Dependence of Radar Emission Spectra on Measurement Bandwidth and
Implications for Compliance with Emission Mask Criteria

Frank Sanders

Institute for Telecommunication Sciences

303.497.5727; fax 303.497.3680

fsanders@its.bldrdoc.gov

**Abstract.** *Radar transmitter emission criteria normally include the
specification of frequency-dependent emission masks. These masks specify
the amount by which unwanted radar emissions (both out-of-band and
spurious) must be suppressed relative to the power levels emitted at the
radars' fundamental frequencies. Compliance with emission masks is
determined through measurements of emission spectra. The measured levels
of radar unwanted emissions and fundamental-frequency emissions both
vary as a function of measurement system bandwidth, B~m~. But the
variation with B~m~ differs between the unwanted emissions and the
fundamental-frequency emissions. Moreover, the variation of unwanted
emission levels varies as a function of frequency as well as B~m~. This
creates a problem for radar emission mask-compliance measurements.*

*The National Telecommunications and Information Administration (NTIA)
Institute for Telecommunication Sciences (ITS) has explored this problem
by performing emission spectrum measurements on a maritime surface
search (navigation) radar. In the spectrum data that are presented, the
radar unwanted emission levels are found to vary between 12 log(B~m~)
and 20 log(B~m~), depending upon frequency. But the measured power of
the radar fundamental frequency is found to vary as 20 log(B~m~) for all
bandwidths that are less than or equal to 1/(radar pulse width). The
result is that the level offset between the unwanted emissions and the
fundamental-frequency emission level depends upon the measurement
bandwidth and the frequency of the unwanted emissions. This result
implies, at a minimum, that measurement personnel must take the effect
of B~m~ into account when performing radar emission spectrum
measurements for the purpose of determining emission mask compliance.
Based upon the results of these maritime radar spectrum data, some
technical strategies for measuring radar emission spectra for emission
mask compliance are proposed. Possible technical implications for future
development of radar emission masks are also discussed.*

**1. Introduction**

Pulsed radar transmitters produce broadband spurious emissions at
relatively high levels as compared to emissions from narrowband
communication systems. As documented in \[1\], radar spurious emission
levels are typically tens of decibels higher than theoretically
predicted sinc^2^ spectra of pulsed emissions. These emissions in the
frequency domain probably correspond to short-term transient behavior in
the time domain characteristics of the rising and falling edges of the
radar pulses.

For spectrum management purposes, radar out-of-band and spurious
emissions are limited in the U.S. by Government regulations such as the
radar spectrum emission criteria (RSEC). RSEC spurious emission masks
are specified in terms of amplitude suppression relative to the power
produced at radars' fundamental frequencies \[2\]. (E.g., suppression
might be required to be at least 60 dB below the fundamental at
frequencies of 100 MHz or more away from the fundamental.)
Mask-compliance limits are computed on the basis of a theoretically
perfect pulsed emission plus a margin that allows for realistic
performance of an economical transmitter design.

Compliance with emission masks is determined through measurements of
emission spectra. The measured levels of radar unwanted emissions and
fundamental-frequency emissions both vary as a function of measurement
system bandwidth, B~m~. But the variation with B~m~ differs between
unwanted emissions and the fundamental-frequency emissions. Moreover,
the variation of unwanted emission levels varies as a function of
frequency as well as B~m~. This presents a problem for radar emission
mask-compliance measurements.

In a peak-detected power measurement, completely non-coherent
(noise-like) emissions should vary as 10 log(B~m~), while coherent
emissions should vary as 20 log(B~m~). Since the bandwidth progression
coefficients of 10 and 20 represent limiting cases, radar spurious
emissions should have a coefficient somewhere between these two values.
But no study is known to have been undertaken to determine the actual
coefficient for radars.

The National Telecommunications and Information Administration (NTIA)
Institute for Telecommunication Sciences (ITS) has begun to address this
problem by performing emission spectrum measurements on a maritime
surface search (navigation) radar. The purpose of the measurements has
been to determine the actual variation of spurious emission levels of a
radar relative to the measured fundamental level as a function of the
measurement bandwidth, B~m~. With this value known, NTIA will be better
able to specify appropriate bandwidths for RSEC-compliance measurements,
as well as any post-measurement correction factors that might be
required.

**2. Approach**

A magnetron-based X-band maritime surface search radar was set up with
an antenna height of approximately 4 m on a mast at a prairie location
free of local obstructions. A measurement system contained in the
RF-shielded enclosure of the NTIA Radio Spectrum Measurement System
(RSMS) was positioned at a distance of 105 m from the radar. The RSMS
received radar emissions via a 1-meter diameter parabolic dish antenna
with a linear, matched-polarization log-periodic feed. Figure 1 shows
the radar transmitter and the RSMS during the measurement.

