for Rsrch Svc Inc
Attn E Carr

1745 Jefferson Davis Hwy Ste 500
Arlington VA 22202-3402

Director

US Army Rsrch Ofc

Attn AMSRL-RO-D JCI Chang
Attn AMSRL-RO-EN WD Bach
PO Box 12211

Research Triangle Park NC 27709

22

Distribution (cont'd)

US Army Rsrch Lab

Attn AMSRL-CI-AI-R Mail & Records Mgmt
Attn AMSRL-CL-AP Techl Pub (3 copies)
Attn AMSRL-CI-LL Techl Lib (3 copies)
Attn AMSRL-DD JM Miller

Attn AMSRL-SE-DP L Cheskis

Attn AMSRL-SE-DP L Dilks

Attn AMSRL-SE-DP M Litz (20 copies)
Attn AMSRL-SE-DP N Tesny

Attn AMSRL-SE-DP RA Kehs

Attn AMSRL-SE-EO D Smith

Attn AMSRL-SE-RU M Ressler
Adelphi MD 20783-1197


The gain plots for the cross-polarized setup show that the gain is 15 to 20 dB
below its level in the EE and HH setups. The EH radiation pattern is weighted
toward the low end of the 0 to 2 GHz range while the HE pattern is weighted
toward the upper end. Again, it is not clear why this should be so. Further
investigation is necessary to understand this result and to determine if it is
reproducible at night when there is less background noise.

3.4 High-Voltage Limit of UWB-UWA Antenna

In the chamber, we ran a test to determine how high of a voltage the UWB-
UWA antenna could handle and still function properly. When the antenna
was fed a 7500-V signal, it functioned properly. When the antenna was fed
a 15-kV signal, corona was discharged at the antenna feed. This discharge
blocked EM transmission from the antenna. We performed this stress test
after taking all radiation measurements. This antenna was checked after
the stress test to determine if it suffered damage and was found to still be
operational.

3.5 Noise Reduction

If we compare the EE and HH gain plots for the commercial LP antenna in
figure 6 with those plots in figures 15 and 16, we observe that the gain plots
in figure 6 exhibit little noise, whereas those plots in figures 15 and 16 ex-
hibit a high-noise level. Figure 5 exhibits the same phenomenon but in a
different manner. The plots on the left are much smoother than their coun-
terparts on the right, which are grainy, especially the lower one. This graini-
ness is symptomatic of a high noise level. We also saw in figure 10 how
increasing the number of waveforms in signal averaging reduces noise.
There, the frequency components of the signal, which diminish in ampli-
tude as the frequency increases, were not obscured by noise until 4 GHz
was reached

What accounted for the reduced noise in all these plots was increasing the
number of waveforms underlying an average. In the TD for each average,
there were three waveforms, each consisting of 2048 points, so that 6144
data points were recorded at each angle. By contrast, the FD plots had 801
frequency points, with each point being the average of 200 readings, so that
160,200 data points were recorded at each angle. This noise reduction can
be explained by a statistical theorem that states that if a random variable is
derived by taking the mean of n independent random variables, each with
the same mean and standard deviation (0), then the derived random vari-
able has a standard deviation that is o/ n. Furthermore, if each of the ran-
dom variables can be described by a Gaussian distribution, which is the
case here, than 2/3 of the variable values fall within 1 standard deviation of
the mean, 95 percent fall within 2 standard deviations, and 99 percent fall
within 3 standard deviations. This means that as you increase the number
of measurements that underlie the mean of a random variable, then the
variability of that mean is going to diminish. Less variability over the range
of the plot translates into a smoother plot with less graininess.

7



3. Experimental Results

Figure 11. TD
waveforms for UWB
UWA inductively
loaded LP antenna 1:
(a) H-plane transmit,
H-plane receive (HH1)
(copolarized); (b) H-
plane transmit, E-plane
receive (HE1) (cross-
polarized); (c) E-plane
transmit, E-plane
receive (EE1)
(copolarized), and (d)
E-plane transmit, H-
plane receive (EH1)
(cross-polarized).

