Millimeter-Wave Magnetron Transmitters

for High-Resolution Radars

D. M. Vavriv and V. A. Volkov

Department of Microwave Electronics Institute of Radio Astronomy of the National Academy of Sciences of Ukraine

4, Chervonopraporna Str., Kharkov-61002, Ukraine Tel.: +380-57-7203718, Fax: +380-57-7203427, E-mail: vavriv@rian.kharkov.ua,

http://radar.kharkov.com

Abstract. Experience in the development and application of transmitters for coherent radar systems based on millimeter-wave magnetrons is summarized. Design approaches to the development of highly efficient transmitters for frequencies of 36 GHz and 95 GHz with power level of 30 kW and 4 kW, respectively, are discussed. Meteorological Doppler radar systems based on such transmitters are described as examples of applications of the transmitters.

Keywords: Magnetron, transmitter, millimeter waves, radar, meteorological radar.

I. INTRODUCTION

There are two main approaches to the development of coherent radar systems. The

first consists of building radars using the so-called “truly coherent” scheme, where the

transmitter is based on an amplifier driven by a highly stable oscillator. Such systems

provide fairly good clutter suppression, a high Doppler resolution, and give rise to a

number of possibilities for the introduction of sophisticated methods of pulse

compression. However, the lack of cost effective, high power, millimeter-wave

amplifiers – especially at frequencies of 95 GHz and higher – essentially limits the

development of such types of radars intended for a long-range operation.

In order to tackle this problem, the second approach, based on the “coherent-on-

receive” technique, can be used for the development of coherent systems. This

technique proposes storing in some way the values of the phase of the RF pulses

emitted by the transmitter and comparing these values with those of backscattered

signals. In this case, the transmitter can be based on a free-running oscillator, such as a

magnetron. In addition, recent advances in microprocessors and digital signal

processing techniques enable the development of coherent-on-receive systems with

capabilities similar to those offered by truly coherent systems.

In this paper, we summarize our experience with the development of millimeter-

wave transmitters based on conventional magnetrons and with spatial-harmonic

magnetrons with cold secondary-emission cathodes [1, 2] for coherent radar systems.

Our studies have shown that the application of specially-designed magnetron

modulators and microprocessor control systems can successfully solve the problem of

the development of highly efficient transmitters for applications at frequencies of 36

GHz and 95 GHz.

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II. MAGNETRON TRANSMITTER DESIGN

We have found that, among the various potential schemes of magnetron modulator

design, the scheme that uses a partial discharge of the storage capacitor is the one most

suitable for driving both 36 GHz and 95 GHz magnetrons [1, 2]. To develop the high

voltage switch for such modulators, we have used both hard tubes and solid-state

devices based on series-connected MOSFETs.

A block-diagram of the transmitter using a hard tube modulator is shown in Fig. 1.

The transmitter includes the following main parts: a high-voltage power supply, which

includes a power factor corrector, a filament power supply, a driver for the modulator

tube, and a controller. The high power supply utilizes a flyback converter with current

feedback. Such a scheme allows us to obtain a rather small voltage ripple at the

harmonics of the AC power line voltage. The output voltage can be set within a range

of 3 to 18 kV and the maximum output power is 600 W. The output stage of the hard

tube driver is based on a two-pole scheme and provides a voltage swing of 1500 V

with rise and fall times of less than 15 ns. All power supplies in the modulator are

synchronized at frequencies that are multiples of the pulse repetition frequency of the

transmitter.

FIGURE 1. Block diagram of 35.5 and 95 GHz magnetron transmitters using a hard tube modulator.

A block-diagram of the transmitter using a solid-state, high voltage switch is shown

in Fig. 2. It is comprised of a high voltage power supply, a magnetron filament supply,

a high voltage switch, and a controller. We used a floating, high voltage switch, which

enables us to use only a single high voltage capacitor. In addition, we use an advanced

scheme for a direct and precise measurement of the magnetron filament voltage. This

scheme is implemented in a loop that automatically adjusts the filament voltage

depending on the magnetron operating conditions.

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FIGURE 2. Block diagram of a 35 GHz magnetron transmitter using a solid-state modulator.

Built-in controllers are used in both types of transmitters to enable local and remote

transmitter control and to provide diagnostics.

III. TRANSMITTER CHARACTERISTICS

The above solutions have resulted in the development of transmitters which

produce output pulses with desirable characteristics and negligibly small jitter. The

intra-pulse phase change during the high power period within the 200 ns pulse is about

10° (36 GHz transmitter) and 20° (95 GHz transmitter), and the pulse-to-pulse frequency chirp is reproducible to within about 100 Hz (36 GHz transmitter) and 300

Hz (95 GHz transmitter).

A typical build-up of the RF pulses in a 95 GHz transmitter is shown in Fig. 3a.

This photo was obtained by superposition of 20,000 successive 200-ns wide pulses. It

can be seen that the pulse jitter is less than 2 ns. The jitter occurs only at the initial

stage of the pulse formation. Figure 3b illustrates the power spectrum on a logarithmic

scale of the same train of pulses seen in Fig. 3a. The level of the first “frequency

sidelobe” is about −30 dB, and the spectral width (between the two first minima) is

2/T, where T is the pulse duration.

