6 meter Moxon

I got the bug to try some 6 meter VHF operation both for CW, my usual mode, and to get back into Wsjt-X digital modes.  To go any distance, wire dipoles are pretty marginal for any mode in current solar conditions.  There are still some good but brief openings that the big guns find to make decent contacts. They all have gain antennas, generally multi-element yagis, some stacked 3 high.  I knew I needed some kind of beam to stack the odds in my favor and have some VHF fun.

I’d been contemplating what to do with an old 2-element tribander in pretty rough shape (see December 2013 Ice Storm at KB1ZMX video) so I decided that cutting it down to a 6M Moxon was something I could get done while summer conditions last.   It actually worked out pretty well as I was able to use the undamaged center sections of 1-inch tubing for both elements.

I started with Moxgen, an excellent but simple to use antenna design software.  I set the center frequency to be 50090, the nominal calling frequency for CW 6 meters. I then exported the Moxgen data into EZNEC 5.0 Plus to see how it modeled.  SWR in free space was less than 1.5:1 for the 50000 to 51000 segment of the band, leaving lots of room for all the weak signal calling frequencies including the digital modes. Gain will probably be 5 to 6 dB with good F/B.

I substituted a PVC boom for the old pipe boom and used PVC elbows with 6-inch PVC stubs for the hose clamps to grab onto.  I then used common aluminum construction flashing cut into strips inside and outside of the elbow bends to make electrical continuity.   The element ends were notched to get a tight fit on the PVC tubing pieces.  I also taped the tails on the hose clamps down to the tubing to avoid snags from wire antennas, having learned the hard way that Murphy’s law always applies when a wire antenna falls across a beam antenna.

Installed SWR came in at 1.7 for 50276 and 1.6 for 50090, a touch more than what I saw when testing with the tower down and the beam aimed up.   The entire 50000-51000 band segment swept at less than 2:1 so I’m happy with outcome.  The lowest SWR of 1.5 was close to 50000. I read somewhere that it’s a good idea to set the center frequency higher than you really want because the finished beam will be resonant somewhat lower. That seems to be what happened here so when I build a 17M moxon soon that’s what I will do.

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