As N8AU pointed out, you wouldn't need the coil on the Butternut if you were
only using it for the higher bands. But, it needs it on the lower bands
(80m in particular) where the feedpoint of a short vertical is too low to
connect directly to 50 ohm coax.
With a poor radial system, the short vertical may match up ok (doesn't mean
its working well though!) But, with an efficient radial system the
feedpoint of a 1/8-wave vertical will be around 12 ohms. The HF6V and HF2V
will exhibit that on 80 meters, which is why they use the coil as a hairpin
match.
If you are building 1/4-wave verticals, you shouldn't need the coil. Even
with a very efficient radial system, the feedpoint impedance will be about
36 ohms. But, unless you're doing 120 1/2-wave radials it should turn out
higher than that. It should match up to your 50 ohm line without the need
for an additional matching network.
For tall verticals where lightening may be a concern, a coil for DC
grounding of the antenna is a good idea. But, if you put a coil similar in
size to the HF6V's hairpin matching coil on the base of a full-size 40m
vertical it wouldn't match anymore. You'll want significantly more
inductance for it to bleed off static charge whilst having no effect on the
feedpoint impedance.
Take a look at Array Solution's page and look for their "static bleed
chokes". They are about 6" long (well, 6" of wire turned on an 8" form),
1.5" diameter, using #16 or so AWG wire. They are easy enough to build
yourself, but I use theirs on my 80m verticals.
You can also use a coaxial stub. It should be an electrical 1/4-wave long,
with one end accross the feedpoint (center to the radiator, shield to the
ground). On the other end the shield and center conductor are soldered
together. I guess this would get tricky with multi-band systems, but I used
these on the verticals in my old 40m 4 square at the recommendation of
Comtek after some lightening problems. Never had a problem after that.
73,
Tom, NI1N
----- Original Message -----
From: "Lee Buller" <k0wa@swbell.net>
To: "TowerTalk Reflector" <towertalk@contesting.com>
Sent: Tuesday, December 13, 2005 4:02 PM
Subject: [TowerTalk] Verticle Antenna Question
>
> Liberal Arts Major type of question.....
>
> On some verticle antennas (i.e. Butternut) there is a
> coil between the feed point and ground. The coil is
> about 1 1/2" in diameter and about 10 to 12 turns. I
> think it is used for a DC ground return for
> lightening. Then again....I may be wrong.
>
> If I was to fabricate my own 1/4 wave verticle...is
> such a thing needed for safety since my verticle I am
> planning does not have an actual ground-ground...but a
> ground plane instead. I would say the the antenna is
> going to be a 1/4 wave ground plane.
>
> Lee - K0WA
>
>
> In our day and age it seems that Common Sense is in short supply. If you
> don't have any Common Sense - get some and use it. If you can't find any
> common sense, ask for help from somebody who has some common sense.
> _______________________________________________
>
> See: http://www.mscomputer.com 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.
>
> _______________________________________________
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>
>
_______________________________________________
See: http://www.mscomputer.com 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.
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