Paul-- I think it can be done a LOT easier than relays -
I saw a suggestion for a trap - that can definitely be done - look at
OPTIBEAM 30-40 M dipole -
I have a M squared 40M1L linear loaded 40 M dipole - I played with nothing but
a parelled 30 M dipole hanging under the 40M1L and connected right at the
balun . It appeared it would work - but I was also trying to get a 12 meter
resonance at the same time . That 40M1L linear loading is really screwy
it has a resonance very close to 12 meters and "sucked" the resonance of any
paralleled 12 meter dipole so it was virtually impossible to make oine I
trusted
for a $350 crane run . (BTW I emailed with K6MYC a couple or three times about
this and he confirmed he had seen that sort of thing ._ I was up against
a scheduled crane run and just dropped the whole idea - the 40M1L has a
"stinger"
at the linear loading that gives a 17 meter resonance and that works great.
Your concern about the voltages across a trap is well founded - if you read
carefully , most of the commercial 30/40 or triband dipoles will NOT handle
1500 watts . M2 has a 30-40 meter dipole they are advertising - might be
worth looking at how they do that . It appears to use the coil approach
that they offer as a non linear loaded 40 M1 dipole. I would have to look
back at my notes or do a quick 30/40 meter paralleled dipole EZNEC and see
how that works . I have to run now but - if I had it to do over again.
I would have bought the OPTIBEAM 30/40 m dipole or make a copy .
I have an old KLM 40 M linear loaded 3 40 m dipole that git hit by lightning
and blew up the center insulator - I have a project to see if that could be
modified to make a 30/40 m dipole by basically just playing between the
linear loading and tip lengths and a capacitor across the linear loading
but seems like that has not made it to top of project piles.
Mi dos Centavos - as many say down here
73 Hank K7HP
Message: 4
Date: Thu, 17 Mar 2016 11:29:47 -0400
From: N1BUG <paul@n1bug.com>
To: towertalk reflector <towertalk@contesting.com>
Subject: [TowerTalk] 40-30m dipole design
Message-ID: <56EACD6B.4010603@n1bug.com>
Content-Type: text/plain; charset=utf-8; format=flowed
While trying to figure out how to move up a notch from my inverted V
antennas on 40 and 30 meters I came up with a crazy idea. I was
looking at building a rotatable dipole for 40 meters, either full
size or possibly shortened. A 70% of full size linear loaded 40
meter dipole is nearly a full size 30 meter dipole. Various articles
claim only about 1 dB reduction in "gain" from a full size dipole on
40. Could relays be used at the dipole feed point to short out the
linear loading, making a two band antenna? It seems good in theory
but if it were that easy, wouldn't someone have done it already? I
have no idea whether RF voltage across the relay contacts when
operating on 40 meters would be high enough to make this
impractical. Any comments, wisdom, or wild guesses?
Any alternative thoughts on building a 40 and 30 meter rotatable dipole?
73,
Paul N1BUG
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