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Re: [TowerTalk] 40/30 Beams - Moxon

Subject: Re: [TowerTalk] 40/30 Beams - Moxon
From: "Dan Zimmerman N3OX" <>
Date: Wed, 12 Mar 2008 20:54:32 -0400
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A 40m Moxon designed using Moxgen for 7.030MHz with 12 gauge wire
construction (like copperweld + fiberglass spreader type?) needs a
20pF capacitor in the center of the reflector for best F/B (only 13dB
at 180 degrees, but it has 22dB nulls at 110 and 250 degrees ) .  It
needs a step-down L network with a shunt capacitor of 56pF across the
feedpoint and a series inductor of 6.2uH to match to 50 ohms.

30m is such a narrow band that SWR and pattern will be flat across the
band.  The free-space gain is 6.2dBi, comparable with the 6 dBi of the
antenna on 40m.  3dB forward beamwidth is 72 degrees to the 79 on 40m.
 In other words, except for the 10dB poorer  180 degree F/B, it works
much like the 40m Moxon.

At 200W power on 30m, the L-network capacitor has 800 volts across it
and the reflector capacitor, 500.

Make them both 1kV doorknobs and you'll be good to go.

Add a SPST relay to short the reflector capacitor out for 40m
operation and a DPDT relay to switch the L-network in and out, and
you're done.  Due to the low power limit on 30m and the fact that the
relays are subjected to modest voltage and current even at legal limit
power on 40m, I would think you could just use regular old 20A open
frame relays... but, of course, you do have to worry a little bit
about RX dropout with the relays, so standard antenna-switching-relay
considerations would apply.

I won't be building one of these soon so I can't verify the real world
performance but the 20/17 one really works.

If someone would like to buy me an 80 foot tower on a nearby plot of
land I will consider building and testing one of these ;-)


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