Topband: Noise Radiation from Long Cables

[Jim Brown]

On Tue, 29 Jan 2008 17:45:50 -0500, GEORGE WALLNER wrote:

> I rebuilt it during 
>day further up the beach about 200 feet north of the tent. 
>The coax going to the antenna now formed a straight line 
>with the power cable. Strong generator noise was now on 
>the vertical, which took a lot of filtering to get rid of. 

I suggest you study my material on common mode current and RFI. 

EARTH is NOT a sink into which noise is poured, to disappear 
forever. 

What was probably happening is that the generator was putting 
common mode current onto the power line, which radiated it as a 
long wire antenna. The coax was probably not isolated from the 
antenna at the feed point, so common mode current on the coax (the 
coax acting as a receive antenna for the noise) was coupled 
straight to the antenna. 

I suggest that your problem would have been solved by either or 
both of the following, the benefits of which will be additive:

1) A serious common mode choke on the antenna's transmission line 
at the feed point. This can be as simple as 7 turns of coax 
through five #31 Fair-Rite 2.4 inch o.d. toroids (commonly called 
FT240). The DXE chokes and 1:1 baluns are excellent, but much more 
expensive. 

2) Effective common mode filtering of the power line connected to 
the noise source. The filter should be right at the generator 
(with what my old EE prof used to call "zero length" leads), and 
the shielding enclosure ground for the filter should go straight 
to the safety ground (green wire) of the generator. You could wind 
turns through the same cores, or you could buy a commercial 
filter. 

Details for all of these filters are in my RFI tutorial.

http://audiosystemsgroup.com/RFI-Ham.pdf

And thanks for the Q during CQ160 -- we worked the first night! 

73,

Jim Brown K9YC