A 'balun' is used to go from 'BALanced' to 'UN-balanced' feed types. A
1/4 wave vertical over a ground plane is already un-balanced and so it
coax so no, a balun is not what you need. An rf choke on the feedline
may help to uncouple the shield from the radiation. You may have some
rf on the shield that is messing up your swr meter reading. It could
also be that when you were standing right at the feed point making your
measurement you increased the capacitance and dragged the resonant
frequency down a bit... to prove this try remeasuring at the feed point
and while measuring back away or squat down or wave your arms around and
see if the measurement changes... the fix in that case is to add more
radials, not a bad thing in any case.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Lew
> Sent: Saturday, October 16, 2004 06:50
> Cc: towertalk@contesting.com
> Subject: [TowerTalk] How to best determine the need for a balun
>
> Here's the lash-up: a mostly vertical 1/4 wave vertical up through a
fir
> tree over 27 radials that has best SWR (1.0 : 1) at 1828 when measured
> at the feed point. After running roughly 350' of RG8 equivalent to the
> feed the best SWR (again 1.0 : 1) is now at 1858 when measured at the
> operating position. There is no matching system employed.
> Does this mean:
> a) I need a 1:1 balun as the feedline is part of the radiator?
> (Doesn't make sense as the sweet spot should've gone down toward 1800
as
> the radiator is lengthened?)
> b) I've created a wiry outside dummy load? (the few tests done
> comparing a 1/4 sloper and a dipole at 70' with the 27 radial beauty
> with stations on the right coast indicate anywhere from a 6 to 18 dB
> improvement over the dipole and sloper so maybe it's just slightly
less
> of a dummy load than the other two?) I do realize that such tests are
> frivolous and mean about as much as some presidential polls given all
> the variables, but gives me a reason to bust in on scintillating
> discussions regarding bodily functions and favorite now dead
> transmitters and new antennea, etc. etc.
> c) I should just shut up and try the thing out and first time I
> can't work a particular station, blame it on the lack of a balun and
> then install one?
> Tnx for any insight from the learned or even the opinionated
among
> us.
> 73 and I remain,
> Lew W7EW/W7AT
>
> _______________________________________________
>
> 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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