> 2. The main reason (as Tom W8JI suggested) was the
switchbox with shields
> connections trough relays. When I connected all shields
together the local
> AM station on 227 khz, which is 200km from my QTH is
15-18db down! Now the
> switcher is modified and makes no difference if the coax
is connected
> directly to TRX or trough this switchbox.
Mac,
What you find is exactly what I find in commercial
applications and in my own stations. The shields should
NEVER be switched. Even if they are behaving OK at the
start, they eventually will be a problem. Now you can help
me correct the rumors and advice saying shield switching it
is a good idea!! I hope we can stop Elmers from repeating
that myth.
If the cable measures good for loss and impedance you can
pretty much bet that changing it won't help anything
significantly. A common mode choke can be a large
difference.
For the cable entering the house, try using a common mode
choke at either or both ends. That could be a long string of
beads or better yet a winding on a toroid. (By the way,
another myth is a cross over winding increases isolation. Do
not bother using a fancy cross-over winding. A simple single
layer is just perfectly fine and works just as well.) You
also could use an air core choke of coaxial cable, but it
would have to be perhaps 30-50 feet of coax.
I would be sure the house connections are good. Bad house
connections can make it do what you describe.
73 Tom
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