On 11/19/2012 9:33 AM, Martin wrote:
Should i install another common mode choke at the feedpoint?
Absolutely! And bond the shield to ground on both sides of the choke.
I totally disagree with that advice.
1.) If the feedline is bonded to the same ground on both sides of the choke,
any choke would do no good at all. It would be shorted.
2.) If a proper Beverage feed transformer is used, the feedline already has
no common mode connection. He could ground it or let it float, and common
mode coupling would be nearly zero with a proper beverage feed transformer
(isolated primary and secondary).
3.) He said the feeder is buried. If the feeder is buried for any distance,
or even laid on earth for any distance, the feeder already has a good CM
attenuator.
The only thing necessary in almost any installation is an isolation
transformer at the antenna feedpoint. There could be an exception with a
feeder routed or suspended in the air, or grouped with things carrying
significant common mode, out to the antenna area.
At my QTH, the only isolation is at the antenna feed transformer. There are
modest impedance CM ferrite sleeves on phono and BNC connected cables to
help mitigate less-than-zero connector ground resistances inside the shack,
but nothing at all outside.
Even when the feeder elevates to cross roads near power lines, sometimes a
span of 20-30 feet and 20 feet high, I've never found a reason to install a
CM choke or sleeves. I doubt there are many cases when needed outside the
ground panel.
73 Tom
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Topband reflector - topband@contesting.com
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