Saturday, October 1, 2011

20 Meter Vertical Moxon

20 Meter Vertical Moxon

The beam is supported by two cross arms mounted to a push up GRP mast and the pictures below show the mounting arrangements. To eliminate as much metal as possible from within the beam I used a form of GRP plate called TUFNOL to fabricate the mountings, with hydraulic pipe clamps used for the tube clamps. These are also plastic only leaving two bolts and a plate that are metallic.

At the end of the cross arms I bolted in eye bolts so that the wire element can be quickly clipped into place. The wire is supported at each corner by a dog bone insulator with a cable tie either side of it to hold the insulator in place while the antenna is being erected. The dog bones are attached to a short length of Kevlar line with a snap link at the far end.

The push up mast I use was supplied without any provision for guying so again I made some guying rings out of TUFNOL. These are used with Kevlar guys and one up the whole thing is very stable and has stood up to some high winds with no problem.

This is the antenna erected and you can clearly see the upper cross arm. The lower one isn't as visible but it's the same idea. This arrangement works much better than I imagined - it keeps the wire in tension in all dimensions and can be put up by one person easily. It also packs up into a convenient size as the cross arms are each 3 sections of 1 metre long.

The sections for the cross arms came from a company called spiderbeam and I can recommend their stuff (no association).

The antenna worked exceptionally well adjacent to the ocean and as a result we managed over 4500 QSO's in 6 days with Japan and Asia contacts coming very easily with our 100W.

Building the Moxon is fairly easy and lends itself to imagination. No two Moxon's are identical save the dimensions of the elements and spacings. If you view the various antennas in the What's New section on the homepage, you'll see what we mean. You can also view a variety of different hubs and designs here.

Since the Moxon is a 50 ohm antenna, you may or may not want to use a 1:1 balun. It's your choice. If you are planning on running any power, you might want to install a balun at the feedpoint. If you're not running any power, then feed the antenna directly from the coax (see Balun Notes below).

Once you've decided on what type of wire to use, and have used the Moxon Software to calculate the dimensions according to your band, you're ready to start constructing your Moxon. So, let's build one. Shown below are the dimensions for the 20 meter band.:

Here we've started the moxgen software, used 14.175 as center frequency, selected our #12AWG wire and hit calculate. The results are shown in inches, but you could have selected other units. You can even generate an eznec or nec antenna model (shown elsewhere on this website). So, looking at the diagram, we can see that the overall antenna will be 303.47 inches, or 25 feet long and just over 9 feet wide.

Sample Construction materials:
1 1/4 Inch conduit (for pole brackets)
U bolts/pipe hangers
4" x 1/4 plate (Mast bracket)
1/4 or 5/16 bolt
1/4 or 5/16 nylock nuts
16 gauge tinned wire
16' Crappie Pole spreaders

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