**JRG-36**

**United States of America**

**Working Document Towards the Determination of the Feasibility to
Achieve More Stringent Out-of-Band Emission Limits for Primary Radar
Systems**

Introduction

At the first JRG 1A-1C-8B meeting, the JRG Work Program (JRG-25)
identified several specific areas requiring studies. One of the subject
areas was to identify through measurements the achievable roll-off from
the --40 dB bandwidth for radars with solid state output devices as a
function of the class of amplification. This task was identified
recognizing that the trend in radar systems was toward the use of
solid-state output devices in radar systems.

Measurements were conducted on two solid state radar systems. Both
systems were large ground-based radar systems. One system uses a bottled
solid-state output device, and the second system uses a mechanically
steered phased array, individual solid-state transmit/receive (T/R)
modules feeding each antenna element.

**Radar 1**

Radar one uses a bottled solid-state output device feeding a waveguide
to a reflector type antenna. The system has two channels. One channel
transmits an FM-pulse, chirped pulse, for long range coverage, an a
non-FM (CW) pulse for short range coverage. The pulse modulator included
pulse shaping. The measured pulse waveform characteristics were:

Channel 1: FM-Pulse Characteristics

Pulse Width (t) = 55.0 μs

Pulse rise/fall time (t~r~) = 1.0 μs

Frequency Deviation (B~c~) = 1.3 MHz

Figures 1 shows the measured emission spectra for channels 1. The figure
also shows the emission masks based on Recommendation ITU-R SM.1541.
Recommendation ITU-R SM.1541 calls for a 20 dB/decade roll-off from the
-40 dB bandwidth. In addition to the current required roll-off from the
-40 dB bandwidth of 20 dB/decade (Green Mask), the figure shows the -30
dB/decade roll-off (Blue Mask) and -40 dB/decade roll-off (Red Mask)
from the -40 dB bandwidth. The -40 dB bandwidth, based on Recommendation
ITU-R SM.1541 (Equation 40, K = 6.2 and A = 0.105) for channels 1 is
3.65 MHz.

![](media/image1.wmf){width="6.499305555555556in"
height="3.7604166666666665in"}

Figure 1. Radar 1 - Channel 1 emission spectrum and emission masks

Figure 1 shows that with pulse shaping, it may be possible to achieve a
40 dB per decade roll-off for FM-pulse systems with long pulses (greater
than 50 μs). It should be noted that there are FM-pulse radar systems
that have pulse lengths in the order of only 1 μs. It is not known at
this time if pulse shaping of FM modulated pulses with pulse lengths in
the order of only a few microseconds would result in as clean emission
spectra.

**Radar 2**

Radar two has a mechanically steered phased array antenna consisting of
individual solid-state transmit/receive (T/R) modules feeding each
antenna element. At the output of each T/R module, a filter is inserted
prior to each antenna element. The system has multiple pulse waveforms
interleaved, and frequency hops over a range of 185 MHz. For
determination of the appropriate -40 dB bandwidth, the -40 dB bandwidth
was calculated for each waveform type, and the widest -40 dB bandwidth
was used in determining the emission mask. The pulse waveform
characteristics are given below:

FM-Pulse characteristics:

Pulse Width (t) = 51.2 μs

Pulse Rise/Fall Time (t~r~/t~f~) = 0.15 μs

Frequency Deviation (B~c~) = 1.25 MHz

Frequency Hopping Range (B~s~) = 185 MHz

Figure 2 shows the measured emission spectrum for Radar 2. The figure
shows emission masks based on Recommendation ITU-R SM.1541.
Recommendation ITU-R SM.1541 calls for a 20 dB/decade roll-off from the
-40 dB bandwidth. In addition, to the current required roll-off from the
-40 dB bandwidth of 20 dB/decade (Green Mask), the figure shows the -30
dB/decade roll-off (Blue Mask) and -40 dB/decade roll-off (Red Mask)
from the -40 dB bandwidth. The -40 dB bandwidth was based on
Recommendation ITU-R SM.1541 (Equation 41, K = 6.2 and A = 0.105), and
determined to be 191.14 MHz.

![](media/image2.wmf){width="6.499305555555556in"
height="3.7604166666666665in"}

Figure 2. Radar 2 emission spectrum and emission masks

Figure 2 indicates that solid-state phased array antenna radar systems
using filters between the T/R modules and each antenna element may be
able to achieve a 40 dB per decade roll-off at the allocated band edges.
It should be noted that only limited measurements have been conducted on
this type of solid-state system at this time. Also, at this time,
factors such as cost, platform size, and mission have not been addressed
in assessing the feasibility of implementing filters between the T/R
modules and each antenna element.

**Summary Finding**

Additional measurements and studies are required taking into
consideration radar mission, cost, type of platform, and additional
range of pulse parameters and pulse modulation types to assess the
feasibility of more stringent emission limits than currently in
Recommendation ITU-R SM.1541.