Topband: Outdoor cable trunking - opinions pse

[Tom W8JI]

> 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