12

Over a period of two days, many waveforms were recorded. The conven-
tion among the figures that will be discussed is that those showing TD plots
are based on measurements taken in the cathedral while those showing FD
plots are based on measurements taken in the chamber. Data were acquired
in both the E-plane and the H-plane. These modes correspond to the princi-
pal planes of the antenna, where for example, in the E-plane, the electric
field of the antenna is in the plane of rotation of the pedestal. In each plane,
the antenna pairs were also measured in copolarized and cross-polarized
orientations.

The UWB-UWA antenna under test contains two cross-polarized LP anten-
nas (UWB-UWA 1 and UWB-UWA 2) in the same cube package. Each was
evaluated separately, and the data from each are labeled EE1 for the
copolarized E-plane of antenna 1 and EE2 for the copolarized E-plane of
antenna 2 (see figs. 11 and 12; see, specifically, fig. 11(c) and fig. 12(c)). The
data from these new inductively loaded antennas are compared to radia-
tion patterns of the commercially available LP antenna labeled LPEE for the
copolarized E-plane and labeled LPHH for the copolarized H-plane, respec-
tively (see fig. 13). All measurements on the UWB-UWA LP and the
commercial LP used the commercial LP as the receiving component in the
configuration.

a) 0.5;
0.5
0 50 100 150 200 250
(b) 04 - +
°
ry oA ) AMA aie aney
2
8 0.1 i 1
© 0 50 100 150 200 250
0.
co) 2g % - - +
(c) 8
~0.5\ i i
0 50 100 150 200 250
(d) 0.2 7 7 ; 7
0.2 i i i
50 100 150 200 250
Time (ns)



receiving antenna, would lag a direct signal from the transmitting antenna
to the receiving antenna by approximately 22 ns. The reflection with the
shortest route bounces off the sand midway between the antennas. This
midpoint was 45 ft from either end of the sandbox. In other words, this
reflection adds to the received signal at the receiving antenna 22 ns after it
begins picking up the direct path signal. Reflections came from the metallic
garage door behind the receiving antenna, but they arrived after the reflec-
tions from the sandbox and are not considered here. The garage door is 35 ft
behind the receiving antenna, so any reflections off this door would arrive
70 ns after the direct signal.

An HP 214B pulse generator-fed 11-V trigger pulses with a rise time of less
than 10 ns into a Kentec transmitter. The HP 214B operates in a frequency
range of 50 to 250 Hz. The 11.5 kV, 2-ns-width output pulses of the Kentec
transmitter passed through two attenuators in sequence, attenuating the
amplitude by factors of 2 and 3, respectively. This output was then fed to
the UWB-UWA antenna, which served as a transmitting antenna. In the
cathedral, the UWB-UWA antenna was mounted on an inverted rotating
pedestal. It was rotated in 5° increments from ~90° to +90° in the horizontal
plane.

An antenna consisting of a single dipole can emit a pulse that accurately
replicates the shape of the original pulse if the center frequency of the pulse
is at or near the resonance of the dipole. If the antenna consists of a se-
quence of dipoles, as this UWB-UWA LP antenna does, the transmitted pulse
is spread out over a much longer time frame than the original pulse, as each
dipole radiates near its resonance. As the pulse travels down the center of
the antenna, each dipole emits a pulse in turn. In addition, since the receiv-
ing antenna at the other end of the sandbox is also LP, the transmitted sig-
nal is picked up by a sequence of dipoles and, as a result, is spread out even
more.

The electric field has an amplitude of 12.5 V/m at the input of the receiving
antenna. Because of the effective length of the antenna, this voltage is mul-
tiplied by a factor of 0.2, yielding a voltage signal. This signal passes through
a 5x attenuator and a delay line. A signal travels directly to the scope to
trigger it. The delay line attenuates the signal by a factor of 2. The resulting
signal of 250 mV feeds into a Tektronix 11801 sampling scope.

