[Top] [All Lists]

Re: [TowerTalk] Coil loss

To: "Michael Tope" <>,"Tom Rauch" <>, <>
Subject: Re: [TowerTalk] Coil loss
From: Jim Lux <>
Date: Wed, 02 Jun 2004 15:06:20 -0700
List-post: <>
At 01:28 PM 6/2/2004 -0700, Michael Tope wrote:

If the portion of the coil inside the tube isn't used, couldn't you just
add extra finger stock contacts at regular intervals along the inside
length of the tube to short out the unused turns?

Mike, W4EF...........................

> > 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) >

You could... but I'll bet that it's a non issue (as in, there's other losses or problems that will get you first...)

I suppose I could take my antenna and hook it up to a suitably beefy PA and do a (potentially destructive) test.

I think Tom's advice to just hook up a SWR meter and turn down the power if it looks like it's shorting might be the optimum strategy. The amp I'm thinking about in this application is the SGC-500, which is moderately kill proof, so if it arcs, raising the VSWR, the amp will shut down, and if the antenna melts, it's an excuse to buy a new antenna without agonizing about replacing a perfectly good one.


However, the theoretical discussion and measurements are interesting. Tom's done a fine job addressing some of those oh-so-hard-to-quantify effects in a mobile installation.


See: for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>