Date sent: Wed, 07 May 2003 08:22:43 -0400
To: les@highnoonfilm.com, topband@contesting.com
From: ABowenN4OO <abowen@nettally.com>
Subject: Topband: 160 Meters Mobile Antenna
>
> The ARRL Compendium 7 has a very detailed article on how to make a 160M
> base loading coil for mobile operation.
>
> The very old ARRL Antenna Anthology also has an article by Barry Boothe,
> W9UCW (The Minooka Special), on loading coils for 160, at least one of
> which could be used for a mobile antenna. This was a repeat of an article
> that appeared earlier in QST.
>
> Be aware that the coils shown in the W9UCW article are quite lossy. A local
> friend built one some years ago and used it very successfully at 100 W,
> while parked near salt water. When he increased power from 100 W to 400 W
> however, he started the PVC coil form on fire. He told a very funny story
> about all the attention he got. He showed us the remains of a burned coil
> form at a subsequent hamfest.
>
> INK N4OO
>
PVC and PBT and other such plumbing pipe plastics are very
lossy for RF. Best to stick with some of the better plastics such as
Lexan (polycarbonate), Plexiglass, etc. or even Teflon stock if
you can afford it. Since air is generally the best dielectric of all,
it is probably best to stay with coils that self-support and are not
wound around any 'coil former' cylinder You could possibly
fashion an "X"-shaped coil former out of two deeply-notched,
interlocking plates of teflon or polycarbonate with small outer
notches to hold the coil turns every 90 degrees- but self-support
and air dielectric is definitely best.
One good design might use a length of solid fiberglass rod
between two aluminum antenna mast sections for insulation
plus great mechanical strength. You mount your coil spanning
the rod. An outer cylindrical cover made from polycarbonate
sheet and film would be good to keep rain off the coil, since
rain likely would severely detune the antenna.
73, David K3KY
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