I have built full sized 40m and shortened 80m driven elements by insulating
them from an aluminum plate with no known issues however if this is a
concern you can use other materials. McMaster-Carr should have any material
that your heart desires to make an insulated driven element. I also insulate
the parasitic elements on all of my yagis. The DX Engineering BEB-1 polymer
element bracket may also work for this antenna.
Perhaps you should model this antenna so that you have a better
understanding of how it works. Like many Yagis these days it probably has a
50 ohm impedance and requires no match. A choke has nothing to do with SWR.
John KK9A
To: TowerTalk <TowerTalk@contesting.com>
Subject: [TowerTalk] Building a W6NL Moxon 40 Meter Beam.
From: "L L bahr " <pulsarxp@embarqmail.com>
Date: Sun, 3 Aug 2014 16:06:43 -0400 (EDT)
As mentioned on another post and subject, I am going to build a 40 meter
W6NL
Moxon Beam. The center of the Director is split and is not at ground
potential.
I see a lot of builders have used a two foot long aluminum channel to mount
the
Director and Reflector to the boom. They use a plastic or fiberglass sleeve
to
isolate the director from the grounded Boom. I can see this would make for a
very strong support.
1. All this to me means, 2 foot of the radiating part of the Director is
close
coupled to ground. I'd think this is Bad!! Seems to me the use of a Delrin
spacer rod or fiberglass sleeve with just the use of U-bolts without the 2
foot
of aluminum channel would dramatically reduce the Director coupling area to
the
grounded aluminum channel.
2. Also, the feed point on all the versions I have looked at couple an RF
choke right to the center point area of the Director. When I built a couple
plain Yagi antennas, I used a Gamma Match or T match thus tapping the
Director
element at a 50 ohm point rather then at say at a 15 ohm point or so right
at
the center of the director. Why is this not done on the W6NL building
examples? Is it because they think and hope the RF balun choke is going to
keep the mismatch SWR in check? Can anyone explain this to me? I'm not
sure,
but I suspect the impedance right at the center of the Director is not 50
ohms
or even close to being 50 ohms.
Anyway, I thought I would throw these two concerns out for you experts to
comment upon. Am I thinking clearly and correctly or am I wrong somewhere?
Lee, w0vt
Houston
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