> Last weekend I've done a test described in ON4UN's book -
I shorted the coax center and shield at the bev. feed point
(after disconnecting it from the antenna) and found that
there are still some signals present on my receiver, even
some of them are quite strong. That happens mostly on AM
broadcast stations, but on 80/40m I can also hear a lot of
signals... Well, in most of cases they're not moving S-meter
at all, but they are there. John ON4UN says in his book that
after such a test "..your receiver should be completely
dead". So, mine is not.>>
That's probably from a bad coax connector or bad cable or
perhaps a poorly designed piece of equipment in the shack
connected to that feedline. Either open or shorted,
terminated or not terminated, there should be very little
reception with no antenna connected to the center conductor
at the far end.
Unfortunately that does not test for common mode problems.
It also does not test for feedpoint problems. It only tests
for a bad cable or bad connections in that cable.
With my 2500 foot long run of coax they are virtually dead
when open, shorted, or terminated.
> By switching beetween the coaxes I found that the strenght
of those signals does not depend on lenght of the feedline,
but on the direction where it goes.
> - each coax's shield is grounded 15-20m away from the
antenna feed point, the longest one is grounded every 40m;
the main coax (between the external switcher and shack) is
also grounded when it enters the house
> - of course the equipment in the shack is grounded with an
independent ground rod
> - what is NOT grounded is the external remote switcher. I
decided not to ground it, because it's placed very close to
my K9AY and it's ground rod
Ground rods make no difference in this type of test. With
the cable grounded only at the device in the shack signal
pick up should be almost zero. This is true even with single
shield cables.
What do the cables connect to in the shack? Are the shield
connections all good low resistance connections with short
leads? What about the remote switcher? Is it well
constructed? Is it RF bypassed on control cables?
73 Tom
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