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Re: [TowerTalk] Coil loss

To: Tom Rauch <>, <>
Subject: Re: [TowerTalk] Coil loss
From: Jim Lux <>
Date: Wed, 02 Jun 2004 13:12:55 -0700
List-post: <>
At 02:17 PM 6/2/2004 -0400, Tom Rauch wrote:
>          It turns out that the shorter an antenna is
relative to a
> wavelength is, the higher the Q of the resonant antenna
system (antenna,
> transmission lines, coils, inductors and tuner)  must
become to effectively
> radiate RF energy. And in achieving that higher Q using
coils and/or
> capacitors there is a good chance of increased losses due
to the higher AC
> currents and voltages involved.

The loss isn't that bad. See:

Ground loss dominates the system.

The real issue is all those extra turns hanging dead when
you go up in frequency. The same thing happens in antenna
tuners in roller inductors. You can get a series-resonance
in the coil and then voltages (and circulating currents) are
incredible. The antenna can also become very narrow banded
at the point were loss is highest. It really is a complex
interaction where the unused area of inductor behaves like a
pair of back-to-back L networks with extremely high values
of L and very low capacitance.

When an inductor is self-resonant or near self-resonance it
does not have anywhere near even current throughout. That's
what happens when all that extra coil isn't used. The same
thing happens in tank circuits and RF plate chokes.

73 Tom

You're talking about the circuit formed by the unused turns (inside the lower tubing) and the C to the surrounding tube? I would think that the C is going to be pretty big (the tube is close to the coil). Say, if you've got 30cm inside the pipe, with the coil being 5cm in diameter and the tube being 6cm, I get 92 pF. Assuming 100 turns for the coil inside the pipe (about 8 TPI), I get 77 uH using Wheeler's formula. These could be off by quite a bit because there is an interaction between the coil and the tubing wall. But anyway, the resonance would be around 1.9 MHz, which is close enough to be of concern (that is, if the L or C were off by a factor of 4, you'd be resonant at 3.8)

And, you're assuming the magnetic field from the part of the coil sticking up above the tube (with the antenna current flowing in it) couples into the part below. There's probably also some coupling to the threaded rod that actuates things (both Capacitive, and the magnetic field)

I think on the higher bands, though, where more of the coil is inside the tube, the problem is less, since the L and C would both be bigger, reducing the resonant frequency. The coupling from the active part of the coil would also be smaller.

Hmm.. maybe a reason to use a iron tube (plated with copper)?


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