[TowerTalk] Balun question

[Tom Rauch]

>    Can anyone tells me what happens? Should I be using  a 
> current-type balun
> ala' W2AU or should I wind the choke? If so, what size is 
> it?

At six meters I'd use a sleeve balun of beads (like 43 
material several inches long along the cable) and then tape 
the cable to the boom after that point.

You could simply space the coax  an inch or so above the 
boom away from the feedpoint for just slightly under 1/4 
wave and then ground the shield to the boom. Years ago I 
designed some TV antenna feed systems for single channel TV 
antennas and the spaced feedline worked very well in actual 
measurements!

Be careful about other schemes. For example a 1/4 wl sleeve 
or Bazooka is highly dependent on the ratio of coax shield 
to inner diameter of the sleeve, and there must be a good 
air gap inside in order to have significant choking 
impedance. It's certainly possible to build a good one, but 
it would be a lot of work to do it right on six meters.

>    Part deux. In my meager understanding, a balun  is a 
> transformer that
> goes from balanced-to-unbalanced (bal-un, get  it?) 
> vis-a-vis a feedpoint. It
> also functions as an RF choke to prevent RF  from spilling 
> down the outside of
> the shield. A coax wound RF choke  doesn't really 
> "transform" anything but does
> choke the stray RF trying  to come down the outside of the 
> shield. Am I close?

Yes. The important part is it have fairly high common mode 
impedance.