40 Meter Moxon

Tom, W0IVJ, built a Moxon antenna for Field Day using fiberglass poles from SteppIR.  He designed it so it could be raised on ropes between trees.  He changed bands by lowering the antenna and hooking the desired element(s) in place and re-raising the antenna.  This allows for only having one set of elements in play at any given time thus eliminating element interaction.

He offered to loan me the antenna to try at my location.  In order to tower mount the antenna I needed to create a hub for the elements.

The antenna performs pretty much as one would expect.  It has a little over 3db of gain in comparison to a dipole located at the same height with over 20db of front to back.  The SWR as measured with an AIM4170 and correcting for feed line characteristics is 1.2 to 1 and usable across the entire band.  The antenna is at 52 feet and one would expect substantial change if it could be raised to 65 feet.

The mounting hub is made of steel with a 3/16 inch back plate with two 1/8 inch end plates bent to form a 120 degree angle.  Top, bottom and a side plate were welded in to complete the center section.  I used 1/8 inch wall by 2 3/8 inch ID tube for the element boots.  .4 inches was removed the length of each tube section and then drawn in and welded so the the tube provided a good fit for the PVC tubes.  The tubes were then cut to 3 1/2 inches long at a 6 degree angle to provide some rise.  In retrospect, I think the tubes should have been 6 inches long and cut at 8 degrees for better support.   The white fiber glass rods are to provide strain relief on the long wire lengths.  Because of the balun and coax at the center of the driven element, I  knew that would need some help and included a boot for that pole.  The one to the rear was an afterthought and had to be bolted on as shown to clear the mast.  I added overhead truss lines to try to take a bit of the strain off of the PVC extenders.  It uses a HyGain mounting block to attach to the mast.

 

BTW, the support for the rope trusses is a 1 inch pipe coupling butt welded to the top of the hub.  A three foot piece of galvanized pipe with an eye welded at the top completes the support.  This is a LOUSY method!  While it supports all of the vertical weight one could put on it, if one of the trusses were to break under load it would put very large forces on the pipe right where the pipe is weakest, the threads.  Placing the trusses on a mast attachment would be a much better idea.

The antenna has performed very well on state side contacts and been successful working into Europe.