Each time a signal waveform enters the scope, the scope samples one point
from the waveform and stores it. The next time a waveform enters the scope,
the scope samples a point that is a specified time further down the wave-
form. That is, the scope delays sampling a given waveform with respect to
the point in time that it sampled the previous waveform, and that delay isa
fixed period of time from one waveform to the next. In this manner, 2048
waveforms are sampled in succession to generate one waveform consisting
of 2048 points. The effective sampling rate therefore is the inverse of 250 ns/
2048 points or 8.192 GHz. We take this approach because the technology
does not exist to sample 2048 points from a single transient waveform at


1.

Introduction

The Mission Research Corporation (MRC) designed and built a proof-of-
principle ultra-wide-band, ultra-wide-angle (UWB-UWA) log-periodic (LP)
antenna for use on the U.S. Army Research Laboratory (ARL) impulse ra-
dar. MRC shipped the antenna from its laboratories in Dayton, Ohio, to
ARL to characterize it at the ARL Electromagnetic Research Facility (EMRF),
which will be referred to as the “cathedral,” and in the ARL Anechoic Cham-
ber, which will be referred to as the “chamber.” This report describes the
time-domain (TD) and frequency-domain (FD) techniques used to evaluate
the gain and radiation patterns of the antenna.

The new UWB-UWA LP antenna is an inductively loaded LP antenna (see
figure 1(a)). The longest arms of the LP antenna are inductively loaded,
which reduces the physical size of the longest arms while maintaining their
electrical length. In addition to the size reduction, two cross-polarized
LP antennas share the same central support arm but are rotated 90°
with respect to each other. The compactness of this system motivates this
investigation.

The other antenna, shown in figure 1(b), is a commercial LP antenna. This
antenna always served as the receiving antenna in both the cathedral and
the chamber, since its properties are well known and reproducible. It re-
mained fixed while the test antenna was rotated.

The relatively small size of the chamber limits its use on the low-frequency
end to 200 MHz. To measure the new UWB-UWA LP antenna below this
frequency, we used the cathedral, which has no radio frequency (rf) ab-
sorber on the walls. Using a 3-ns full-width pulse that includes a frequency
content from 50 MHz to 2.5 GHz, we can measure the impulse response of
the antenna and calculate its gain. Because the cathedral is large, we can
time-gate any reflections that may occur within it. Where measurements
provide overlapping results between TD (cathedral) and FD (chamber) tech-
niques, they are compared in this report.

Another objective of this investigation includes determination of the high-
voltage limits of the antenna. The LP antenna is not usually considered to
be a high-power antenna because of the breakdown that can occur between
the many arms of the antenna or in the feed structure. Just how practical is
the antenna in a high-voltage, short pulsewidth application? We applied
successively larger voltages to the antenna to determine the high-voltage
limits.



Figures

iv

a kw Ne

a

10

H.

12
13
14

15
16

Antennas used in experiments .............:eseeeeeeeeeeeeeeeeeeeeeee eee eennes 2
Inside view of EMRE ...........::::ccceceeeeesseeeeeeeeteeeeeeeeetenneseeeeeees 3
Electrical configuration of experiment in EMRF .............:: seers 5
Antenna configuration in Anechoic Chamber ........-::::::eereeeeeee 6
Commercial LP antenna radiation patterns measured with HP8510c
DN ESE EEOEEEOEEOOOOSOOOSOOSSSOSOSOSOSOSOSOOOOOON 7
Comparison of HH and EE in FD o..... ec eeeseeeeeeeeeseeteeseeeereeees 8
UWB-UWA LP antenna showing tapered envelope, inductive loading,
and dual polarization feeds . 9
Kentec impulse, FFT, and joint time-frequency plot... 10
Spectrogram of UWB-UWA pulse after its emission from UWB-UWA
ET e Co 010: SEES SESOSSOOTOSOSOOOOS 10

FFT of signal average of 3 waveforms and FFT of signal average of
3000 waveforms and Fourier transform of original Kentec output
|e EEO OEEOOOOEEOSOESEOEOOSOON 11

TD waveforms for UWB-UWA inductively loaded LP antenna 1 ... 12
TD waveforms for UWB-UWA inductively loaded LP antenna 2 ... 13

