Category 2 Emission Spectra

The measured and modeled emission spectrums for radars
with pulse widths between 0.7 to 1.0 usec are shown in
Figures B-10 through B-16. In general, the measured FAA ASR
radar emission spectra agreed closer (within 5 dB) with the
model than the military AN/MPN-13 radars. The measured
emission spectrum of the Travis AN/FPN-55 radar (Figure B-16)
agreed closely with the modeled emission spectrum. However,
the AN/FPN-55, with the exception of the antenna subsystem,
is a military version of the ASR-5 radar.

The measured emission spectrum of the Burbank ASR-6
radar (Figure B-14) on the +AF side was as much as 15 dB
less than the model. Comparison of this emission spectrum
with the other 3 ASR emission spectra indicates that this
fast roll-off is untypical of ASR radars.

Category 3 Emission Spectra

The measured and modeled emission spectra for 2.0 usec
pulse width radars are shown in Figure B-17 through B-19.
The greatest difference between measured and modeled emission
spectra occurred for the San Pedro AN/FPS-90 radar (Figure B-19),
with differences as great as 20 dB at some frequencies. However,
some of this difference is believed to be due to measurement
error. The San Pedro radar spectrum was measured from the
Long Beach ASR-5 radar site, located about 20 KM away. The
signal level variation introduced to the emission spectra due
to propagation short term fading is estimated, based on half
hour propagation loss measurement over this distance, to be
less than 2.5 dB. In addition, the low grazing angle of this
line of sight path barely cleared the trees and building tops
surrounding the measurement location. This is believed to
have introduced selective multipathing error to the measured
emission spectrum. For the above reasons, the San Pedro emis-
sion spectrum was not used in the statistical comparison between
modeled and measured emissions.

Category 4 Emission Spectra

The measured emission spectrum of the Sacramento WSR-57
radar with 4.0 psec pulse width setting is shown in Figure B-20.
The category 4 (4.0 ysec) modeled emission spectrum agrees
well with the model down to the -75 dB level from the peak.

The measured emission spectrum appears to flatten rather
abruptly beyond this point.


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empirical correction to the magnetron emission model, based
on the algebraic mean, is not recommended. It is believed
that the overall standard deviation between modeled and
measured emission spectrum of 6 dB may be the limit in
obtainable accuracy for conventional magnetron emission
modeling.

B-24


Ki P

(£-£,)2 [0 (£-£¢) ]2 (B-1)
Ba _ P
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K3 P
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_ [:- Se a 2 (B-4)
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and
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P = Peak power of emission spectrum (watts)
67 5¢ = Pulse rise and fall times (sec)
tT = transmitter pulse width (sec)
= = frequency (Hz)
Be = total amount of frequency deviation during the rise

and fall times of modulating pulse (Hz)

The portion of the emission spectrum that equation B-1l

through B-4 depict, and the relationship between the modulating

pulse amplitude and magnetron frequency shift are shown in

Figure B-l.


Statistical Comparison of Measured and Modeled Emission Spectra

Statistical techniques were employed to gauge the dis-
agreement between the measured and modeled emission spectra
for the four radar pulse width categories. This was accom-
plished by computing the difference between the modeled and
measured emission spectra (EP-EM) at every 10 MHz increment
over the measured emission spectra indicated in Figures B-6€
through B-18 and B-20. The differences, EP-EM, were used to
compute the statistics indicated in Table B-3. Statistical
comparisons were made for the negative (-AF) and positive
(+4F) side of the emission spectra for each pulse width cate-

gory.

The absolute mean of the difference between modeled and
measured emissions, and the standard deviation of the pre-
dicted (modeled) about the measured emissions are indicators
of the overall agreement between measured and modeled emissions.
The smallest values of these indicators occurred for the cate-
gory 2 radars.

The increase in the category 2 absolute mean from 1.4
to 3.8 dB, from -4F to AF respectively, was primarily due to
the fast emission spectrum roll-off of the Burbank radar
(Figure B-14) which seems to be untypical for FAA radars.
The largest absolute mean and standard deviation of the
modeled about the measured emissions occurred for category l
radars. The greatest contributors to this disagreement were
the Los Alamitos AN/CPN-4 and Sacramento WSR-57 radar emission
spectra indicated in Figures B-6 and B-9, respectively.

The negative algebraic means, X, for the category 1, 2
and 3 radar emission spectrums indicates that the modeled
emission tended to be lower than measured emissions for these
radars by 2 to 5 dB.

