[Elecraft] KAT 500 and open wire

[K9MA]

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

k9ma at sdellington.us