Comparison of the Emission Characteristics of Conventional and Coaxial Magnetrons

This paper compares the electromagnetic compat­ ibility (EMC) characteristics of conventional and coaxial magnetrons used in radar transmitters. The characteristics of major concern in this comparison are, emission spectrum, spurious emission, pulse shape and frequency stability. The data presented were obtained under the Department of Defense spectrum signature program. Measurements were made in accordance with MIL-STD-449D, and were monitored by the Electro­ magnetic Compatibility Analysis Center, (ECAC). The equipments measured were the AN/FPS-6 B (2.7-2.9 GHz), and the AN/FPS-16 (5.4-5.9 GHz) radars, located at the U.S. Army Electronic Proving Ground (USAEPG), Fort Huachuca, Arizona. The measurements show a significant improvement in the emission characteristics of the transmitters when they are retrofitted with coaxial magnetrons. The emission spectrum envelope of the coaxial magnetron is about one half as wide as that of the conventional magnetron in the region of 0 to 50 dB below the carrier. Below this region, the emission envelope of the coaxial magnetron was 10 to 20 dB below that of the conven­ tional magnetron. The radiated output levels of the second and third harmonic of the coaxial magnetron used in the AN/FPS-6 B radar were approximately 30 dB lower (82 and 90 dB down respectively) than those from the conventional magentron. A spurious output at a level approximately 50 dB below the carrier was detected in the output emission of the coaxial magnetron used in the AN/FPS-16 radar. This spurious output was at a frequency 110-120 MHz above the carrier frequency and is attributed to in­ adequate suppression of an undesired mode in the cavity. Short term frequency stability of the two types of magnetrons tested was comparable. In a four hour test, after 15 minutes warm-up, a total variation of + 250 KC was detected. Measurements showed an increase in the rise and fall times of the detected R.F. pulse of the coaxial magnetrons over that of the conventional magnetrons. This is a result of the higher Q of the coaxial type magnetron which also accounts for the narrower spectrum measured. Several characteristics of the two types of mag­ netrons which relate directly and indirectly to EMC are discussed in the paper. Also discussed are factors which should be considered when planning for use of coaxial magnetrons in future systems.