"Ed G" wrote in message
. 192.196...

I think I misunderstood by what you meant by INPUT. The choke goes
between the antenna and the tuner. Generally at the antenna feed
point.

Tam/WB2TT


Tam,

Wouldln't that would be a very bad idea, at least in our particular
case? As I pointed out in the start of this thread, we are feeding the
antenna with about 14' of 600 ohm ladder line from the output of the SGC
antenna coupler. Isn't a choke balan used to prevent the flow of current
in the outer braid of a coax feedline? We certainly wouldn't want to be
choking any RF coming out of the SGC coupler going to the antenna!

Ed

The choke suppresses the common mode signal. There will still be current
flowing on the shield which will have the same magnitude as the current
flowing in the center conductor. You are not throwing away any energy by
doing this. If you put the choke at the antenna feed point, the transmission
line will not radiate and act as part of the antenna. The choke as you
described it could also be called a 1:1 current balun. You might want to
take a look at the ARRL Handbook.

As for the 600 Ohm line, it is not obvious that 14 feet of 600 Ohm line with
an SWR of 120:1 is better than 14 feet of RG8 with an SWR of 10:1. (I came
up with an impedance at 3800 of 5 Ohms, assuming a perfect ground. If the
roof is not that good a ground, you will want to add a couple of Ohms to
that).

You guys could solve a lot of the mystery by measuring the SWR as close to
the feedpoint of the antenna as possible.

Tam/WB2TT