![](media/image1.png){width="3.0in" height="1.7666666666666666in"}

Figure 1. Maritime radar and measurement system.

The measurement system is shown in block-diagram form in Figure 2.
Following the antenna, a variable (0-70 dB) attenuator was adjusted on a
frequency-dependent basis to keep the received signal level from the
radar within the dynamic range of the measurement system. That is, the
attenuation level was zero when radar emissions were close to
measurement system noise, but was gradually increased (in 10 dB steps)
to as much as 70 dB as the measured frequency approached the radar
fundamental. The attenuation was gradually decreased at frequencies
above the radar fundamental, being finally reduced to zero at the upper
end of the measured spectrum.

![](media/image2.png){width="3.0in" height="2.5590277777777777in"}

Figure 2. Measurement system functional block diagram.

Following the attenuator, a tunable bandpass filter based on
yttrium-iron-garnet (YIG) technology isolated the system's broadband
low-noise filter (LNA) from the high-power radar fundamental energy as
the spectrum measurement progressed. The LNA provided the sensitivity
required to measure out-of-band and spurious emission levels as much as
100 dB below the measured fundamental power level.

The LNA output was fed into a spectrum analyzer. The critical spectrum
analyzer stages are shown in Figure 2, including frequency
downconversion; IF filtering; envelope and peak detection;
analog-to-digital conversion; and final output to a data-recording
computer.

As noted above, the requisite dynamic range of the measurement (about
100 dB) demanded that the RF attenuator setting be varied as a function
of frequency. Therefore, the measurement system could not be operated in
a swept-frequency mode. Instead, the measurement system was fixed-tuned
to a single frequency with a single attenuator setting. A peak-detector
circuit was operated at the single frequency for a period of time (3
sec) slightly in excess of the radar's antenna rotation period (2.7
sec). This ensured that the radar beam maximum output would be sampled
at each measured frequency.

With the emission amplitude measured at a single frequency, the
measurement system was tuned to another frequency, the RF attenuator was
adjusted (if necessary) and the measurement process was repeated for
another 3 sec. This stepped-frequency measurement process was used to
acquire all data presented in this paper.

With 100 dB dynamic range available in the measurement, the radar
spectrum was measurable across 4.2 GHz of spectrum, from 7300 MHz to
11500 MHz. Because the out-of-band and spurious emissions are a
continuum, lacking discrete carrier frequencies, those emissions could
be sampled at arbitrary spacings between frequency steps. For this
measurement, the step interval was set at 6 MHz, for a total of 700
steps (and 701 measured frequencies) across the range 7300-11500 MHz.
Each spectrum run therefore required about 35 minutes for completion.

Because the 6 MHz step spacing could result in the omission of the radar
fundamental frequency from the measured spectrum, a supplemental
measurement was made at the radar fundamental for each spectrum. This
ensured that radar peak power was accurately included in each spectrum
output.

The radar can transmit multiple pulse widths, ranging from 80 nsec to
800 nsec. The radar spectrum was measured in two pulse modes (80 nsec
and 800 nsec) in four IF bandwidths (where B~m~=300 kHz, 1 MHz, 3 MHz,
and 8 MHz). Thus a total of eight spectrum measurements were performed.

**3. Results**

The measured emission spectra for short-pulse and long-pulse radar modes
are shown in Figures 3 and 4. Measurement system internal noise occurs
as a flat floor at frequencies between 7300-8000 MHz, and as a flat area
between 11000-11200 MHz. It is observed that the spurious emission
levels are changing with a progression that is somewhere between 10 log
and 20 log bandwidth, depending upon frequency.

Figure 5 shows measured power at the radar fundamental frequency
(approximately 9410 MHz) as a function of measurement IF bandwidth. The
data in this figure confirm that the radar fundamental power follows a
20 log(B~m~) rule for values of B~m~ that are equal to or less than
(1/pulse width).

In the short-pulse mode, the 80 nsec pulse width results in a predicted
fundamental-frequency 3-dB emission bandwidth of 12.5 MHz, which is
wider than the maximum measurement IF bandwidth of 8 MHz. Consequently,
the measured power increases as 20 log(B~m~) for all points. But in the
long-pulse mode of 800 nsec pulse width, the 3-dB emission bandwidth is
predicted to be 1.25 MHz. As a result, the 20 log(B~m~) progression
breaks down for 3 MHz and 8 MHz B~m~ values.

