NTIA Technical Memorandum TM-05-430 

Comparison of Radar Spectra on 

Varying Azimuths Relative to the 

Base of the Antenna Rotary Joint 

Frank H. Sanders 

Bradley J. Ramsey 

U.S. DEPARTMENT OF COMMERCE 

Carlos M. Gutierrez, Secretary 

Michael D. Gallagher, Assistant Secretary 

for Communications and Information 

August 2005

DISCLAIMER 

Certain commercial equipment, instruments, or materials are identified in this report to specify 

the technical aspects of the reported results. In no case does such identification imply 

recommendation or endorsement by the National Telecommunications and Information 

Administration, nor does it imply that the material or equipment identified is necessarily the best 

available for the purpose. 

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CONTENTS

Page 

1 INTRODUCTION........................................................................................................... 1 

2 APPROACH .................................................................................................................. 2

3 RESULTS AND ANALYSIS ......................................................................................... 4 

4 CONCLUSION ............................................................................................................... 6 

5 REFERENCES................................................................................................................ 7

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COMPARISON OF RADAR SPECTRA ON VARYING AZIMUTHS RELATIVE TO 

THE BASE OF THE ANTENNA ROTARY JOINT 

Frank H. Sanders1

 and Bradley J. Ramsey2

This technical memorandum describes the results of radiated measurements of the 

emission spectrum of a maritime radar at varying azimuths relative to the base of

the radar transmitter antenna rotary joint. The measurements were performed to 

address the question of whether the emission spectrum of such a radar might vary 

as a function of the pointing azimuth of the radar antenna as it rotates due to 

variation in the joint’s voltage standing wave ratio (VSWR) with azimuth. If such 

variation were to occur, then the radiated spectrum would need to be 

characterized as a function of azimuth. The measurement results indicate that the 

emission spectrum of this radar does not vary as a function of transmitter antenna 

azimuth. It is concluded that this issue is probably not a concern for radar 

emission measurements in general, and that the radiated spectrum measurement 

procedures described in NTIA Report TR-05-420 are adequate. 

Key words: radar emission measurement techniques; radar rotary joint voltage standing wave 

 ratio (VSWR); radar spectrum engineering criteria 

1 INTRODUCTION 

According to published procedures for the measurement of radiated emissions from radar 

transmitters [1-2], 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 radar transmitter antenna 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 radiolocation 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 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. 

 

1 The author is with the Institute for Telecommunication Sciences, National Telecommunications and 

Information Administration, U.S. Department of Commerce, 325 Broadway, Boulder, CO 80305-3328.

2 The author is with the RF Metrics Corporation, Westminster, CO.
2 APPROACH 

A maritime surface search radar, operating in the 8,500-10,500 MHz band, 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 located at a height of approximately 4 m above the ground. 

Table 1. Characteristics of the Maritime Radar Used in the Rotary Joint Study 

Pulse width 

(ns) 

Output 

device 

Transmitter peak 

power (kW)

Antenna type, 

length, and gain 

Radiated EIRP 

(MW) 

80 magnetron 12 end fed slotted array

1.8 m, 30 dBi

12 

The measurement antenna was a 0.5-m diameter parabolic unit mounted about 4 m above the 

ground. Measurement system calibration (with a noise diode and a standard Y-factor 

computation) and data collection were performed in conformance with published procedures of 

NTIA and the International Telecommunication Union Radiocommunication Sector (ITU-R) [1-

2]. 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. 

The radar antenna rotated normally during all measurements. After 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. Then the procedure of rotating the base of the radar transmitter and 

measuring the spectrum was repeated twice more, with the base of the radar oriented at angles of 

180 degrees and 270 degrees relative to the initial orientation. 

2
Figure 1. Block diagram of the radar spectrum measurement system. The spectrum analyzer 

was an Agilent E-4440A. The RF front end was designed and built by NTIA/ITS. 

3
3 RESULTS AND ANALYSIS 

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. 

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. In Figure 2, the radar emission 

spectra are shown, the only corrections being drawn from the noise diode Y-factor calibration 

that had been run on the measurement system before the data were collected. In Figure 3, the 

spectra are normalized to the same power level at the radar fundamental frequency. In both 

cases, the variation between the spectra, about 1 dB, is less than the resolution of the 

measurement system [3]. 

Figure 2. Non-normalized radar emission spectra measured at four azimuths relative to the base

of the radar transmitter antenna rotary joint. 

4
Figure 3. Radar emission spectra on four azimuths relative to the base of the rotary joint, 

normalized to 0 dB maximum power at the fundamental frequency. 

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4 CONCLUSION

No 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 inherent error of the measurement system. 

While it could be hazardous to generalize this conclusion (as it is based upon a sample set of 

one), it should be noted that the radar transmitter used for these measurements was a relatively 

inexpensive unit that had been handled somewhat roughly at some times in its operational 

history. In particular, the radar antenna was mechanically stressed on one occasion, resulting in a 

slight vertical oriented bend near the center of the antenna, directly above the rotary joint. 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 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. Published procedures for measurements of radiated emission spectra from radar 

transmitters [1-2], which currently specify measurements of such emissions from a single 

azimuth, are therefore presumed to be adequate. 

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5 REFERENCES 

[1] F.H. Sanders, R.L. Hinkle, and B.J. Ramsey, “Measurement procedures for the radar 

spectrum engineering criteria,” NTIA Report TR-05-420, Mar. 2005. 

[2] ITU-R Recommendation M.1177, “Techniques for measurement of unwanted emissions 

of radar systems,” International Telecommunication Union, Radiocommunication Sector, 

2003. 

[3] J.W. Allen, “Gain characterization of the RF measurement path,” NTIA Report TR-04-

410, Feb. 2004. 

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