TD boresight waveforms of commercial LP antenna

Radiation patterns for two copackaged, inductively loaded UWB-

UWA antennas and commercial LP............::::ccesreeeeeteteeeeeeeee 15
LP EE radiation pattern ...........:::c:ceeeeesseeeeeeeeeeeeeeeeetteneteeeeees 18
LP HH radiation pattern .........:.cccceceseceeeseeesteeeeeeeeetnteeenneeees 18



Figure 12 Tl) (a) 05,- _ a

waveforms for UWB
UWA inductively 0 mn LA cel cee eicenepeomeceenneiened
loaded LP antenna 2: (a) 05
HH2, (b) HE2, (c) FE2, x 50 700 150 200 250
and (d) FE2 (6) o 0 7 t r
8 —~etINAN Npbeetnel yel ene tne
fo} te) .
B : : :
@ 02 50 T00 150 200 250
2 05 + T
(c) s : i :
Of YIN ent nme
05 H 4 : :
0. a 50 100 150 200 250
d T T
(4) °
-02 H : : i
50 100 150 200 250
Time (ns)
Figure 13. 1D boresight (a) 1 ae ana
waveforms of
commercial LP antenna: 0
(a) LPHH4, (b) | PHE4,
(c) LPEE1, and -19
(d) LPEH1
(b) 0.2
OL.
®
3 -0 a
a
()e 1
Z
Ko} OL..
>
aa
(0)
(d) 0.2 t r r ;
of--
02 i i i i
0 50 100 150 200 250
Time (ns)

Insummary, we evaluated three antennas: the commercial LP and both units
of the inductively loaded UWB-UWA LP. Each antenna was measured with
TD techniques. In addition, the commercially available LP was measured
with the FD techniques, and the results were compared directly with those
in the TD. With each technique and for each antenna, a radiation pattern
was generated for both the copolarized and the cross-polarized configura-
tions. Results are discussed in the next section

13



2. Experimental Configuration

2.1 TD Setup

Figure 2. Inside view of
EMRE. Large wooden
structure is very useful
for TD analysis of
impulse signals.

Two experimental configurations are described in this section. First, the TD
experimental configuration and technique used in the cathedral are de-
scribed. Then the FD experimental configuration in the chamber and the
corresponding instrumentation and techniques are discussed.

The interior of the EMRF is shown in figure 2. A sandbox appears at the
bottom of the figure. The dimensions of the sandbox are 90 x 60 x 4 ft, and
the weight of the sand is over 300 tons. Characteristics of the cathedral make
it appropriate for testing the UWB-UWA LP antenna. Because the building
is constructed of wood, it is transparent to those frequency components
below 500 MHz. This construction contributes toward reducing reflections
picked up by the receiving antenna. The length of the EMRF (130 ft) means
more time can elapse before a reflection off a wall can be detected by the
receiving antenna. The antennas were placed at opposite ends of the sand-
box, hoisted to 32 ft, and pointed so that they were facing each other.

The UWB-UWA LP antenna was the transmitting antenna in this configura-
tion, and the commercial LP antenna was the receiving antenna. Initially
the two antennas directly faced each other. The UWB-UWA antenna, how-
ever, was rotated, as will be discussed in the next paragraph. Since the length
of the sandbox is 90 ft, the transmitting and receiving antennas were ap-
proximately 85 ft apart. This means that the reflected wave, taking the short-
est route from the transmitting antenna down to the sandbox and up to the



4. Conclusions

The UWB-UWA 1 and UWB-UWA 2 antennas had similar radiation pat-
terns for various combinations of the E- and H-planes in transmitting and
receiving modes. For all three antennas, the H-plane radiation covered the
full azimuthal range while F-plane radiation was spread out approximately
from —60° to +60°.

We saw that the SNR in the FD was much higher than in the TD. In the FD,
160,200 data points (801 points x 200 measurements) were taken at each
angle setting, whereas in the TD, 6144 points (3 waveforms x 2048 points)
were taken at each angle setting. Taking the time to collect additional data
points benefited in smoother radiation pattern plots in the FD as compared
to the TD. This smoothness was the consequence of eliminating much of
the noise. It also shows that the FD plots represent more information than
the TD plots. The fact that increasing the number of waveforms going into
an average reduces noise was strikingly illustrated when a gain plot based
on 3000 waveforms was compared with one based on just 3 waveforms. All
frequency components above 2 GHz were swamped by noise in the plot
based on 3 waveforms while frequency components up to 4 GHz were clearly
visible in the plot based on 3000 waveforms.

