A Reduced Uncertainty Method for Gain over Temperature Measurements in an Anechoic Chamber

Author: Vince Rodriguez and Charles Osborne

P Gain over Temperature (G/T) is an antenna parameter of importance in both satellite communications and radio-astronomy. Methods to measure G/T are discussed in the literature [1-3]. These methodologies usually call for measurements outdoors where the antenna under test (AUT) is pointed to the “empty” sky to get a “cold” noise temperature measurement; as required by the Y-factor measurement approach [4]. In reference [5], Kolesnikoff et al. present a method for measuring G/T in an anechoic chamber. In this approach the chamber has to be maintained at 290 kelvin to achieve the “cold” reference temperature. In this paper, a new method is presented intended for the characterization of lower gain antennas, such as active elements of arrays. The new method does not require a cold temperature reference, thus alleviating the need for testing outside or maintaining a cold reference temperature in a chamber. The new method uses two separate “hot” sources. The two hot sources are created by using two separate noise diode sources of known excess noise ratios (ENR) or by one source and a known attenuation. The key is that the sources differ by a known amount. In a conventional Y-factor measurement [4], when the noise source is turned off, the noise power is simply the output attenuator acting as a 50 ohm termination for the rest of the receive system. But by using two known noise sources, the lower noise temperature source takes the place of T-cold in the Y-factor equations. The added noise becomes the difference in ENR values. An advantage of this approach is that it allows all the ambient absorber thermal noise temperature change effects to be small factors, thus reducing one of the sources of uncertainty in the measurement. This paper provides simulation data to get an approximation of the signal loss from the probe to the antenna under test (AUT). Another critical part of the method is to correctly define the reference plane for the measurement. Preliminary measurements are presented to validate the approach for a known amplifier attached to a standard gain horn SGH) which is used as the AUT.

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