Topband: 160 Meters Mobile Antenna

[David Sinclair]

From:           	Bill Turner <w7ti at ispwest.com>
To:             	topband at contesting.com
Subject:        	Re: Topband: 160 Meters Mobile Antenna
Date sent:      	Wed, 07 May 2003 20:06:06 -0700
Send reply to:  	w7ti at ispwest.com
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> On Wed, 07 May 2003 20:00:52 -0400, you wrote:
> 
> >PVC and PBT and other such plumbing pipe plastics are very
> >lossy for RF. 
> 
> The traditional test for lossy plastic is to put a section of about
> six inches in the microwave for 20 seconds or so and see if it gets
> hot under full power.  To protect the magnetron, also put a small
> amount of water in there to absorb the microwaves if the plastic
> actually is loss free.
> 
> -- 
> Bill, W7TI
>
Hi, Bill-
Seems I wrote a hidden, implied absolute: "PVC and PBT...(always)...
lossy for RF..."  Of course, I should have used the more politically
correct verbage:    "... often are..."   or   "...can be..."      :o)

There's likely a lot of variability in PVC loss depending on
manufacturer. Anyway, there are a lot of diverse applications
where we see these forms used at times. I personally avoid
PVC for antenna work, with some few exceptions- for example,
K6STI and W6KUT used PVC tubing for corner insulators on the
STI loop (receive only). So did I. Further, there are at least two
different basic formulations- PVC and the higher temperature-
rated CPVC. I have read a number of humorous accounts of
melting and burning PVC tubing in ham antennas.

Someone pointed out to me off-reflector that he has an antenna
with a top loading (?) coil using a PVC former that has held up OK
at 1KW. Or perhaps it acts like a trap/choke for 80 and a center
inductor on 160, inverted L style? I am rusty and will have to
go back and remind myself of how the "Minooka Special"
design works. His is apparently a Barry Boothe,  W9UCW creation,
and works just fine. Lessee- maybe more voltage across the coil,
higher up, but probably *way* less current than one mounted
lower down in a vertical element- right? All I could respond was
to point out that there are plenty of lossy inductor designs that
nonetheless work just fine for their application, but that I would
not personally use PVC formers in most cases. As I remember,
most of the current gets pinched off rapidly above even a center
loading coil- much moreso than in a full length quarter wave
without 'lumped constants' (inductive loading), right?

Yes, the microwave test ought to be a fairly good go/no-go,
and adding the water is a real good idea!  :o)   Personally, I
would rather spend that time making a stiff, self-supporting
coil or else use one of those "X" formers out of poly-
carbonate sheets with the edges notched. I suppose you
can get by with what is essentially a giant Hustler resonator
by using a PVC former, teflon-insulated wire, and a heat
shrink tubing outer wrap to keep rain from detuning the coil
so much. You'd probably get away with it if you were real
careful where, electrically, you place it in the antenna wire.
It would have to be more lossy than better-designed coils,
but maybe for most hams the criterion would be simply
whether it holds up at their power level or not. That may 
be fine for all but the most dedicated topbanders, who are
trying to cumulatively wring out every last fractional dB on
the theory that it actually does add up to a useful 
difference- they may be right.  :o)   73, David K3KY