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a) b)

FIGURE 3. (a) The build-up of RF pulses in a 95 GHz transmitter and (b) the associated power spectrum of transmitted pulses.

Typical characteristics of transmitters developed for Ka- and W-band Doppler

radar systems are given in Table 1.

TABLE 1. Parameters of Ka- and W-band Transmitters.

Operating Frequency, GHz Parameter

35.5±0.2 35±0.2 95±0.3

Output pulse power, kW 30 2.5 4 Output average power (max), W 80 2.5 4 Pulse duration, ns 100…400 50-500 50-500 Pulse repetition rate, kHz 2.5, 5, 7.5 1…10 1-10 Supply voltage, V 230 AC 27 DC 230 AC Power consumption (max), W 700 60 500

FIGURE 4. Power spectrum (detected logarithmic power versus frequency in Hz off of the carrier frequency) from a stationary target at the distance of 8 km obtained with a 35.5 Doppler radar.

In order to illustrate the quality of the 30 kW magnetron transmitter used in a 35.5

GHz radar system, the Doppler spectrum from a stationary ground-based target at a

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distance of 8 km is shown in Fig. 4. A dwell time of 0.1 seconds and a pulse repetition

frequency of 5 kHz were used for the measurement. Note that the noise floor is about

−53 dB, which corresponds to a pulse-to-pulse frequency stability of about 10−8

. The

target’s spectral line width is 10 Hz, and here it is determined only by the dwell time.

IV. METEOROLOGICAL RADARS

The above described transmitters are used in 36 GHz and 95 GHz Doppler

meteorological radar systems [5, 6]. These radars have been dopplerized by using a

digital coherent-on-receive technique. The radars have been designed for long-term,

unattended operation, and provide high-resolution, real-time measurements of profiles

of reflectivity, Doppler spectrum, mean radial velocity, velocity variance, and

polarimetric characteristics. Such a 36 GHz transmitter has also been used in a

recently developed scanning meteorological radar with improved measurement

capabilities. A photo of this radar is shown in Fig. 5, and its characteristics are

summarized in Table 2.

Figure 5. Photo of a 36 GHz scanning meteorological radar.

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TABLE 2. Parameters of a Scanning Meteorological Radar.

Frequency, GHz 35.5 ± 0.15 Peak power (max), kW 30 Tube type magnetron Pulse width, ns 100, 200, and 400 Pulse repetition frequency, kHz 2.5, 5, and 10 Receiver noise figure, dB 3.2 Losses, dB Transmitting path Receiving path

1

2.5 Antenna diameter, m 1.2 Sidelobe level, dB −25 Precision of the antenna positioning, deg 0.1 Scan range, deg

Azimuth direction Elevation direction

−183…+183

−45…+45 Scan velocity (max), both directions, deg/sec 10 Acceleration (max), both directions, deg2/sec 10 Polarization isolation, dB −35

V. CONCLUSIONS

In this paper, we have summarized our experiences with the development and

implementation of millimeter-wave transmitters for high-resolution, coherent radar

systems. The obtained results demonstrate the advantages of these transmitters for the

creation of high quality transmitting pulses whose attributes include high pulse-to-

pulse frequency stability, low intra-pulse phase variation, and negligibly small jitter.

Transmitters that operate at 36 GHz and 95 GHz with output power levels of 30 kW

and 4 kW, respectively, have been developed and successfully implemented in

Doppler meteorological radars.

VI. ACKNOWLEDGEMENTS

The authors are grateful to V. Bormotov, V. Semenuta, B. Trush, R. Kozhin, and A.

Belikov for their contributions to this work.

REFERENCES

1. V. Naumenko and D. Vavriv, “Millimeter wave magnetrons with secondary-emission cathode: Theory and Experiments,” in Proc. of the 21st Int. Conf. on Infrared and Millimeter Waves, Berlin, 1996, paper ATh14.

2. V. D. Naumenko, K. Schünemann, and D. M. Vavriv, Electronics Lett. 35(22), 1960-1961 (1999). 3. V.D. Naumenko, K. Schünemann, V. Ye. Semenuta, D.M.Vavriv, and V.A. Volkov, “Transmitters based on

magnetrons with cold secondary-emission cathode,” in Proc. of the 22nd Int. Conf. on Infrared and Millimeter Waves, Wintergreen, VA, 1997, pp. 42-43.

4. M. Jenett, V. Kazantsev, A. Kurekin, K. Schünemann, D. Vavriv, V. Vinogradov, and V. Volkov, “Dual 94 and 36 GHz radar system for remote sensing applications,” in Int. Geoscience and Remote Sensing Symposium, Hamburg, 1999.

5. D.M. Vavriv, et al., Radio Physics and Radio Astronomy, 7(2), 121 – 138 (2002). 6. V. Bormotov, G. Peters, K. Schünemann, D. Vavriv, V. Vinogradov, and V. Volkov, “36 GHz Doppler radar

for remote sensing of the atmosphere,” in Proc. of Millennium Conf. on Antennas and Propagation, Davos, p. 319, 2000.

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