[TowerTalk] Balun question shield leakage

[jeremy-ca]

Hi Ian, nice to here from you, its been ages!

As far as the ship problem "Not at all" still holds. The network did not use 
PL259's, they are not even constant impedence. The installer used inferior 
cable but the proper F connectors for that cable and the ones we checked 
were all installed correctly.

I was actually looking foreward to the catapult; I had been thru it 4-5 
times while on active duty and loved it. While traveling around the US on 
business I also made it a point to enjoy the latest and baddest roller 
coasters.


----- Original Message ----- 
From: "Ian White GM3SEK" <gm3sek at ifwtech.co.uk>
To: <towertalk at contesting.com>
Sent: Friday, June 15, 2007 4:39 AM
Subject: Re: [TowerTalk] Balun question shield leakage


> 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
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