[TowerTalk] BALUN revisited again

[Marsh Stewart]

Patrick,

You might start with a 1:1 "balun" which is a common mode choke. There is no
impedance transformation. The purpose of the common mode choke is to prevent
the outside of the coax shield from becoming part of the antenna. In other
words, to "decouple" the transmission line from the antenna.

Here's where is gets a a little complicated, there is no single common mode
choke I know of that will be optimum for 40M through 6M. For suggestions on
ferrite cores and number of turns of RG58 or RG8X, take a look at page 36 of
Jim K9YC's excellent paper on this subject http://k9yc.com/RFI-Ham.pdf

I hope this helps.

73,
Marsh, KA5M


-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
Patrick Greenlee
Sent: Thursday, April 07, 2016 10:07 AM
To: towertalk at contesting.com
Subject: [TowerTalk] BALUN revisited again


What I don't know about baluns would fill volumes.  I'm trying to help a
fellow ham, even less knowledgeable than I (if you can believe that
possible) but are running into a lot of confusion.

Here is the application of interest:

My friend is building a portable one man operable sectional mast (guyed when
he uses enough sections to make it required) with an adjustable length
horizontal dipole on top.  Recall in the olden days how automotive antennas
were telescopic metal whips that could be manually lengthened and shortened?
Well he has a pair of these ON STEROIDS.  They collapse short enough to make
a 1/2 wave dipole for 6m and extend long enough to be a 1/2 wave dipole on
40 m.  They are robust stainless steel (yeah, I know not the best conductor)
but they will survive his use and the weather.

I told him he needed a balun to interface the unbalanced coax feedline to
the balanced antenna. ...and then massive confusion ensued...  he was sold
something I have determined is not going to work.  I think the term balun is
a description of what a device does but not how it does it.  
Anyway he will be running low power, i.e. no more that 100 watts and
typically QRP.  I'm taking him with me to a ham fest departing for it before
sun rise Saturday (day after tomorrow)

If someone could fill me in regarding what he needs maybe we can buy one
then.

I have a UNIDILA balun that is coax in and flying leads out but his dipole
arrangement has an SO-239 so he wants to connect the balun to the antenna
via a double male UHF (double ended PL-259, I forget the
number)  We can adapt as required, even flying leads to UHF connector.

Please tell me what the best couple choices are for this application and
I'll try to steer him in the right direction.

Thanks for any consideration,

Patrick        NJ5G
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