|To:||Tom Rauch <firstname.lastname@example.org>, <email@example.com>|
|Subject:||Re: [TowerTalk] Coil loss|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Wed, 02 Jun 2004 13:12:55 -0700|
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.
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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