I did some searching on the web and came across the Moxon Antenna which looked to be just the job and fairly easy to build. I didn’t fancy the standard wire antenna with the cross support poles, I wanted something a little sturdier, and found this website which describes all the details for constructing a tubular Moxon including the Moxgen software for working out the dimensions.
I inputted the information but setting the thickness was a bit tricky because I had different aluminium tube sizes throughout the rectangle (bought on the cheap from the local hardware store). Bending the tubing proved interesting but I managed to borrow a small pipe bender that proved very useful. Eventually I set it at an average of 12 mm on the Moxgen calculator thinking that if I overestimated I could always trim. (Better to have the length than it being too short).
I got to the stage that the rectangle has been built and I had used an old TV antenna as the boom and pinched the plastic feeder complete with circuit board, but when testing I think I had over complicated the build so I removed the TV feeder (the circuit was shorting) and literally just attached the end of the coax using jubilee clips to each side of the antenna. First results showed the SWR to be very high at 28.100 but an acceptable 1.3 at approx 26.500 so some trimming was required.
Having done further checks on the thickness of the tubing I soon realised I had over compensated. The tubing was a mixture of 8, 10 and 12mm with the majority being at 10mm. So I recalculated at 10mm and of course my original dimensions were slightly out, hence the high SWR. So I dismantled the rectangle and began to re-trim to the correct lengths for 10mm, lets hope that makes the difference, I’m informed that the space between the reflector and driven elements are very important so I had to make sure I got that absolutely correct;
Finally it was completed with a mish mash of bits and pieces mostly nicked from the junk box. The only expense was the tubing and two plastic garden stakes for the spacers which I got from a local hardware shop for the princely sum of £24.00.
The SWR readings were spot on at 1:2:1 from 28.050 right up to 28.450 which did nicely considering I’ll was for using it for CW
A few pictures below; two from the beginning of the rebuild when assembling the old TV boom and making the reflector and one of the finished product in the advancing evening dusk (shows how keen I am)!
was bought from Conrad Electronics for £49.00 and is a standard TV rotator, but
ideal for something light like the moxon. The only problems I faced were that
the instructions were in German (my German is not good), and so in my haste I
did not realise that the separately supplied 3 pin plug just allowed the
European 2 pin plug to just slot in to it. I thought I had to rewire and so I
cut the 2 pin plug off..........oops! A quick bit of reconnecting and
soldering fixed the problem and I soon had it all attached to my mast and
was rotating away to my hearts delight.
The reason I say 17 or 20 is because of size, whether it will fit in with the surrounds and not take over the paddock. So I'm going to have to measure up and see how things will fit!
The twenty meter version is nearly 8 metres across so it's fairly big and obviously the 17 meter is not that much smaller. Two factors will need to be ticked off, the XYL's approval and the locals in the village passing by in their cars not being able to notice without a really good look, so a bit of camouflage will be needed! To be honest I like to go for the 20 but we'll just have to wait and see.
The tricky bit is getting the separation of the director and reflector wires the right distance, but again the measurements are all in the calculation tool on the website. So it's a case of measuring up twice and working on the antenna once!
The SWR is rubbish when set at low height. you'll find as you raise above 15ft things start to change.
At first I was testing at about 10 feet and my SWR was awful but then raising I got some results, peaking at 1:2 at 17.400. I thought I'd really screwed up on something. After avidly reading up on the Moxon forum, I raised the antenna and the SWR still looked no better, At 20 feet I got 1:9 at 18.100, rising to 2:0 at 18.160. It certainly needed a trim!
While we are at it below is a picture of my Delta loop for 20m, but this will come down to be replaced by the Moxon when its completed. The 35 foot mast is a tilt over, so adding the Moxon should be relatively easy, plus at a later date when I have some 5 core cable I can add to the rotator.
Deciding to be cautious and to not have any accidents I shortened the mast by 5 foot removing the added section, although now only 30 foot it didn't seem to effect the antenna too much.
Happy with the set up I then turned to the SWR which was now reading a 1:2 at 17.185 and a 1:9 at 18.068, getting higher to 2:2 at 18.130. Taking the 17.185Mhz as a template I trimmed the director by 150 mm and the reflector by 50mm, this brought down the SWR to 1:2 at 18.100, perfect for my needs! the difference was because I hadn't added for the PVC coated wire I was using (42 stranded speaker wire) you have to be exact for the Moxon calculations.