The standard deviation, Oger is a measure of the variation

or repeatability of the absolute difference between modeled
and measured emissions. The o of 7 dB for category 1 radars
is about 2 dB greater than other category radars. This is
because the difference between modeled and measured emission
spectra varied from radar-to-radar the most for category 1
radars.

SUMMARY OF EMISSION SPECTRUM MODELING

Radars with 0.83 usec pulse widths, particularly FAA ASR
type radars, can be modeled with an average absolute error of

B-21


of about 3.0 dB. Radars with pulse widths of 0.5, 2 and 4 usec,
which included all military radars with the exception of one
WSR-57 weather radar, had a mean absolute difference between
predicted and measured that ranged from 3 to 7 GB. For

these cases, the modeled emission level was on the average

2 to 4 dB less than the measured emission level.

The greatest absolute mean difference (6 dB) between
modeled and meausred emission occurred for 0.5 ysec pulse
width category radars. The measured emission skirt level
varied the most (maximum 15 dB) between individual radars
in the 0.5 usec pulse width category.

This large variation even occurred between electrically
identical, including type magnetron, AN/CPN-4 and AN/MPN-11
radars. The most likely explanation for this paradox are the
radar-to-radar differences in the magnetron age and calibra-
tion of the pulse shaping circuitry that controls the magnetron.

The following factors are believed to have caused the
radar-to-radar variations of the differences between measured
and modeled emissions:

1. Magnetron age

2. Modulating pulse shape

3. Magnetron Type

4. Magnetron Anode Voltage and Current (peak power) setting
5. Magnetron Load

The factors are listed in their suspected order of sig-
nificance. Only the first two factors are believed to have
caused the large difference in measured emission spectrum levels
for AN/CPN-4 and AN/MPN-11 radars since these radars are
electrically identical, including the type magnetron employed,
and were operated at the same peak power setting. The degree
that a conventional magnetron emission spectrum increases with
age depends on the tube type. The shape of the pulse that
modulates the magnetron must be calibrated and maintained within
the manufacturer specifications to insure a clean emission spec-
trum. Frequent calibration of the pulse rise time on narrow
0.5 usec pulse width radars is particularly critical.

It is difficult to analytically include the above factors
into the emission spectrum model. Because the standard devi-
ation of the differences between predicted and measured emis-
sions were significantly greater than the absolute mean, an

B-23


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Width Mode)

Figure B-20.

B-18


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The magnetron model does not include the high level of
harmonic and nonharmonic spurious emissions that occur with
conventional magnetrons. Conversations with several experts
concerning magnetron spurious emissions (References 45 and 46)
indicated that it is difficult to analytically predict the
frequency and amplitude of magnetron spurious emissions. A
practical approach is to use an empirical model for the spectrum
skirts. This is usually accomplished by increasing the lower
level skirts of the model emission skirts by a fixed number
of 4B for |AF|> 100 MHz. The adjusted skirt level should be
based on spurious emission measurement data on the particular
radar type being modeled. The measured radar emission spectra
in this investigation confirmed that refinement to the magne-
tron model for spurious emissions was not necessary for
|AF|< 100 MHz. Since this frequency range covered the 2.7 to
2.9 GHz band of interest, in most cases, the spurious emission
refinement to the magnetron model was not employed in this in-

vestigation.

Emission Spectrum Model Categories

To simplify the calculations of radar emission spectra,
radars operating in the Los Angeles and San Francisco areas
were categorized by pulse width in the manner indicated in
Table B-2. The asterisks identify weather radars with two
operational pulse widths. The computed emission spectra,
for the various category radars are shown in Figures B-2
through B-5. The parameters used to compute the emission
model are indicated in the upper right hand corner of each
figure.

MEASURED EMISSION SPECTRA

Radiated emission spectra of the Los Angeles and San
Francisco area radars were measured intermittently over 1975.
The results of these measurements are presented in Figures
B-6 and B-8 through B-20. The AN/CPN-4 emission spectrum
shown in Figure B-7 was measured at the Norfolk, Virginia
Naval Air Station in June 1974.

These emission spectra were measured with the test re-
ceiving antenna positioned in the far field and in line-of-
sight of the radar antenna. When possible, the emissions
were measured no further from the radar than necessary to
be in the radar antenna far field.

This procedure reduced the chance of error contribution
by frequency selective multipath and short term propagation
fading.


