At 01:27 PM 1/4/99 +0100, Peter Wardenier wrote:
>We're considering using the "Spitfire modification" for our
>35m vertical during the CQWW160 contest
>(see http://www.yccc.org/Features/Spitfire/spitfire.htm).
>Does anyone have some practical experience with this type of
>antenna on 160m?
>What dimensions of vertical and parasitic elements do you use?
>
>I'm a bit worried about the drop in radiation resistance caused by
>the addition of a reflector/director (ELNEC), as our ground resistance
>is pretty 'bad' (not *all* of Holland is below sea level, you know...)
>
Peter and others,
I am the originator of the Spitfire antenna and co-author, with K1VR, of
the presentation which was given last year at Dayton and now appears on the
Web. K1VR has one running on 160, but we are in the process of modifying
it now to fit in 2 more elements. This is the only one I am aware of at
the moment on 160, although a number of others indicated they had plans to
construct versions for 160 and other bands. For those who are not familiar
with the antenna, it is a parasitic array of somewhat unusual geometry
which was designed to be a simple upgrade to an existing quarter-wave
vertical. It fits in the same space as the original vertical plus its
radials and has four-quadrant coverage like a 4-square with a little less
gain.
According to the computer models, the radiation resistance drops only
slightly with the addition of the parasitic elements. My model shows a
feedpoint impedance of 26.6+j35.9 if using series feed at the base of a
quarter wave driven element radiator. For most situations, shunt feed is
more practical. K1VR's array uses shunt feed of his tower and we had no
problems obtaining a match this way although shunt feed doesn't change the
sensitivity to resistive ground losses.
To those interested in constructing a Spitfire, I first recommend that you
have some experience and technical knowledge in the area of antennas since
this is a somewhat unconventional design that requires some care in
construction and tuning to obtain optimal results. If you are an EZNEC
user and would like my modeling file, send e-mail directly to me and I will
e-mail you the EZNEC file. The "current" model of the Spitfire uses a
somewhat different geometry than that originally presented at Dayton (and
on the Web) to improve low-angle performance. I recommend you do your own
modeling first if you intend to construct an array which deviates from this
"baseline" model because performance is sensitive to geometry changes.
73, John W1FV
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|