On 2007-09-21, Richard Clark (67.168.144.41) wrote in
message


On Fri, 21 Sep 2007 14:28:47 GMT, Navtex-Fan wrote:

I'm a SWL and am specially interested in Navtex DX-reception.My problem
is (was) the heavy QRM from about every TV-set, monitor, PC in the
neighbourhood.
...
In despair - and against all advise in antenna textbooks (ground
loops!) - I decided to make an additional earth point at the balun. ...
QRM totally gone!
Can anyone explain this?

Hi Dirk,

The coax to your antenna was suffering common mode currents. These
currents came from noise sources in your home (by coupling directly
through conduction, or over-the-air). Those currents traveled towards
the antenna along the outer surface of the coax's braided shield. When
those currents got to the antenna feed point, they reversed direction
and entered INTO the transmission line and went back to your receiver.


Thanks for the clarification Richard.
So in fact the coax shield was part of the antenna?

When you grounded the far end, those currents went to ground the easy
way instead of through your receiver's front end.

A "magnetic" BalUn is not an effective choke. Chokes are used to
decouple the transmission line from the feed point. The W2DU choke
BalUn (1:1) would also reduce noise.

Google gives many hits on this, thanks for the tip.



You were LUCKY with ground loops. When the weather changes, you may
see your luck change.


I'll keep my fingers crossed... :-)
The soil here is partly clay/ partly sand. In winter it becomes quite
moist. Would that adversely affect the situation?