Posted by
Charlie T, K3ICH on
Jan 07, 2019; 4:17am
URL: http://elecraft.85.s1.nabble.com/KAT-500-and-open-wire-tp7647776p7647810.html
I should probably know this, but what is the old dual air coil Heathkit B-1
balun, a current or voltage type?
If I remember correctly, they are 4:1 ratio.
Would this be a better choice for open-wire feed to a dipole intended for
use on several bands?
73, Charlie k3ICH
-----Original Message-----
From:
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[hidden email]> On
Behalf Of K9MA
Sent: Sunday, January 06, 2019 10:24 PM
To:
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Subject: Re: [Elecraft] KAT 500 and open wire
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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