The collection of 160,200 data points at each angle setting for FD plots took
240 min. for the entire range of -90° to +90°. The collection of 6144 data
points at each angle setting for TD plots took 30 min. for the entire azi-
muthal range. This would suggest that collecting FD data is 3 times more
efficient than collecting TD data. If we consider that the azimuth settings in
the chamber were 2.5° apart, whereas those in the cathedral were 5° apart,
then FD collection is 6 times more efficient than TD collection. If we want to
apply in the future what we learned about noise reduction, then the same
experiment can be performed in the cathedral as before, except that instead
of averaging with 3 waveforms, we would average with 3000. Some prob-
lems with this approach would be greatly increased measurement times
and wearing out of the Kentec impulse transmitter under the stress of such
prolonged use.

Another possible refinement would be to apply time-gating. Time-gating
would greatly reduce the contamination of the received signal from
multipath reflections. In the cathedral, the first multipath reflection arrived
22 ns after the beginning of the signal, so time-gating might be applied 25 ns
after the signal begins.

The cathedral environment offers a large volume that greatly eliminates the
effects of reflection and scattering; however, in the relatively open EM envi-
ronment that the cathedral provides, we find an ambient noise background
of radio, communications, and radars of the Baltimore-Washington area that
reduces the sensitivity with which we can measure the calibration signals
of interest.

19



ARMY RESEARCH LABORATORY

Characteristics of an
Ultra-Wide-Band
Ultra-Wide-Angle Log-Periodic
Antenna

Leland M. Cheskis, Marc S. Litz, Neal Tesny, and Lillian Dilks

ARL-TR-2254 February 2001

Approved for public release; distribution unlimited.

20010402 02?



2.2 FD Setup

Figure 4. Antenna
configuration in
Anechoic Chamber.

We utilized the experimental setup in the chamber to collect data in the FD,
as opposed to the TD previously described. This setup is depicted in figure
4. The transmitting antenna is connected to port 1 of the HP8510c Vector
Network Analyzer (VNA), and the receiving antenna is connected to port 2.
The CPU controls the rotation of the pedestal as in section 2.1, except that
the angle increment here is 2.5°. For each azimuthal setting, the VNA feeds
801 frequencies into the transmitting antenna. The frequencies range from
50 MHz to 10 GHz. At each frequency, 200 measurements of the power gain
are taken and averaged. The entire set of measurements from -90° to +90°
takes 240 min. The power gain of interest here is the ratio, expressed in
decibels, of the power received to the power fed into the transmitting an-
tenna. Averaging in the FD, as in the TD, cancels out as much noise as pos-
sible, ie., enhances the signal-to-noise ratio (SNR). The results are shown in
figures 5 and 6, which show gain versus frequency plots and gain versus
azimuthal angle and frequency plots.

The chamber also served as a test environment for the UWB-UWA antenna
evaluations. It is 50 x 25 x 30 ft. Ithas an rf absorber attached to its walls that
reduces the power of the reflected signals by at least 20 dB in the frequency
range of 2 to 18 GHz (and ~10 dB below 1 GHz). Since this chamber is 50 ft
long, any frequency components of a signal below 200 MHz are in the near
field; i.e., those frequencies picked up at the receiving antenna may not pro-
vide meaningful far-field radiation information. The VNA is used in the FD
measurements in the chamber. It was also configured in its time-gating mode
to eliminate the calculated reflections from the chamber walls.

