RCG-26

Comparison of Radiated Emission Spectra of Maritime Radiolocation Radars
with Rotating Versus Non-Rotating Transmitter Antennas During
Measurements in the Bands 2 900-3 100 MHz and 8 500-10 500 MHz

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

Robert Sole, +1-202-482-1245, <rsole@ntia.doc.gov>

**Introduction.**

Emission spectra of radiolocation radars may be measured via direct
radiation from the transmitter antenna. The direct technique allows
frequency-dependent (and possibly important) effects of the radiating
antenna to be automatically included in the final set of emission
spectrum data.

The radiated mode of measurement ordinarily implies that the radar
antenna will rotate while the measurement is performed. In this case, a
single spectrum frequency point needs to be acquired for each revolution
of the radar antenna, as described in ITU-R New Recommendation M.1177.
This means that the time required to acquire the spectrum is equal to
the rotation time of the antenna multiplied by the total number of data
points. If, for example, the total number of data points were 1000 and
the rotation interval of the radar were 3 seconds, then the total time
required for the measurement would be 3000 seconds, or 50 minutes. And
in the case of emission measurements of 9000 MHz maritime radars, in
which a data point may need to be acquired with a spacing of 1 MHz and
in which the measurement might need to cover a frequency range of
7-26 GHz (a total span of 19 GHz), the total measurement time required
with a rotation interval of 3 seconds would be 15.8 hours.

The measurement pace could be considerably accelerated if the radar
antenna could be suspended during data acquisition. Then, the spectrum
data could be acquired at a rate determined by the maximum speed of the
measurement system's frequency-stepping capability. If, for example, the
data points could be collected at the rate of one point every 0.5 sec,
the total time required for the 19 GHz span described above would be 1/6
of 15.8 hours, or 2.6 hours.

In principle, it might be expected that the characteristics of a radar
emission spectrum should not depend upon whether the radar antenna is
rotating or stationary, but this hypothesis has needed to be tested.
This document describes the results of a study that compared the
emissions of 3 GHz and 9 GHz maritime radars when their antennas were
rotated versus not rotated during radiated spectrum measurements.

**Experimental Setup.**

Radiated emission spectrum measurements were performed at an outdoor
radio measurement facility, in the Table Mountain radio quiet zone north
of Boulder, Colorado in the US on two maritime radars, one with a
fundamental operating frequency in the 2 900-3 100 MHz band ("S-Band
Radar 1") and the other with a fundamental operating frequency in the
8 500-10 500 MHz band ("X-Band Radar 1"). The radars were provided by
the Administration of Japan and the measurements were performed jointly
by the Administrations of Japan and the US in accordance with the
methodology described for the Direct (radiated) Method in M.1177. The
measurement system block diagram is shown in Figure 1.

![](media/image1.png){width="4.002083333333333in"
height="3.136111111111111in"}

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

The measurement antenna was a 1-m parabolic dish located at a height of
\[4 m\] above the ground. The radar antennas were mounted approximately
\[3 m\] above the ground. The distances between the measurement antenna
and the radars, the spectrum analyzer models that were used, and the
measurement bandwidths are shown in Table 1. Radar characteristics are
shown in Table 2. All measurement parameters except rotation versus
non-rotation were held constant during measurements for each radar
model.

Table 1. Measurement distances, spectrum analyzer models, and
measurement bandwidths used for emission spectrum measurements.

+--------------+-------------------------+---------------+------------+
| ## Radar     | **Distance between      | **Spectrum    | **M        |
|              | radar and measurement   | analyzer      | easurement |
|              | antenna**               | model**       | b          |
|              |                         |               | andwidth** |
|              | **(m)**                 |               |            |
|              |                         |               | **(MHz)**  |
+--------------+-------------------------+---------------+------------+
| S-Band Radar | 366                     | ### HP-8566B  | 3          |
| 1            |                         |               |            |
+--------------+-------------------------+---------------+------------+
| X-Band Radar | 83                      | Agilent       | 8          |
| 1            |                         | E4440A        |            |
+--------------+-------------------------+---------------+------------+

Table 2. Emission characteristics of radars in this study.

