Re: [TowerTalk] Coil loss

[Jim Lux <jimlux@earthlink.net>]

At 01:28 PM 6/2/2004 -0700, Michael Tope wrote:
> Jim/Tom,
>
> 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.

Jim,W6RMK

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.


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