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Re: [TowerTalk] 80/40 coax trap dipole design

To: towertalk <>
Subject: Re: [TowerTalk] 80/40 coax trap dipole design
From: Dan Zimmerman N3OX <>
Date: Wed, 15 Dec 2010 15:10:32 -0500
List-post: <">>
On Tue, Dec 14, 2010 at 9:16 PM, WW3S <> wrote:

> Anyone have a good design for one of these? I got the 40m length ok, on a
> fence about the traps...some designs say to make them resonant, W8JI
> recommends again if they aren't resonant on 40m, what freq do
> you
> make them resonant for? And does that affect the length of the 80m leg?

I don't know how to decide on the resonant frequency (besides doing a
systematic modeling study) of the traps to absolutely optimize it, but I
anything in the vicinity of a few hundred kHz below the band is OK.

The thing that effects the resonant length of the 80m legs most with a
traditional trap design is the
L/C ratio of the trap.  Let's say you have two traps, both resonant around
6.5MHz, but one is 3uH and 200pF and the other is 10uH and 60pF.

If you work out the parallel combinations at 3.5MHz, the 3uH || 200pF trap
is about +j50 ohms, something like a 2.3uH inductor (not the full 3uH
because it's shunted by the capacitance).  The 10uH || 60pF trap is +j170
ohms, more like a 7.7uH inductor.  Both of them are high impedance on 40m
and it doesn't much matter what the resistance or phase angle is, so both
traps will make 40m work right.  But the higher the inductance and the lower
the capacitance for a given resonant frequency, the more loading you get.

So you can pick, depending on whether or not you're trying to achieve
shortening (and can live with the bandwidth reduction inherent in that).


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