Build a Phasing Harness For Stacking Antennas

Build a Phasing Harness For Stacking Antennas

Build a Phasing Harness For Stacking Antennas

Recently at work, I had the opportunity (or need) to make a phasing harness for a two bay antenna. But I had a problem. I found on the web a few years back how to do this for my particular antenna situation, but now the web page was gone! So I had to figure it out on my own. After many hours of looking through my (very poor) notes, I figured it out.

What I had was a pair of circular polarized antennas that were to be set up as a two bay antenna system. Each antenna had an impedance of 100 ohms. Here is what I came up with, and it seems to work.

In a transmission line, such as coax, the impedance of the load repeats itself every half wavelength. Since each antenna is tuned to 100 ohms at resonance, all I have to do is cut two lengths of coax exactly to a multiple of a half wavelength, and connect them to a Tee connector. What this does is takes the two 100 ohm impedances of each antenna and put them in parallel with each other. The end result is a 50 ohm feed point, which allows me to connect my 50 ohm coax for proper match.

However there is one problem. When coax is manufactured, there is a 10% tolerance in the velocity factor of the coax. So as far as I am concerned, just taking the published velocity factor of the coax could be problematic. So I needed a way to measure or tune the individual lengths of coax to some multiple of a half wavelength.

By way of background, here is a diagram that is similar to the antenna system I was setting up. The two pieces of coax I had to precisely cut are labeled “Phasing Harness“:

Build a Phasing Harness For Stacking Antennas

Build a Phasing Harness For Stacking Antennas

So what I had on hand was Belden 8237 RG-8-U Type coax. This has a velocity factor of 0.66 and a characteristic impedance of 52 ohms. So based on these numbers, and the spacing between the two antenna bays, I chose to use a length of coax that is 7 half wavelengths long. Actually this is way too long for my needs, but that’s ok.