Posted by
K9MA on
Jan 07, 2019; 3:24am
URL: http://elecraft.85.s1.nabble.com/KAT-500-and-open-wire-tp7647776p7647809.html
On 1/6/2019 20:12, Jim Brown wrote:
> We add one or more common mode chokes to minimize the effect if the
> common mode circuit, which is radiation and reception, and to do that,
> a very good choke must be AT THE FEEDPOINT. I don't know of a way to
> EFFECTIVELY choke a feedline that is not matched to the antenna. Yes,
> you can put something there, but if it did anything useful it would
> fry (unless yo only ran QRP to the antena), and a choke that wouldn't
> fry with TX power won't do anything useful!
I have to respectfully disagree a little here. A choke specifically
designed for the antenna impedance at the feed point could effectively
eliminate the common mode current. The problem is, we often use balanced
line with antennas which have a large range of impedances over frequency
range at which they are used. (Think G5RV.) It may be impossible to
design a single choke which can work over the whole range. However,
feeding balanced line with a balanced tuner can work pretty well, even
though the feedline may radiate a bit due to common mode current. I've
only attempted this with link-coupled tuners. I'm not sure it works as
well with the double T-network tuners with a choke at the input, like
the MFJ-976, but it may.
For years, I've used a warped, lop-sided, short, 80 meter "dipole" fed
with window line and the balanced tuner. I never bothered to try to
figure out the impedance at either the feedpoint or the tuner, but the
tuner matches it on several bands. I don't use it much on 80, though, as
it's so close to the roof of my house that the RF gets into everything.
I'm sure the feedline radiates some, but that's not where the RFI is
coming from.
73,
Scott K9MA
--
Scott K9MA
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