[TowerTalk] To Shunt feed or Not to Shunt feed??
jim at audiosystemsgroup.com
Mon Jul 7 08:11:43 EDT 2008
On Mon, 7 Jul 2008 02:40:06 -0500, AD5VJ Bob wrote:
>I am searching the index for the reflector as well as the internet on
'shunt feeding' for my new 30' tower which is soon to have a
>10' mast on top of it.
>For 40 thru 160 meters
>Would I be wasting allot of time and effort if I started this (not so)
little project without anything to measure impedance or an
>antenna analyzer to measure tuning? Also where does one get the
capacitor necessary for the shunt feed?
Break down and buy the ON4UN book and the ARRL Antenna Book. Then study
them. This is not new stuff, and you can learn it if you work at it.
For capacitors -- it's easy to roll your own using transmitting coax
(RG8, RG11). The ARRL Handbook and Antenna Book list the capacitance per
foot of standard cables. Ballpark is 20-30 pF/ft, depending on type.
These will handle enough voltage for a low impedance feedpoint.
You can use NEC to get a handle on this. A version that nandles simple
antennas comes with the ARRL Antenna Book, you can buy a version that is
more powerful from W7EL for not a lot of money.
You're going to want an antenna analyzer when you're working on this. If
you haven't done so already, find and join a local ham club that includes
some serious contesters and/or DXers. There's a good chance that these
guys will have an analyzer you can borrow, AND they're likely to be
willing to help you do the work! I found that support from a club in
Chicago, and again when I moved to CA two years ago.
It's worth the time and cost to work with an analzyer -- you'll learn
from it, because you can see on the analyzer what the textbooks are
You'll need a LOT of top-loading to make that tower work on 160, but it
can be done with an umbrella top-hat. You'll also need radials to make it
Jim Brown K9YC
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