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November 24th 08, 04:52 AM posted to rec.radio.amateur.antenna

Owen Duffy Owen Duffy is offline

Owen Duffy is offline

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A question about dummy load

ve2pid wrote in news:66e9aa1e-ad1c-4dc8-b324-
:

Hi to all

In a document from Roy Lewallen, we read this:

When a balanced antenna is fed with coaxial cable (Fig.1),
the outside of the shield appears as an extra, separate
conductor connected to the right side of the antenna at the
feed point. (page 157. What They Do And How They Do lt)

My question is this: Can we say the same thing for a dummy load (i.e
can we replace the words 'balanced antenna' with 'dummy load'?) so the
dummy load would 'see' three conductors?

If the dummy load is effectively screened, and to the extent that it is
effectively screened, there is very little current coupled to the outside
surface of the outer conductor. That is very different to the feedline of
an antenna which usually has significant mutual coupling to the nominal
radiator.

We are trying to explain why some dummy load/Tx line systems radiate.
Perhaps, tthe current outside the coax (common mode current) exits,
while being very small, so the radiation could come from a leaking
coax... ?

The through braid loss (ie from inside of outer conductor to outside of
outer conductor) is not infinite, so there is some leakage and it depends
on cable construction and frequency. There is also leakage from the dummy
load enclosure, transmitter enclosure, and other interfaces to the
transmitter (eg power cable).

Owen

Thanks de Pierre VE2PID

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