RELATIVE POWER IN dB

FREQUENCY

AMPLITUDE

TIME

PULSE ENVELOPE

as

t
2

Figure B-l.

f fo

FREQUENCY

Emission Spectrum Model for, Radar “ith
Conventional Magnetron

B-6


The RSMS van does not have the capability in all measurement
situations to completely eliminate undesired radar signals
from interfering with emission spectrum measurements. An example
of this interference appears at 2752 MHz on the measured
emission spectrum of the Los Angeles ASR-7 radar in Figure B-15.
This frequency corresponds to the Los Angeles ASR-4 radar and
is, in fact, a low level emission spectrum of the ASR-4. The
frequency location of the interfering radars are noted on the
measured emission spectra.

COMPARISON OF MODELED AND MEASURED EMISSION SPECTRUMS

The modeled emission spectra are shown superimposed on
the measured emission spectra in Figures B-6 through B-20.

Category 1 Emission Spectra

The measured and modeled emission spectra for 0.5 usec
pulse width radars are shown in Figures B-6 through B-9.
The measured Los Alamitos AN/CPN-4 emission spectrum skirts
(Figure B-6) are 10 to 20 dB above the model. However, the
Measured emission skirts of the Norfolk AN/CPN-4 (Figure B-7)
and Alameda AN/MPN-11 (Figure B-8) agree with the model to
within 10 dB. The AN/MPN-1l radar is electronically identical
to the AN/CPN-4 radars. The radars at the above locations all
employ the same model, 5586, magnetron tubes. There are two
possible reasons why the measured emission spectrum skirts of
the Los Alamitos radar were 10 dB greater than the measured
emission spectrum skirts of the two other electronically
identical radars. When the magnetron tube begins to fail the
level of the emission skirts tend to increase. Second, the
magnetron modulation pulse shape must be maintained within
manufacturer specifications for a clean emission spectrum.

The measured emission spectrum of the Sacramento WSR-57
radar for 0.5 usec pulse width setting (Figure B-9) was as
much as 15 dB greater than the model; however, for the 4.0 usec
pulse width setting (Figure B-20), the measured emission spec-
trum agrees with the model to within 10 dB. The same Ratheon
model QKH729 is used for both pulse width transmissions of the
WSR-57 radar. The manufacturers catalog rates the tube for
4.0 usec operation. However, the manufacturer has stated that
the tube will operate with a clean emission at 0.5 usec pulse
width. Therefore, the high emission spectrum is attributed
to the difficulty of achieving and maintaining required pulse
rise time for narrow pulse widths.

B-19


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B-S



APPRENDIX B

EMISSION SPECTRUM MODELING
AND MEASUREMENTS

INTRODUCTION

Modeling transmitter emission spectra is an essential
step in performing an EMC analysis. This appendix discusses
factors that affect emission spectrum characteristics, and
the model selected for predicting emission spectrum. Modeled
emission spectra for particular radar characteristics and
transmitter output tubes are presented, and compared with
measurements made during the Los Angeles and San Francisco
area investigation. A discussion of the RSMS van system
hardware and software used to measure radar emission spectra
is given in Appendix G.

FACTORS AFFECTING EMISSION SPECTRUM CHARACTERISTICS

The emission spectrum of a pulse radar is determined by
the modulating pulse shape and width, transmitter RF tube,
and RF output tube load. The transmission waveguide, rotary
couplers, and antennas also affect the emission spectra
but to a much less degree.

Magnetron Characteristics

All the 2.7 to 2.9 GHz band radars operating in the Los
Angeles and San Francisco areas currently employ conventional
magnetron tubes. The magnetron tube types employed by each
radar in the Los Angeles and San Francisco area are indicated
in Tables 4-2 and 5-2. The tuning range and nominal operating
characteristics of the employed magnetron tube types are in-
dicated in Table B-l1. The inherent wideband asymmetrical
emission characteristics of conventional magnetrons are ex-
hibited by these tubes. The wide emission spectrum is due
to the low Q of the tubes resonant cavity. The operating
frequency of the tube increases with modulating voltage ampli-
tude, which results in a lower tube operating frequency during
the rise time of the modulating pulse. This instability causes
an asymmetrical spectrum with greater power on the low side
of the carrier frequency. In addition, conventional magnetrons
emit high levels of both harmonic and nonharmonic spurious
emissions. These outputs result from the excitation of cavity
modes, in addition to the desired Il mode, during the rise time
of the modulating pulse. The freauency of the undesired modes


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TABLE B-2

EMISSION SPECTRUM CATEGORIES

PULSE WIDTH RADAR PULSE WIDTH
CATEGORY NEMENCLATURE (u SEC)
1 AN/CPN-4 5
0.5 AN/FPS-41* 5

: msec) AN/MPN-11 “5

WSR-57* ‘5

2 AN/MPN-13, 14, 15 7

F AN/MPN-5 8

iG. Seusec} AN/MPS-19 .8
AN/FPN-47, 55 833
ASR-4,5,6,7 833

ASR-3 1,0

3

(2.0 usec) AN/APS-20 2.0
AN/FPS-6,90 2.0

AN/MPS~-14 2.0

4 AN/FPS-41* 4.0
(4.0 usec) WSR--57* 4.0

*Weather Radars which have two operational pulse widths.



