Out-of-Band Antenna Response

The amount of power coupled to an antenna in a noisy radio frequency interference (rfi) environment is an important factor in determining system interference levels. Because the interfering frequencies may be outside the operating band of the system antennas, we must be able to predict the out-of-band response of antennas. The unavailability of wide-band radiation density patterns requires us to estimate the out-ofband response of antennas using limited data. By combining generic equations for out-of-band antenna effective aperture with input impedance mismatch and radiation pattern measurements of typical system antennas, we have developed a methodology for predicting the out-of-band response of antennas. Our measurements are restricted to the microwave band between 2 and 12 GHz, but the methodology is general and is applicable to other out-of-band frequencies. Even if no measurements are available, worst-case response estimates can be made by use of the generic equations. is about -2.7 dBi. (We have observed median gains to be within a few dB of mean gains for all of our data and therefore use the median because it is also the 50$ point of the cumulative probability distribution.) Our measurements indicate that it is not unusual for an antenna to be nearly lossless and even to exhibit significant gain at out-of-band frequencies. DEFINITIONS OF ANTENNA PARAMETERS Definition of Effective Aperture In order to simplify the analysis of antenna re­ sponse, we consider parameters affecting antenna effective aperture separately. The general equation for antenna effective aperture is [1,2]