[RFI] Cable TV Interference

james.skjervem@hqasc.army.mil james.skjervem@hqasc.army.mil
Thu, 6 Dec 2001 14:08:08 -0700


All:

Some interesting reading regarding cable TV leakage can be found on the Web
from Communications Technology magazine (a cable industry publication, from
the SCTE that Dale mentioned) in August-December 1999 issues, titled
"Training: Detecting Signal Leakage", available beginning at
http://www.cabletoday.com/ct2/archives/0899/ct0899i.htm and the July 2000
issue, titled "Signal Leakage & Harmful Interference, A Ham Radio
Perspective" available at
http://www.cabletoday.com/ct2/archives/0700/0700fe4.htm.

Jim, WA7SLD 

-----Original Message-----
From: dgsvetan@rockwellcollins.com [mailto:dgsvetan@rockwellcollins.com]
Sent: Thursday, December 06, 2001 11:50 AM
To: wb3fsr@home.com; rfi@contesting.com
Subject: Re: [RFI] Cable TV Interference



Peter,

I have fought the battle before, in the Chicago suburban area, prior to
coming out here to Iowa.

As a point of interest, I imagine that if you tune your ham FM receiver up
to 149.750, you will hear the audio portion of the channel signal.  In my
case, I found that the local cable company (to whose services I did not
subscribe) had the Home Shopping Service on that particular channel.  (I
identified the program by listening to the audio portion.)

The interfering signals were 10 over 9 when my yagi was pointed toward the
pedestal near which the obvious leak was occurring.  (CATV was all buried
in my neighborhood.)  With a groundplane, the sigs were still S-7 to S-8.
The sync pulses made it impossible to monitor repeaters at 145.23, .25, and
.27 MHz.  They sometimes caused receive problems at 145.33 MHz, which was
the local ESDA repeater freq.  In talking with other hams in the area at
that time, I found that this was not an unusual problem.

You appear to at least have gotten some response from the cable company.  I
got nowhere with calls, but I think I caught their attention when I
operated the VHF contest in January, 1998.  I ran both SSB (only 25 watts
around 144.2) and FM (50 watts in the region of 146 to 147, which is right
in the passband of that cable channel).  The contest started early on
Saturday afternoon, my time.  By late Saturday evening, the interference
was gone.  However, it re-appeared 2 days later at the same strength.  I
think they temporarily killed the channel in my area, probably due to
significant overload of the line amp from my sigs (and the probable pile of
irate phone calls from viewers).  (Remember, leaks are bi-directional.  I
suspected that the leak was at the output end of an amp near my house,
either a line booster or one feeding signal to most of my neighbors to the
east..  If correct, that means my sigs got onto the cable and wiped the
next amp down the line.)

After several weeks of interference, with the level varying in strength,
the signals finally went away for good.  I, of course, had continued
operating on 2m during the same time.  I saw a cable company truck in the
neighborhood once during that time, but he never stopped at my house, nor
did he go into any neighbor's homes.  He did, however, wander around by the
pedestals.  My guess:  the leak was probably at a connector, and varied
with temperature.   Tightening the connector was most likely the fix.  This
is common in areas with wide seasonal temperature variations.

In your case, the signals seem bad enough to interfere, but are not S-meter
crunchers.  The bottom line is that you are licensed to radiate, and they
are not.  (In my professional life back in Illinois, I provided signal
tracking and analysis to some local telco folks who had complaints from
public safety officials that the pilot tone on T-1 carrier, which happens
to be at 1.54 MHz, was interfering with police and fire dispatchers.  That
is when I discovered that the pilot tone is generated at 100 times its
actual frequency - 154.xx MHz - and divided down so as to very very stable
and accurate.  Well, in some cases, the 154 MHz fundamental got carried
along by the T-1 lines for hundreds of yards from the central office, thus
wiping out nearby radios tuned to that frequency.  Again, the Telco was not
licensed to radiate.  The telco engineers were amazed when I tuned my
mobile ham receiver to 154.xx MHz and drove around the area.  The pilot
carrier was copyable as far as 2 blocks from the central office with the
mobile whip.)

My suggestion is that you remind the cable folks of their responsibility,
although they might very well be within allowable limits.  In that case,
two thoughts come to mind:  1) Try to minimize antenna aiming in the
direction of the leak source; 2) transmit and operate normally.   If they
experience "intrusion" upon their system by your licensed transmitter, I
suspect they'll be out there to plug the holes sooner or later.

As for obtaining info on CATV specs, there is an organization called the
Society of Cable Television Engineers (SCTE), and I once had some
communication with them on a matter unrelated to ham rfi.  I believe they
are out east, and I am quite certain they have a website.  Perhaps you can
get some information from them.  Beware however:  I suspect that, like most
organizations that issue standards (including the IEEE, UL, IEC, etc.),
there is usually a charge for documents.  Typical prices range from $25 to
nearly $100.  So, it might be worth checking with them, but be prepared.

Good luck.

73, Dale
WA9ENA





"Peter D. Vouvounas" <wb3fsr@home.com>@contesting.com on 12/06/2001
08:20:21 AM

Please respond to <wb3fsr@home.com>

Sent by:  rfi-admin@contesting.com


To:   <rfi@contesting.com>
cc:

Subject:  [RFI] Cable TV Interference


Morning ALL,

Can anyone share their thoughts on how best to determine if cable TV
radiation is within spec...

I am receiving a carrier on 145.250 MHz generally around S7-S9 signal
strength = approx. 16-17 microvolts at the antenna port.  Using a VHF
collinear on my tower.

I've also been monitoring 121.2625 the cable snifer test frequency with my
spectrum analyzer connected to a VHF yagi (horizontal polarization)- Noted
peaks in at least five directions.

After a bit of prodding I received a call from the Comcast Engineering
Supervisor stating that they have passed ALL of the fly over tests in the
past with flying colors.  He went on to say that they could very well be
within legal limits for cable radiation  - due to the sensitivity of the
radio  and apparent gain of my antenna system the interference signal my be
normal.

I have two interests one is the interference into my VHF equipment and
secondly any leaks allowing my high power HF rf into the cable system
leading to neighborhood TVI issues.

The supervisor did share some others names and numbers to work with me.  I
think it's best to be up to speed before I engage them one on one.

Specifically tests & measurements that my be performed by the HAM - I have
no way of knowing how good or poor the cable companies test equipment
designed, maintained or utilized.  In other words I want to have my ducks
in
a row hihi..

Regards,

PeterV [REN] WB3FSR

p.s. I see a constant carrier at 110.630 MHz that appears to have different
peaks then the ones indicated above.  Any idea what this signal may be?
=========================================
Peter D. Vouvounas  Ofc: 732-701-1130
McLeod USA               Cell:732-674-4001
98 Meridian Dr.
Brick, NJ 08724
Mailto: wb3fsr@home.com
InterActive 2way: wb3fsr@imcingular.com
=========================================




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