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Re: [TowerTalk] Good vacation antenna

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Good vacation antenna
From: David Gilbert <ab7echo@gmail.com>
Date: Thu, 3 Jul 2025 19:34:53 -0700
List-post: <mailto:towertalk@contesting.com>

Use the "Currents" data from EZNEC to plot the current profile of a base loaded vertical and compare that to the "Currents" profile of a non-loaded vertical of the same length.  Calculate the loss in the base loaded coil and subtract that from the drive level for the example without the base loaded coil.  If you can't understand the effect of the difference between the two current profiles I don't know what to say.

Or you can take the easy route and just don't believe me.  I really don't care.

Dave   AB7E


On 7/3/2025 6:27 PM, john@kk9a.com wrote:
You're not alone Wes.  I also do not understand how having an inductor at
the tuner for an electrically short antenna has a different effect from
putting the inductor at the antenna base.

John KK9A


Wes n7ws wrote:

I'll confess some difficulty (probably my failing) in understanding your
point(s).  To try and simplify the situation to something my pea brain may
understand, let's use a toroid at either location.  Now tell me what the
difference is.

Wes  N7WS



On 7/3/2025 3:39 PM, David Gilbert wrote:

Because whatever current is left after the loss in the tuner still makes
it to
the antenna but doesn't have a major portion of it compressed inside the
coil
where it matches and also suffers loss but doesn't radiate.  Radiated RF
is a
function of BOTH current and length.  If it was otherwise we'd all be
using
parallel tuned circuits with large coils on 40m instead of aluminum tubes
and
wires.

Dave   AB7E





On 7/3/2025 2:11 PM, Wes Stewart via TowerTalk wrote:
    So you move the loading coil from the base of the antenna to inside
the
tuner in the radio (with some coax in between).  How does that affect the
radiation efficiency/?
On Thursday, July 3, 2025 at 01:57:33 PM MST, Jim Brown
<jim at audiosystemsgroup.com> wrote:
https://www.dxengineering.com/parts/cha-ss25
That looks like a great solution, Dave! I strongly suggest against a
loading coil for 40M, using a tuner built into the radio instead, with
or without some top loading. Loss in short lengths of decent-size coax
on 40M is pretty low, even with significant mismatch. An excellent study
in QEX about ten years ago showed that inductive loading at the base
greatly reduces the radiation efficiency, because it's at the current
maxima! The study included the construction of multiple configurations
of loading, and very rigorous measurements.

73, Jim K9YC

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