The question is...to use galvanised steel or PVC?
I'm favouring steel because when earthed, I believe it will give extra
noise screening for the RX feeders but are there any down sides such as
"oven effects" in hot summer sun or "diode effects" between lids and
sections, or any other problems?
Once shielding (just like when common mode suppression) goes beyond a
certain point as a ratio to levels inside the cable, any additional
shielding is useless. The sole exception is lightning or another sudden
severe unusual excitation.
I looked at all this with common cable types I planned on using (dual shield
with braid and foil). Unless fields exciting the cable were beyond the
levels caused by my own transmitters into a dipole above and parallel the
cable, any additional shielding was useless.
The test for common mode is more involved than just open circuiting the
cable or terminating it, but with normal "non-exceptional" cables nearly all
CM issues would come from coupling in at a gap in the system, either at
plastic boxes that are absent large backplanes between shields, or poor
shield connections at a connector or connector.
PVC is a bit cheaper but wont offer the extra screening.
Does anyone have any relative experience please?
Metal conduit, with proper grounding, will improve lightning survivability a
great deal if you are blowing shields out in storms somewhere out in the
middle of the runs. Of course there are other ways to reduce shield damage,
but a metallic pipe forms an effective bypass for high CM currents that
might melt the cable shield. It can or will improve shielding from the
outside along the run, but I can't imagine a case where that is an issue
unless your cable runs past a high power transmitter antenna or your receive
antenna levels are near thermal noise floor limits in feedline conductors.
It does not mitigate CM noise or end-damage, however. The same or more
current and voltage will exist at the ends.
Unless running past in-band very high level noise sources, or past something
causing lightning to enter the shield, I think is a waste of worry, time,
and money.
Tom
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Topband Reflector
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