ve2pid wrote:
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?

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... ?

Thanks de Pierre VE2PID

Hi Pier
There can be several reasons for a dummy load radiating a signal but
common mode current is not likely to be one of them. In a good dummy
load the load resistor is inside a shielded container. If the container
is RF tight the RF current is on the inside of the box and not on the
outside, just like a coax transmission line all the signal of interest
is on the inside surface of the shield and the surface of the center
conductor. Skin effect works on shield boxes too. There should be little
to no RF current flowing on the outside of container or the coax shield.
One place the radiation could come from is from the coax itself. The
shielding on typical coax is 95% coverage. This is a source of leakage.
Connectors are a problem too. Pl259's are not a very RF tight connector.
Type N's are better but there are some that are even better. Hard line
is 100% shielded it would not be likely to radiate but it can be spoiled
by sloppy connector installation. The most likely reason for any
radiation is poor shielding of the dummy load. Typical ham grade dummy
loads are not very well shielded. Often when using a dummy load and one
finds that a signal is being radiated it easy to blame the dummy load
when in fact it's the transmitter. Transmitters are not often all that
RF tight. The only way to tell is to probe around with a field strength
meter and find were the signal is coming from. This rambles all over the
place, hope it makes some sense to you.

73

John Passanau W3JXP