>It is a little difficult to measure common mode ingress when
>an antenna is connected. You could inject current by placing
>a toroid over the shield with a few turns of wire connected
>to a signal generator or antenna analyzer and see how strong
>that signal is. There is no reference point, however, unless
>you compare it to other systems.
>
If you have long runs of coax cable hooked to a monitor you can
actually visualize the common mode ingress watching the hums bars roll
across the screen. The 60 hz bar intensity may be directly related to
the amount of common mode ingress. One easy cure is by using Triax
coaxial cable or with the construction of a "Faraday Shield" on the
outside of the coax jacket with braid, foil, or even metal conduit. Only
one end of the coax outside Faraday Sheild is connected to earth and the
other end floats while the internal coax is connected to the load in the
conventional manner. This works in the mitigation of common mode ingress.
Belden makes a RG-59/U Triax (p/n 1856a) and also a RG-11/U Triax (p/n
7803A) that is flooded for direct burial. Both are much higer in cost
than the conventional CATV RG-6 feed of choice for Beverages. But if you
want to kill that last vestige of unwanted cable signal ingress, Triax
cable with the independant outside shield earthed at only one end, would
be IMHO a good place to start.
Herb Schoenbohm. KV4FZ
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