RCG-33

Investigation into the UE implications of a radar's Ro Jo using the
direct method

S. W. Moss

QinetiQ Funtington

QinetiQ/D&TS/SEA/TWP051471

Abstract

It has been shown that variation in VSWR seen by the magnetron in marine
navigation radars can have a significant effect on the emission
spectrum, specifically the 2^nd^ harmonic for X band radars. QinetiQ
Funtington has undertaken an investigation to ensure that this
phenomenon can be measured and subsequently taken into account when
using the direct method for Unwanted Emissions type approval testing.

Introduction

Concerns have been raised in recent times about the effect that
different VSWR values can have on the spectral efficiency of magnetron
driven radar systems. Two separate studies \[1\], \[2\] have shown
significant variation in harmonic power levels seen when the VSWR seen
by the magnetron is changed. The largest variation seen was 13dB with
one system \[2\]. This would not be a problem from a type approval point
of view if the system was unchanging during normal operation. If this
was the case then a simple pass/fail decision could be made easily. The
problem in this case is that the VSWR varies continuously with antenna
rotation due to mechanical imperfections in the rotating joint (Ro Jo)
which carries the RF energy from the base unit into the antenna. This
variation is not random however and changes in a periodic way with
antenna angle. The purpose of this paper is to show that this effect can
be measured using the direct method and therefore does not represent a
problem in using the direct method for type approval testing.

Current Measurement Methods

There are two methods described in \[3\] for Unwanted Emissions
measurements of radars. The indirect method is a two stage method that
involves removal of the antenna to measure its gain at a number of
frequencies across the band. These measurements are then combined with a
spectral measurement carried out directly on the output of the Ro Jo.
This setup allows an easy check on the effect of the VSWR on the
harmonic level simply by rotating the joint with the measurement
equipment still attached. This was one of the methods employed by
Cetecom for BSH \[1\] where a ± 6.5dB variation in harmonic level was
observed. There is concern however that this measurement method alters
the load conditions on the magnetron itself and may not give an accurate
reading as a result.

The direct method of measurement employed by QinetiQ Funtington leaves
the radar intact and operating normally and then measures the radiated
power in the far field. As the Rx antenna is in a fixed location only
the emitted spectrum at one value of Ro Jo VSWR is measured. This
depends entirely on how the base unit is orientated to the range when
mounted.

The solution

The solution to the problem is to measure the region of the spectra most
influenced by Ro Jo VSWR for all angles of base unit and hence Ro Jo.

To provide a rotating facility the device under test (DUT) was mounted
on top of a Scientific Atlanta antenna positioner system originally
installed for making accurate RCS measurements of objects for the UK
MOD. The positioner is driven remotely from the Rx cabin by a positioner
controller. This is in turn controlled by the measurement PC allowing
precise control of the base unit angle.

Software was developed to control the positioner system while measuring
the harmonic level as a function of base unit angle. Once the worse case
angle has been identified the standard unwanted emission measurement can
be made at that angle of base unit.

A picture of the positioner with a DUT on top is shown below in Figure
1.

![](media/image1.jpeg){width="4.502083333333333in"
height="3.6708333333333334in"}

Figure 1 Picture of the positioner with DUT mounted

The Test

The DUT was a 10kW X band marine navigation radar with a 6ft open array
antenna. It was operating normally in long pulse mode to maximise the
received power in the spectrum analyser. The UE receiver system was
fixed tuned to the harmonic frequency of 18.828 GHz. The sweep time on
the analyser was set to 10s which allowed for a little over 4 rotations
of the radar antenna. The peak value of the trace was then recorded by
the PC. The base unit was rotated by 1º and the spectrum analyser
allowed to re sweep. This process continued until the full 360º had been
measured. The experiment was then repeated at the fundamental frequency
and at in the centre of the pi -1 emission to see if any effect could be
observed.

Results

The results of the harmonic and fundamental frequency measurements are
shown together in Figure 1. A variation in harmonic signal level of
±1.5dB was observed. The pi -1 emission showed no variation with angle
and is not shown in Figure 2 below.

![](media/image2.wmf){width="5.7659722222222225in" height="3.5375in"}

Figure 2 Signal level variance vs. Base unit angle

On completion of the emission measurements the radar was dismantled and
the return loss of the Ro Jo was measured at 9.41GHz. The return loss
was found to vary by 0.5dB in amplitude and approximately 8º in phase
over the 360º of rotation.

Conclusions

The effect of varying Ro Jo VSWR on the harmonic emissions of an X band
marine radar has been measured using the direct method in the far field.
For the radar tested the variation in harmonic level was found to be ±
1.5dB.

By using the facilities available on site Funtington has developed a
practical automated measurement technique to identify the worst case and
hence measure the maximum radiated emissions.

Recommendations

Any future direct method measurements should begin with an assessment of
the Ro Jo variation as carried out here. The worst case emission angle
could then be automatically aligned to the range prior to the main
spectral measurement. This would add about an hour to the overall
measurement time.

It is important to note that the quality and type of Ro Jo's used for
different classes of marine radars can vary considerably therefore an
investigation should be carried out into the consistency of manufacture
of the various types. If this reveals large variations then it's
possible that tighter manufacturing tolerances may have to be
considered. This is especially important due to the large numbers of
radars produced using low cost components for the leisure craft market.

Similar quality considerations should also be given to the circulators
used as their isolation value provides protection to the magnetron from
the Ro Jo VSWR and an example with a poor isolation could lead to the Ro
Jo having a greater influence on the magnetron than it would otherwise
have had.

As QinetiQ Funtington did not have access to one at the time, a similar
investigation should be carried out with an S band radar to assess the
VSWR effect on its harmonic levels.

References

1.  Input 8B/84-E to WP8B Sept 04. *Comparisons of Unwanted Emissions
    Measurements*

2.  RayMarine inputs to the 1^st^ meeting of JRG 1A-1C-8B. *Further
    considerations on issues impacting the resultant measured spectrum
    of magnetron based Radar systems*

3.  ITU R-M1177-3. *Techniques for measurement of unwanted emissions of
    radar systems*