  -------------- -------------- ----------- ---------------- --------------
  **Radar        **Pulse width  **Output    **Antenna type** **Antenna
  Designator**   (s)**          device**                     length (m)**

  S Band Radar 1 0.06           magnetron   end fed slotted  4.3
                                            array            

  X Band Radar 1 0.07           magnetron   end fed slotted  1.4
                                            array            
  -------------- -------------- ----------- ---------------- --------------

**Measurement Results.**

The emission spectrum measurement results for the conditions of rotation
versus non-rotation are shown in Figures 2-3.

![](media/image2.png){width="5.0680555555555555in"
height="3.703472222222222in"}

Figure 2. Emission spectra of S-Band Radar 1 with transmitter antenna
rotating versus not rotating. Note waveguide cutoff features between
about 2 300-2 400 MHz. Airport surveillance radar emission at 2 720 MHz
was reduced as much as possible by timing measurement steps to be
anti-coincident with the airport radar beam scanning.

![](media/image3.png){width="4.915972222222222in"
height="3.5409722222222224in"}

Figure 3. Emission spectra of X-Band Radar 1 with transmitter antenna
rotating versus not rotating.

In both cases, the measured emission spectra showed nearly the same
amplitudes at the fundamental frequencies regardless of whether the
radar transmitter antennas were rotating or stationary. But the emission
spectra of unwanted emissions (that is, the out-of-band and spurious
emissions) were markedly lower when the transmitter antennas were
stationary versus when the transmitter antennas were rotated during the
measurements.

To understand this problem more thoroughly, measurements of X Band Radar
antenna patterns were performed at intervals of 8 MHz throughout the
emission spectrum. For each antenna pattern, the measurement was
triggered by the passage of the radar main beam at the fundamental
frequency. These measurements showed that, at most frequencies other
than the fundamental, the radar antenna pattern formed two lobes rather
than one, and that the null between the two lobes occurred where the
main beam was formed at the fundamental frequency. An example is shown
in Figure 4.

![](media/image4.png){width="5.996527777777778in"
height="4.325694444444444in"}

Figure 4. Radar antenna patterns for a slotted array at the fundamental
frequency (upper curve) and a simultaneously acquired antenna pattern
464 MHz higher in frequency (lower curve), which was triggered by the
passage of the main beam. The frequency-offset antenna pattern forms two
lobes, the higher-amplitude lobe in this case being offset 2.7 degrees
and 16 dB in amplitude from the position of the main beam at the
fundamental. This behavior is observed throughout the spectrum.

**Analysis and Interpretation.**

Since both radars provided the same emission power on the respective
center frequencies, the possibility that the rotary joint of the radar
did not perform as well when it was stationary as when it was rotating
can be eliminated. Instead, the effect that is observed is clearly
frequency-dependent. That is, Figures 2-3 show that the difference
between the rotating antenna versus non-rotating antenna measurements
grows progressively larger as measured frequencies diverge further from
the radar fundamental frequencies.

Figure 4, which shows an example of a behavior that is observed
throughout the spectrum, demonstrates the cause of the phenomenon. The
slotted array forms a double beam at frequencies other than the
fundamental, and the null between the two beams occurs at (or near) the
location where the main beam is formed at the fundamental.

As a result of this behavior, spectrum measurements performed on
non-rotating radar antennas will result in lower (and incorrect)
amplitudes relative to the fundamental power than measurements performed
on rotating antennas.

The cause of this phenomenon is the slotted design of the radar
antennas, which produces frequency-dependent lobe structures in the
antenna patterns.

**Conclusion.**

For Direct Method radiated emission spectrum measurements, it is
desirable to ensure that the radar transmitter antenna is rotated during
the measurements if the antenna characteristics cause a
frequency-dependent variation in the direction of the main radiation
lobes relative to the direction of the main beam at the radar
fundamental frequency.

For types of radar transmitter antennas that do not have this
characteristic spectrum measurements can be performed with the
transmitter antennas held stationary.