Transmitting
antenna

»—

Receiving
antenna

8510
VNA

Pedestal

GPIB

CPU



Impulse transmitter .

that sampling rate. The TD plots used to generate the figures int this report
are based on the average of three waveforms, with cach waveform consist-
ing of 2048 successive sampled points as described above. The purpose of
averaging is to reduce ambient noise. Since the EMRF is a wooden build-
ing, it is unable to shield the experiment from the outside world

As can be seen from the schematic in figure 3, the Tektronix 11801 and the
central processing unit (CPU) controller are connected through a general
purpose interface bus (GPIB), a fiber-optic line, and a second GPIB. The
fiber GPIB is used to eliminate metallic cables in the test area that will con-
tribute to electromagnetic (EM) pickup or radiation that will contribute to
the experimental noise background. When the CPU controller receives a
signal from the pedestal notifying it that the pedestal has rotated to a new
azimuthal setting and the antenna is ready to transmit, it will send a signal
through the fiber GPIB to the scope. The scope is alerted to accept signals
from the receiving antenna. For each setting of the pedestal, the scope re-
ceives a stream of waveforms from the receiving antenna, samples them as
discussed in the previous paragraph, and stores the sampled points. When
all the waveforms have arrived at the scope and been sampled, it averages
each of the 2048 sets of three corresponding points to produce a waveform
average and then signals the CPU that it is ready to send the average down.
The CPU, when notified, signals the scope that it is ready to receive the
waveform. The waveform is then sent and stored on the CPU’s hard drive
for future processing. The CPU now directs the pedestal to rotate to the
next azimuthal setting, and the cycle repeats. Collecting data for all angle
settings over the entire azimuthal range takes 30 min

_f 2x attenuator Transmitting Receiving
antenna

antenna
Kentec
3x attenuator i — 5x attenuator
Pulse generator Pedestal
HP214B Tektronix DL11
delay line
Trigger signal
a
Delayed
signal
Tektronix
11801 scope
CPU ~-|GPIB GPIB} P
OtoE Fiber-optic line EtoO

Figure 3. Electrical configuration of experiment in EMRF.



Figure 1. Antennas
used in experiments
(a) UWB-UWA LP
antenna and

(b) commercially
available LP antenna:

(a)

(b)



3.3 Comparison of FD and TD Measurement Results

16

In figure 5, the two radiation patterns on the left are based on FD data taken
by the HP8510c VNA in the chamber, whereas the two radiation patterns
on the right are based on TD data of the transmitted Kentec pulse that has
been sampled by the scope in the cathedral and then processed by an FFT
routine. The plots on the left were generated by having the 8510c step through
and feed 801 frequencies between 50 MHz and 10 GHz to the UWB-UWA
transmitting antenna and recording the response of the system at each fre-
quency. The plots in both TD and FD show that the H-field has a broader
pattern than the E-field over the full azimuthal range. We see that the E-field
radiation pattern is primarily confined to between -60° and +60°, whereas
the H-plane radiation pattern extends to 90° in both directions, as observed
in TD. The H-plane frequency content measured in TD (on the right in fig. 5)
extends to a lower frequency than that measured in the E-plane, a phenom-
enon not observed in FD data (on the left).

The right-hand plots (TD) in figure 5 have less low-frequency information
than the left-hand plots because they exhibit only frequency components in
the original Kentec pulse, whereas left-hand plots were the product of a
VNA generating signals starting at 50 MHz. Perhaps the most important
factor in comparing the two techniques is the presence of background noise
in the open environment of the cathedral. The H-plane radiation pattern on
the right is grainy, indicating more background noise. The E-plane pattern
is also grainy, but not as much as the H-plane radiation. This could be over-
come in the future by raising the voltage output level greater than the 1.9-kV
voltage applied. The practical upper limits that can be used depend on the
antenna under test and the environmental assessment limits under which
we are legally constrained to operate.

Because of ambient noise shown in figure 10, we determined that the an-
tenna patterns contained valid data only up to 2 GHz. We compared the
commercial LP antenna in both principal planes (H-plane and E-plane) us-
ing both TD and FD techniques as described previously. The experimental
setup in this case has both the transmitting and receiving antennas being
the commercial antenna. The FD boresight gain (not shown directly), which
corresponds to a 0° angle in the antenna patterns of figure 5, show that in
both principal planes, the gain falls off at ~1.6 GHz.