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B-22


Magnetron Load

The magnetron in a typical radar is loaded by an
electrically long transmission line that is terminated by an
ATR (anti-transmit-receive) gas discharge tube. At the start
of a pulse, the ATR is unfired and presents an open circuit
to the transmission line which results in a voltage standing
wave on the line. Under these conditions, the impedance that
the magnetron output sees depends on the electrical length
of the line in wavelengths and the magnetron's oscillating
frequency. The impedance that the magnetron sees influences
its frequency mode starting characteristics and consequently
the radars emission spectrum. The transmission line electrical
length is normally optimized during the radar design and in
modern radars ferrite isolators are employed to isolate the
reflected wave and ATR tube input impedance from the magnetron.
However, when a magnetron is being replaced by a different model
than the radar was originally designed for, to prevent emission
spectrum spreading, it is necessary that the replacement tube
operate over the transmission line lengths and VSWR (voltage
standing wave ratio) limits specified by the manufacturer.

The impedance that the magnetron sees changes with
magnetron operating frequency, since the electrical length of
the transmission line changes with frequency. If the VSWR
magnitude on the line is large, tuning of the magnetron will
result in broadening of the spectrum. At some frequencies,
there will be periodic holes in the tuning curve where the
magnetron will not oscillate. This was confirmed by conver-
sations with Air Force personnel who stated that some frequencies
in the tuning range of AN/MPN-13 radars result in poor emission
spectrum. Therefore, the magnetron tuning ranges listed in
Table B-1 do not necessarily imply that the radar emission
spectrum will be satisfactory at all frequencies over this
range.

MODELING

Several modeis (References 23, 24) were considered for
predicting the emission spectrum envelope of conventional
magnetron for this investigation. The model by Newhouse
(Reference 24) was chosen because it takes into account the
frequency shift, characteristics of the conventional magnetron.

Conventional Magnetron Model

The conventional magnetron model used in this investigation
consists of emission envelope segments defined by the following
equation:


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depend on the resonant cavity structure of the particular type
tube and rise time of the modulating pulse.

The tube parameters listed in Tabie B-1 ere typical
operating values specified by the manufacturer and differ from
those used by government users. For example, it was observed
that the magnetron peak power of FAA ASR and military GCA
radars were typically set at 25 to 50 percent lower than the
values listed in Table B-1 to increase tube life. On the
other hand, the magnetron peak power of military range
clearance and height finding radars were set about 20 percent
higher to maximize range. The peak power of a magnetron is
set by adjusting the tubes peak anode voltage and current.
This adjustment affects the modulating pulse ability to excite
resonant cavity modes and consequently alters the tubes emis-
sion spectrum. The magnitude and direction of the emission
epectrum change as a function of magnetron peak power setting
has not been tested by manufacturers. However, it is a known
fact that, if the anode voltage and current are not in the
range specified by the manufacturer, spurious modes and poor
emission will result.

Modulating Pulse

A clean emission spectrum will be realized only if the
modulation circuitry provides a voltage pulse shape that is
within the tolerance (Reference 22) specified by the manu-
facturer. Exciting the proper mode requires a specific rate
of voltage rise between the 60 to 80 percent amplitude levels
of the pulse. This rate of voltage rise must be long enough
to permit oscillations to start in the desired 1 mode, but
net so slow that undesired modes are excited. The tolerance
of this voltage rate increase depends on the excitation time
of the tube and how close the modes are in frequency. Once
the oscillations are started in the desired 7 mode, the
modulating voltage should increase to the maximum pulse ampli-
tude value as rapidly as possible to prevent excitation of
other modes. The voltage should remain constant over the
duration of the pulse to prevent spectrum broadening. It is
also important that the modulating pulse drop to zero as
rapidly as possible, because inverse and post-pulse voltages
can result in undesirable noise radiation. For the above
reasons, different radar types employing the same model magne-
tron at identical peak power settings and pulse widths, can
have different emission spectrums due to the adjustment and
design of the modulating pulse shaping circuitry.



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B-16