reference signals at the transmit/receive modules will have increasingly larger differential time delays with respect to the original reference source. In effect, the modules at the transmitter edges will be excited by a reference frequency which has been time delayed by the radius of the transmitter. This would be of no concern if the VCO frequencies and the reference source were ideal tones. However, these frequencies will have finite spectral widths, and differential time delays in finite spectral width signals cause decorrelating effects which tend to reduce aperture efficiency (reference 15). The amount of decorrelation and consequent loss of aperture efficiency depends on the spectral width of the source, how it is distributed, and the size of aperture. Analyses have shown (reference 15) that an aperture illuminated with a signal having a typical distance L for 63% decorrelation will result in the antenna pattern having a peak power density 20% down from the ideal if the transmitter radius is as much as 25% of the distance L . For smaller radius/L ratios this relationship is approximately proportional so that for losses to be held less than 1% the transmitter radius should not exceed 1.3% of the decorrelation distance, L. The decorrelation distance L can be approximated a variety of ways. One simple method is arrived at by assuming the reference frequency source can be modeled as very narrowband noise. Assuming the reference frequency has a spectral power density which is constant over the spectral width and zero outside this width, an autocorrelation of such a signal would have the form,
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