My club (BRC) is looking at getting in to some fox hunting as a way of creating some shared activities, so I’m going to need a simple, portable beam with a good front to back ratio in order to play.
I had a look around and vk3hra’s blog had a nice step-by-step so I decided to copy his. It’s based on the fairly common PCV pipe & tape measure approach. The idea with steel tape measure is that it bounces back from being folded, so it’s easy to fold the elements for transport etc. Here’s his design:
While I was standing in Bunnings looking for PCV pipe, I decided that using a wooden boom would make for simpler construction, so ended up with that, plus a box of self tappers and the cheapest 8m tape measure they had. I probably spent about an hour on it, in no particular hurry.
I started out with the hairpin match at the recommended 10cm fully expecting I’d need to adjust it, and started looking at the SWR across 144-148 MHz (the national ARDF frequency is 145.300 MHz but I want to use it for SOTA and FM repeaters as well). I must have made some building error, because to get the SWR down I ended up with the shortest possible hairpin, basically straight across. Compare the design, with my finished version:
On the upside, the SWR looked pretty reasonable by the time I had shortened it to there.
W and I tested it out in the park with the UVB5 transmitting into a dummy load, and using a scanner as a receiver. It does have a good null to the back and a clear gain improvement at the front about 30-40 degrees wide. For some reason, the null and the forward lobe did not appear to be straight down the boom. I’ll need to do a bit of testing on this away from metal fences etc as obviously the direction is going to be important.