Figure 6 quantifies the decibel differences between spectra measured in
successive values of B~m~. These are shown as deviations from a 20 log
progression, computed as follows:

$$D = \left\lbrack \frac{P_{x} - P_{y}}{\text{log}\left( \frac{B_{x}}{B_{y}} \right)} \right\rbrack - \text{20}$$

where:

= deviation from 20 log(B~m~) progression;

P~\[x,y\]~ = log power measured in B~x~ and B~y~;

B~\[x,y\]~ = measurement bandwidth;

\[x,y\] are subscripts for successive measurement IF bandwidths (e.g.,
3 MHZ and 1 MHz).

The differences should lie between --10 (corresponding to a noise-like
10 log progression) and 0 (for the 20 log rule that coherent emissions
should follow). Some points lie outside these bounds. Deviations outside
this range could result from the interaction of spectrum peaks and
valleys with a particular measurement bandwidth; uncertainty in measured
values; or variation in shape factors between filters.

![](media/image3.png){width="6.389583333333333in" height="3.9125in"}

Figure 3. Maritime radar emission spectrum measured in four bandwidths
with transmitter operating in short-pulse mode.

![](media/image4.png){width="6.486111111111111in"
height="3.865972222222222in"}

Figure 4. Maritime radar emission spectrum measured in four bandwidths
with transmitter operating in long-pulse mode.

![](media/image5.png){width="3.0in" height="2.3in"}

Figure 5. Variation in measured power at the radar fundamental as a
function of measurement bandwidth and pulse mode.

Some trends are clearly observed in the values for the curves in Figure
6. The values of the deviation curves typically range between -4 and -2,
corresponding to variation coefficients between 16 and 18. Neglecting
the points that exceed zero, the extreme values range between -8 and 0,
corresponding to variation coefficients between 12 and 20, respectively.

**4. Summary and Conclusions**

In summary, spurious and out-of-band emissions measured from a maritime
navigation radar were found to typically vary at a value between
16 log(B~m~) and 18 log(B~m~). Extreme values ranged from a low of
12 log(B~m~) to a high of 20 log(B~m~).

These results indicate that the radar spurious emissions did not vary as
would be predicted for thermal noise (10 log progression). Thus radar
spurious emissions should not be characterized as "noise-like," at least
for the radar measured in this study.

Radar emission spectrum measurements that are performed for the purpose
of determining the suppression of out-of-band and spurious emissions
relative to radar fundamental-frequency power will show a bandwidth
dependent variation. Measurement of spectra in multiple bandwidths is
recommended until this phenomenon is better understood.

It may be desirable for future emission masks to accommodate this
effect. Such masks might include a recommended IF measurement bandwidth,
and possibly a method for computing correction factors for measurements
made in other bandwidths.

**5. References**

\[1\] R.J. Matheson, J.D. Smilley, G.D. Falcon, V.S. Lawrence, "Output
tube emission characteristics of operational radars," NTIA Report 82-92,
Jan. 1982.

\[2\] *Manual of Regulations and Procedures for Federal Radio Frequency
Management*, revised Jan. 2000, Revisions Jan./May/Sep. 2001, NTIA
Office of Spectrum Management, U.S. Government Printing Office, Stock
No. 903-008-00000-8.

**Acknowledgments**

The author wishes to thank the following individuals for their valuable
assistance: Richard N. Statz of ITS for software development; Bradley J.
Ramsey of RF Metrics for assistance in getting the field measurement
system operational and in data analysis; and the technical and editorial
reviewers who commented on this paper on short notice.

![](media/image6.png){width="2.7576388888888888in"
height="2.277083333333333in"}

Figure 6a. 8 MHz -- 3 MHz smoothed difference, short pulse radar mode.

![](media/image7.png){width="2.7576388888888888in"
height="2.204861111111111in"}

Figure 6b. 3 MHz -- 1 MHz smoothed difference, short pulse radar mode.

![](media/image8.png){width="2.7576388888888888in"
height="2.204861111111111in"}

Figure 6c. 1 MHz -- 300 kHz smoothed difference, short pulse radar mode.

![](media/image9.png){width="2.7576388888888888in"
height="2.277083333333333in"}

Figure 6d. 8 MHz -- 3 MHz smoothed difference, long pulse radar mode.

![](media/image10.png){width="2.7576388888888888in"
height="2.204861111111111in"}

Figure 6e. 3 MHz -- 1 MHz smoothed difference, long pulse radar mode.

![](media/image11.png){width="2.7576388888888888in"
height="2.204861111111111in"}

Figure 6f. 1 MHz -- 300 kHz smoothed difference, long pulse radar mode.