Comparison of Multiple Emission Spectra of a Maritime Radionavigation
Radar Measured at Four Azimuths with Respect to the Base of the Radar
Antenna Rotary Joint

Contacts: Frank Sanders, +1-303-497-7600,
[**fsanders@its.blddrdoc.gov**](mailto:fsanders@its.blddrdoc.gov)

**Bradley Ramsey, +1-303-452-6887**,
[bramsey@rfmetrics.com](mailto:rsole@ntia.doc.gov)

Introduction.

According to the procedures described in ITU-R New Recommendation M.1177
for measuring radiated emission spectra of radar transmitters via the
so-called Direct Method, the measurement system should stay at a fixed
azimuth relative to the radar transmitter unit for the duration of a
spectrum measurement. For radar transmitters that utilize mechanically
rotated antennas, this means that such a spectrum measurement would be
performed at a single azimuth relative to the base of the rotary joint.
The question has arisen as to whether the electrical characteristics of
radar rotary joints might be expected to vary sufficiently as a function
of azimuth as to cause the emission spectra of such radars to vary
significantly as a function of azimuth from the radar transmitters.

To address this question, the authors have performed measurements of
emission spectra of a maritime radionavigation radar transmitter,
operating in the band 8 500-10 500 MHz, at four separate azimuths with
respect to the base of the rotary joint of the radar antenna. The four
spectra so obtained have been compared to determine the amount of
variation between those emission spectra. This document describes those
measurements and the results that were obtained.

Approach.

A maritime surface search radar operating in the 8 500-10 500 MHz band,
and property of the US Government, was set up in a normal operating mode
at the Table Mountain Radio Quiet Zone facility north of Boulder, CO,
USA. Pertinent radar transmitter characteristics are given in Table 1.
The radar transmitter antenna was approximately 4 m above the ground.

Table 1. Characteristics of the maritime radar used in the rotary joint
study.

+-------------+-------------+----------+----------------+-------------+
| **Pulse     | **Output    | **Tra    | **Antenna      | **Radiated  |
| width       | device**    | nsmitter | type, length,  | EIRP (MW)** |
| (ns)**      |             | peak     | and gain**     |             |
|             |             | power    |                |             |
|             |             | (kW)**   |                |             |
+-------------+-------------+----------+----------------+-------------+
| 80          | magnetron   | 12       | end fed        | 12          |
|             |             |          | slotted array  |             |
|             |             |          |                |             |
|             |             |          | 1.8 m, 30 dBi  |             |
+-------------+-------------+----------+----------------+-------------+

The measurement antenna was a 0.5-m diameter parabolic unit mounted
about 4 m above the ground. Measurements were performed in conformance
with the Direct Method (radiated) procedure of ITU-R New Recommendation
M.1177. The measurement system block diagram is shown in Figure 1. The
3-dB measurement bandwidth was 8 MHz and the frequency step size was
8 MHz between data points. The data were collected at a distance of
126 m from the radar.

![](media/image1.png){width="5.708333333333333in"
height="4.258333333333334in"}

Figure 1. Block diagram of measurement system used for radar emission
spectrum measurements.

The radar antenna rotated normally during all measurements. When the
first radar emission spectrum was completed with the base of the radar
in its initial position, the radar was temporarily turned off and the
base of the radar was rotated 90 degrees clockwise (as seen from above)
on the mounting tower; the measurement system position was not changed.
Then the radar was reactivated and the spectrum measurement was
repeated, again with the radar antenna rotating normally throughout.
When this second spectrum measurement was completed, the same procedure
of rotating the base of the radar transmitter and measuring the spectrum
with the base in the new orientation was repeated for a third time, this
time with the radar rotated 180 degrees from its original orientation.
Finally, the procedure was repeated for a fourth time with the radar
base rotated 270 degrees from its original orientation.

The result of these measurements was a set of four radar emission
spectra made at four azimuths relative to the base of the radar antenna
rotary joint and separated one from the next by an angle of 90 degrees.

Results.

The spectrum measurement results are shown in Figures 2 and 3. The
figures show the four radiated spectra observed from the four rotary
joint base angles.

![](media/image2.png){width="5.204166666666667in"
height="3.5680555555555555in"}

Figure 2. Non-normalized radar emission spectra measured at four
azimuths relative to the base of the radar transmitter antenna rotary
joint.

![](media/image3.png){width="5.396527777777778in"
height="3.747916666666667in"}

Figure 3. Radar emission spectra on four azimuths relative to the base
of the rotary joint and normalized at maximum measured power at the
fundamental frequency.

The variation between the spectra, about 1 dB, is within the uncertainty
bounds of the measurement system.

Conclusion.

No significant difference was observed between emission spectra at four
azimuths separated by 90 degrees relative to the base of the rotary
joint of this radar. The small variations that are observed are
accounted for by the uncertainty bounds of the measurement system
itself.

It could be hazardous to generalize this conclusion, since it is based
upon a sample set of one. But it should be noted that the radar used for
these measurements was a relatively inexpensive unit that has been
handled somewhat roughly at some points in its operational history. In
particular, the radar antenna was somewhat mechanically stressed at one
time, with the stress having been applied vertically at one end; a
slight vertical bend can consequently now be seen near the center of the
antenna, above the rotary joint, when a sight is taken along the
antenna's length. Thus, if there were any radar antenna rotary joint
that should have shown significant variation in its electrical
characteristics as a function of azimuth, this one was it.

Since this inexpensive and somewhat roughly handled radar does not show
significant variations in its emission spectrum as a function of rotary
joint azimuth, it is concluded that the issue may not be very important
for radar rotary joints in general. The procedures of M.1177 for Direct
Method radiated measurements of radar emissions would seem to be
adequate even if the measurements are performed on only one azimuth
relative to the base of a radar antenna rotary joint.