In a message dated 97-06-10 18:09:34 EDT, trbarber@mt.arias.net (Tommy
Barber) writes:
> the Cushcraft A3 tribander. Is the coaxial coil choke balun described in
> the owner/assembly manual adequate or should I purchase a separate choke
> balun? If your suggestion is to purchase a balun, please state why I
should.
>
A primary purpose of a balun is to keep RF off of the outside of the
coax shield. In other words, it is supposed to keep the feedline 'cold' so
that it doesn't radiate. Feedline radiation can be a major contributor to RFI
problems.
A coaxial coil choke is one attempt at accomplishing this. There are two
main problems with it. First, it is imprecise in regard to frequency. Current
thinking advises that the coil should be wound on an appropriate coil form
(like a round oatmeal carton or something that will work size-wise). Even at
that, I don't think the frequency response is anywhere close to a good
current-type or stacked ferrite bead balun.
The second problem is that the coax termination of the shield and center
conductor is split and attached directly to the driven element studs. It is
damned difficult to weatherproof this type of feed system. It's almost
impossible to keep water from wicking down the coax. Of course this degrades
the performance over time.
The obvious fix to both problems would be to use a nice wx-proof
current-type balun. While I haven't done this myself, I can't think of any
reason why this wouldn't work.
OTOH, I'm a big fan of 'doing what the factory says'. But in this case,
I disagree with the factory.
73, Steve K7LXC
TOWER TECH -- professional tower supplies for amateurs
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|