In doing this comparison, we performed the same calibrations and charac-
terizations on a pair of the commercial LP antennas. The EE boresight gain
is predominantly at a level of 10 dB, with dropouts at 400 and 800 MHz (see
fig. 5). It drops off below 100 MHz. The azimuth ranges from —60° to +60°.
The HH radiation power gain also is fairly flat and extends to a lower fre-
quency than EE before it drops. This result is true of both the UWB-UWA
antennas—the H-plane pattern extends over the full azimuthal range. The
radiation pattern plot is much noisier in the H-plane than in the E-plane. It
is not clear why this should be so, but it could be because of a higher back-
ground noise environment when we took this data. One way to test this
hypothesis is to take the same set of data atnight when environmental noise
is reduced.


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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED
February 2001 Final, 11/99
4,TITLE AND suBTiTLe Characteristics of an Ultra-Wide-Band 5. FUNDING NUMBERS
Ultra-Wide-Angle Log-Periodic Antenna DA PR: —
PE: 62120A
s.autHors) Leland M. Cheskis, Marc S. Litz, Neal Tesny, and Lillian
Dilks
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 4. PERFORMING ORGANIZATION
U.S. Army Research Laboratory
Attn: AMSRL-SE-DP email: Icheskis@arl.army.mil ARL-TR-2254
2800 Powder Mill Road
Adelphi, MD 20783-1197
98. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING
Mission Research Corporation AGENCY REPORT NUMBER

3975 Research Blvd
Dayton, OH 45430-2108

11, SUPPLEMENTARY NOTES

ARL PR: ONX3XX
AMS code: 622120.H16

12a, DISTRIBUTION AVAILABILITY STATEMENT A nproved for public release; distribution] 12>. DISTRIBUTION CODE
unlimited.

13. ABSTRACT (Maximum 200 words}

A compact ultra-wide-band, ultra-wide-angle log-periodic antenna has been designed by the
Mission Research Corporation under a small business innovative research phase II contract with
the U.S. Army Research Laboratory (ARL). A proof-of-principle version of the antenna was
delivered to ARL. The antenna was evaluated within two days and information returned to the
vendor. The calibrations of the antennas included both frequency-domain and time-domain
measurements in the High-Power Microwave Anechoic Chamber and the Electromagnetic Research
Facility, respectively, and high-voltage-limit characterizations. The appropriate conversion of|
time-domain and frequency-domain data into gain curves was performed, and the results were!
discussed. These results are now being applied to the next version of the dual-polarization antenna
pair.

14 SUBJECTTERMS ~Copolarized, cross-polarized radiation pattern, time domain, | 15 baa OF PAGES
frequency domain

16. PRICE CODE

17, SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19, SECURITY CLASSIFICATION 20. LIMITATION OF ABSTR/
OF REPORT OF THIS PAGE OF ABSTRACT |ON OF ABSTRACT
Unclassified Unclassified Unclassified SAR

NSN 7540-01-280-5500 Standard Form 298 (Rev, 2-89)

Prescribed by ANSI Std, 239-18

23



Abstract

A compact ultra-wide-band, ultra-wide-angle log-periodic antenna has
been designed by the Mission Research Corporation under a small
business innovative research phase II contract with the U.S. Army
Research Laboratory (ARL). A proof-of-principle version of the antenna
was delivered to ARL. The antenna was evaluated within two days and
information returned to the vendor. The calibrations of the antennas
included both frequency-domain and time-domain measurements in the
High-Power Microwave Anechoic Chamber and the Electromagnetic
Research Facility, respectively, and high-voltage-limit characterizations.
The appropriate conversion of time-domain and frequency-domain data
into gain curves was performed, and the results were discussed. These
results are now being applied to the next version of the dual-polarization
antenna pair.



Figure 7, UWB-UWA LP antenna showing (a) tapered envelope, (b) inductive loading, and (c) dual
polarization feeds.

The following is a note about the crisscross connection between the dipoles:
Mechanically crisscrossing or transposing the feed between adjacent ele-
ments causes a phase angle of 180° to be added to the terminal of each ele-
ment. Since the phase between the adjacent closely spaced short elements is
almost in opposition, they radiate very little energy, and their interference
effects cancel out. At the same time, the longer and more widely spaced
elements do radiate. The mechanical phase reversal between these elements
produces a phase progression so that the energy is beamed in the direction
of the shorter elements. The most active elements of this feed arrangement
are those that are nearly resonant, and their combined radiation pattern lies
toward the vertex of the end-fire array.

