Rob,
I've been a broadcast engineer for 40 years, and of course radials are
mandatory with a vertical radiator, but one of the main reasons broadcast
facilities do not need CM chokes at the antenna feedpoints is: they are tuned
to one frequency only so the match can be optimized to the point where there is
little to no CM current. Some transmitter manufacturers like Nautel specify a
CM choke at the transmitter itself, but this is largely to provide an
inductance to delay a potential lightning surge on the feedline itself until
the stout grounding system found at commercial transmission facilities drains
most of the charge.
Amateurs are frequency agile and a perfect match is seldom found on a single
antenna on all frequencies used. In this situation a CM choke forces radiation
by the antenna instead of the feedline and the ham will see the true antenna
system match with the feedline out of the equation. Many hams think their
antenna is a good match over a wide range of frequencies when the is feedline
radiating until they install a good CM choke at the feedpoint...then reality
can rear it's ugly head.
Cheers!
Howie / WA4PSC
Not if you use enough radials. I don't think I've ever seen ferrite
cores out on heliax at AM broadcast towers. If you use enough
radials, the return current will be so divided down that the current
on the outside of the feedline (burying it helps but isn't necessary)
will be negligible. If you have to use ferrite cores to keep RF off
your feedline exterior, you don't have enough radials down.
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