>>>We quickly discovered that the installing contractor ignored the specs and
>>>used cheap unbranded RG-11 with what looked to be around 75-80% shield
>> Hmm. that's up to 450MHz, where the holes in the shield start to be
>> significant size (compared to HF) and, is it possible that some of the
>> problem might also be crummy connectors and crummy installation practice?
>Not at all. Besides the range above 200 mHz wasnt even in use yet and
>most of the computer activity was terminal to CPU over modems in the
>5-50 mHz range.
Helluva story, Carl! (It must really focus the mind to know that, one
way or another, you were going to leave that ship by catapult. )
But Jim made two separate points there: the % shield; and also the
possibility of crummy installation practices. I don't believe a "not at
all" reply can cover them both.
A 75-80% shield rating is only the visual % coverage. Its RF shielding
effectiveness varies with frequency. The lower in frequency you go, the
smaller the holes in a 75-80% shield become in terms of wavelength, and
the *more* effective the RF containment becomes. So leakage through the
shield at lower frequencies becomes *less* likely to account for RFI
running riot all over the ship.
But it seems *very* likely that the bozos who used poor-quality cable
had also left behind several PL259s with the shields twisted right off.
Every one of those shielding gaps would transform the "shield" into a
fully functioning antenna with RF currents running all along the
It seems quite likely that most of the solution was your all-round
On a more general point, I don't think we can ever dump lab tests and
practical performance into two separate compartments. Whenever we see a
difference, we've GOT to get our heads around some bigger picture, and
figure out the reasons why.
Finally, some practical shielding results from the different perspective
of European UHF TV:
Check out the whole www.wrightsaerials.tv site, and don't miss the
73 from Ian GM3SEK
TowerTalk mailing list