Radiotelescope Square Law Detectors

If you become a dedicated amateur radio astronomer, you will no doubt hear much talk about "square law detectors". It is interesting that one of the simplest electronic configurations in the radiotelescope show draw so much attention, but all of this attention is due to its' very important role. The radio frequency energy exiting from the earlier portions of the receiver alternates in polarity around some central voltage. If we could just hook up a DC (direct current) meter to this signal it would read zero volts because the positive and negative swings of the voltage would cancel each other out. In order to measure the intensity of the signal we thus must throw half of it away! We need a door which only allows passage of the signal in one direction and this is the semiconductor diode. Even the symbol we use for the diode suggests this quality, an arrowhead pointing towards a line.

There are several types of diodes. The types most commonly used by amateur radio telescope makers are the germanium diode and the schottky diode. If we pass just the right range of current through these diodes, the voltage we measure coming out of them will be the square of the input, and thus will be proportional to the power which is fed to them from the receiver. It is the power received by the radiotelescope antenna that we want to measure and we owe to detector the ability to measure it. If this is a bit confusing, fear not. You will no doubt have time to sort it out in one of those discussions I refer to above.