2.4 Kentec Impulse Source

The frequency content of the voltage output of the Kentec impulse source is
shown in figure 8. The Matrix Laboratory (MATLAB) program
“jtfa_axis_length.m” generated this plot and the one in figure 9. In figure 8,
we see the fast Fourier Transform (FFT) of the output pulse on the left and
its spectrogram on the right. The original shape of the pulse is evident in
the figure 8 spectrogram. The subplot in the upper left-hand corner has a
vertical axis representing frequency in hertz and a horizontal axis for en-
ergy in joules /hertz. The lower right-hand subplot has a vertical axis repre-
senting volts and simply depicts the original waveform. It starts a little af-
ter 2 ns, rises to a peak in ~150 ps, and then exponentially tapers off. Let us
compare that with a plot of a pulse that was emitted from a UWB-UWA
antenna. Figure 9 is the spectrogram of such a spread-out pulse. As can be
seen from the colored intensity subplot, the radiated electric field from the
UWB-UWA antenna takes the form of a frequency chirp. It extends in fre-
quency up to 4 GHz, an observation consistent with the FFT in figure 10.
What this spectrogram illustrates so vividly in figure 9 is that the original
3 ns pulses have been spread out to over ~75 ns. The dispersive design of
an LP structure is responsible for the elongated output.



(b) 50° . Direct”
‘ .. LP-LP

40

30 7
| Hb ine .
pee

~15 206 Lok :
0 2 4 6 8 10 0 2 4 6 8

Frequency (GHz) Frequency (GHz)

Figure 10. (a) FFT of signal average of 3 waveforms and (b) FFT of signal average of 3000 waveforms and
Fourier transform of original Kentec output pulse. Ambient noise floor is significant above 2 GHz.
Signal averaging lowers noise floor, provides smoother response, and moves high-frequency limit up to

4 GHz.

et



Army Research Laboratory

Adelphi, MD 20783-1197

ARL-TR-2254 ____ February 2001

Characteristics of an
Ultra-Wide-Band
Ultra-Wide-Angle Log-Periodic
Antenna

Leland M. Cheskis, Marc S. Litz, Neal Tesny, and Lillian Dilks

Sensors and Electron Devices Directorate, ARL

Sponsored by
Mission Research Corporation

3975 Research Blvd
Dayton, OH 45430-2108

“Approved for public release; distribution u



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21



The findings in this report are not to be construed as an
official Department of the Army position unless so
designated by other authorized documents.

Citation of manufacturer's or trade names does not
constitute an official endorsement or approval of the use
thereof.

Destroy this report when it is no longer needed. Do not
return it to the originator.


(b) Gain, EE (dB)

-80
~60
-40
320 5 g 20
2
2 9 5 0
s 0 5 2
5
2 20 E
z 40 < 40
60 60
80 80
05 1 1.5 05 1 15 2
Frequency (GHz) Frequency (GHz)
pe we Gain, HH (dB) Gain, HH (dB) 15
7 Beret Paka ai
80 | | wy me yt aT
60 Va ah
\ ren Jt: Se]
aol | i 0
£ 2 1
ran cS
$ 20 § 2 5
- ? 5
2 20 o 3
5 E °
< 40 , 2
60 ( 6
80 1 -
i wehbe. ‘ 10
05 1 15 ‘0

Frequency (GHz)

Frequency (GHz)

Figure 5. Commercial LP antenna radiation patterns measured with HP8510c VNA. Comparing results of
(a) FD measurements and (b) TD measurements provides confidence of reproducible findings. EE and
HH denote antennas were set to transmit and receive same orientation E-field and H-field, respectively.



Figure 8. Kentec
impulse (horizontal plot
in TD), FFT (vertical
plot in FD), and joint
time-frequency plot
(spectrogram).

Figure 9. Spectrogram
of UWB-UWA pulse
after its emission from
UWB-UWA antenna.

10

Frequency (GHz)

Frequency (GHz)

Time (s)