NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/sci/electron/18537

Text File | 1992-11-08 | 3.9 KB | 81 lines

Newsgroups: sci.electronics
Path: sparky!uunet!spool.mu.edu!wupost!cs.utexas.edu!qt.cs.utexas.edu!yale.edu!jvnc.net!adx.adelphi.edu!auvax1.adelphi.edu!schmidt
From: [e-mail]
Subject: Re: RF leakage from TV cable system
Message-ID: <[e-mail]>
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Organization: Adelphi University, Garden City NY
References: <[e-mail]>
Date: Sun, 8 Nov 1992 06:47:49 GMT

In article <[e-mail]>, [e-mail] writes:
> I have some questions about RF leakage from a cable TV source:
>
> 1. Why does the cable company check for RF leakage? Is it merely to see who
> is tapping their signal and not paying for it? Is it a health hazard? Or does
> it cause interference to other TV's?

The reason they check is that the FCC REQUIRES them to, and will shut them down
if they leak too much.

The reason the FCC is concerned is that the "cable" channels above 13 are in
spectrum used for other services in "over the air" radio.  The most critical
problem is aviation frequencies.  A plane flying over a leaky cable system can
receive serious interference to its communications and navagation radios, which
are mostly in the vhf "high" band.  This has been a significant problem, and
the FAA has been very concerned, and has brought real pressure on the FCC to
clean up cable systems.  Several real leaky ones have been temporarily shut
down.
>
> 2. How do they detect leakage? I'm suprised there is enough leakage from
> a leaky system to be detectable. How much stray RF actually comes out of
> the leaks?]
>

They use a field strength meter, which is essentially a calibrated
receiver/antenna set tuneable through the frequencies in question, with a meter
to indicate the received signal strength.

Leaks come from many sources.  Shields pull out of F connector crimps, or the
connectors (particularly the "push-on" unthreaded ones) may not be properly
mated, with only the center conductor making contact.  This makes for a big
leak.  Customers sometimes connect the cable to their cable ready TV in
parallel with their outside antenna, particularly if all local channels aren't
on the cable, without the benefit of an A-B switch.  This makes for a REAL BIG
leak ;-).  Trunk cables sometimes get damaged by tree branches, animals,
automobiles hitting poles, etc, either pulling out the shield, or slitting the
cable open (remember how "radiax" leaky coax works in tunnels).  Since the
signal levels on the trunks are pretty high, a small (physical) damaged area
can leak a lot of RF.

> 3. What is the best way to reduce RF leaks in a home cable system.

Use good CAC6 cable, with good, properly crimped, screw-on F connectors, use
FCC certified A-B switches, amplifiers, etc.

It is interesting to note, that when we recently installed cable in this
building, the cable company said we had to pull the cable in the building
ourselves, but they provided the cable, and they came back and put all the F
connectors on themselves.  They are obviously concerned about leaks.

John

>
> Thanks,
> Hugh
> --
> *******************************************************************************
> ** Hugh L. Eaves                      ** Internet: [e-mail]      **
> ** Medical College of Virginia        ** Bitnet: hleaves@vcuruby             **
> ** Department of Human Genetics       ** Voice: [phone]               **
> ***************** All non-incendiary communication welcome! *******************
--
*******************************************************************************
John H. Schmidt, P.E.              |Internet: [e-mail]
Technical Director, WBAU           |Phone--Days     [phone]
Adelphi University                 |       Evenings [phone]
Garden City, New York 11530        |Fax-------------[phone]
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