Hi Andy,
The Wilkinson Power Divider/Combiner is a primary component in the adding (or subtracting) and phase switched interferometers. It combines the signals (and noise) from two antennas and implements multiplication by squaring the sum.
(a + b)^2 = a^2 + b^2 + 2 ab
The LNA noise also gets squared and produces a large DC output that can result in large noise output due to gain variations. The phase switched interferometer greatly mitigates this problem by switching the phase of one antenna signal from 0 degrees to 180 degrees at an audio rate. This puts the desired product signal on an audio carrier and the large DC and low frequency noise can be rejected by high-pass filtering or averaging following synchronous demodulation.
One property of the combiner which seems to be mostly overlooked is that the coherent signals add in power but only half of the incoherent noise appears at the output sum port. In other words if the noise from each of the two LNAs is, say, Pn, the total output LNA noise is not 2 Pn but only Pn. The other half of the LNA noise power is dissipated in the isolation resistor. This means that the effective antenna area of the adding interferometer is the sum of the areas of the two antennas (assuming they are the same size). This is not the case with the usual interferometer that multiplies the two antenna signals. In that case, the effective antenna area is that of a single antenna.
Jim Abshier