70MHz Moxom loop

Les Moxom G6XN certainly hit on a good idea thats been carried on, and has spawned a Moxon experimenters website. There are two parts to a Moxom loop, the Driven element, and the reflector, both have their tails tucked in towards the other, forming a loop (but isolated from each other.

the basic moxon loop

Fig One: The basic moxon loop

You can see from the Fig One to the right the Director with feedpoint in the middle, and the reflector at the bottom, as you would expect the Reflector is larger than the Driven element, if you where not expecting this, you need to learn more about directional beams *No Criticism

This gives the basic idea of how it works its a two element beam, adding Directors to this is not an issue, there is a nice application that will generate accurate Moxon loops for any frequency which will also output any designs to NEC or Eznec (I have the latter),

The Moxom loop we’re looking at here has been generated in Moxom Rectangle Generator (download here)

So the next thing to do was save the model that had been input for 70.200MHz and export it as an .Ez file (Eznec) this was then launched in Eznec and checked for optimisation, no errors in the model occurred. Figure two shows you the reflector to the rear and the currents in purple, and the Driven element to the front again with the currents in purple. Then in Fig Three you have an Azimuth plot which shows your gain listed as 25.56 dBi.

70mhz Moxom Viewed in Eznec

Fig two: 70mhz Moxom Viewed in Eznec

70MHz Moxom Plot

70MHz Moxom Plot

Construction: For making the holders I suggest getting some really cheap fishing poles, the center clamps / holders can be purchased or made, but what I suggest is using thinner (but not the thinnest) segments of the rods, cut a V shape in the thinner pieces and then PTFE tape the whole end, then once you have done this for all your element spacers, again use PTFE tape to close the other end of the rods too. Then using yacht varnish cover the whole rod, and ptfe and leave to dry out. (tip also PTFE any joins between sections that telescope) this will weatherproof them a lot better than most ways.

Then you are ready to assemble the antenna, there are various guides on howto do this, I use nylon guy ropes to fill gaps 1,4 and 3,6 on the Fig two diagram, and let the holders take the strain as they are designed to.

With this antenna there is no way your going to get away with a horizontal matching stub, so this is one place I’d suggest using a air spaced current balun. Wind this with a quarter wave (1/4λ) of 75Ω Coax.

Hope you enjoyed this article and if you choose to build one please feel free to let me know how yours works.

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