LASER beam widths can be seen but RF beams cannot so I have to devise a way of measuring the width. It was required to measure how parallel the RF beam was to check that it indeed does not expand over distance. I set up a diode detector at 15ft and made sure that the detector had a larger capture area than the beam. Took the DC from the detector into a voltage level trip that operated an LED such that it was only on when it saw RF power. I adjusted the trip to set the LED off if we had a 1db drop in signal. By adjusting the source level on the signal generator it was possible to test the drop without moving anything hence removing other errors.

Now by using a metal adjustable measure as in the photo you can straddle the beam (and watching the LED) slowly bring the distance down until you are at the beam width. When the beam is correctly focused and parallel no matter where you go along the beam will the beam width change. Later on I intend to do tests over longer distances but obviously gets more difficult with distance but my view is if its correct over 15ft then there should be no reason why it will be the same at 150ft and so on. Its only going to start falling off in power when the beam gets as big or bigger than the capture area. G3RFL