R.M. Rudish and S.E. Sussman-Fort (USA)
Phased Arrays, Non-Foster Impedance matching
The scan impedance of a dipole that is in an array becomes highly reactive as frequency is decreased. As an example, Fig. 1 shows the scan impedance of an infinite array of dipoles for a 5:1 bandwidth. At the high end of the operating band these dipoles are one-half wavelength long, are located along a rectangular grid with half wavelength periodicity, and are located one-quarter wavelength in front of a reflector plane. Conventional networks cannot match such a widely changing scan impedance over the full frequency band to limit reflection losses to a few dB. Multi-octave-bandwidth phased-array elements that are closely-spaced to avoid grating lobes at the highest operating frequency must be electrically-small at the lowest frequency. Even isolated VHF elements for small vehicles must be electrically-short if they are to be practical. However, electrically-small wideband elements are notorious for their poor gain, which is caused by impedance-mismatch loss. This paper presents experimental verification of a new technique for reducing this loss. We compare the performance of a 12-inch tall, lossy-matched monopole to that of a 12-inch tall dipole which is partially matched using non-Foster (negative) capacitors. Even though the non-Foster capacitor introduces noise, the net improvement in S/N is shown to be at least 10 dB over much of the VHF band.
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