[TowerTalk] Balun question shield leakage

[Ian White GM3SEK]

jeremy-ca wrote:
>>>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
>>>coverage.
>>
>> 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 
outside.

It seems quite likely that most of the solution was your all-round 
better workmanship.



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:
http://www.wrightsaerials.tv/coaxcablequalityhmdim.htm

Check out the whole www.wrightsaerials.tv site, and don't miss the 
Rogues' Gallery!


-- 

73 from Ian GM3SEK