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[RFI] CATVI Problem

To: <rfi@contesting.com>
Subject: [RFI] CATVI Problem
From: martyt@pobox.com (Marty Tippin) (Marty Tippin)
Date: Tue, 07 Jul 1998 15:23:08 GMT
(Tried to send this yesterday but I think it bounced... Sorry if it's
a dupe...)

How do you know when a TVI problem is due to direct pickup at the
television or a leak in the CATV system?

I recently moved to a new neighborhood and have started getting
complaints from the neighbors about CATVI problems. 

My own house is clean - I use a regular TV antenna and mini-dish
satellite system, located less than 15 ft away from the apex of my
G5RV (about 40 ft up), and have no problems with TVI, even at 500
watts.  I use a low-pass filter and the station is well grounded.  So
I'm pretty sure the problem isn't due to harmonics from my

One neighbor has interference on only a single cable-connected TV (out
of 5 in the house), located in an upstairs bedroom about 30 feet from
the end of the G5RV - the end of the antenna points pretty much at
that bedroom.  The interference is when I transmit on 20m, at high or
low power.  Haven't investigated other bands yet to see how widespread
the problem is.  

I tried a high-pass filter to no avail.  Disconnected the
cable-company-supplied preamp, no change.  Haven't tried a common-mode
choke yet (the only toroid I have is a T200-2 iron powder core).  Is
it likely to be a problem of common mode currents?

Another TV in the next bedroom doesn't have any problems, but the TV
with the problem still shows interference when we move it to the other
bedroom.  So I'm thinking the TV is being overloaded by my
transmitter.  How would I know for sure if that was the problem?  At a
distance of at least 25 ft from the end of the G5RV, is it really
possible for 500 watts to cause overload??  All the TVs are newer
model Samsungs, made in 1997; the one with the problem is a 21" (or
so) and the others are a big screen and a 13".

What else might I try to narrow it down between overload and my signal
getting into the CATV system?  FWIW - the cable system is buried in
the neighborhood, with junction boxes for every few houses - the box
in my yard was not locked shut, so I took the cover off and noticed a
4-way splitter fed by hardline, with only only coax run coming off of
it (to my house, I presume) - the other 3 connectors were not
terminated or anything.  I held my 2m HT next to the box and got a
terriffic buzz at 145.25 MHz - only raised the meter to about S4 on
the HT.  The buzz dropped considerably when I put the cover back on.  

I haven't investigated the other neighbor's problems yet, but he said
it seems to affect all TVs.  He is also using cable.  And he related
that the neighbor on the other side of him has had problems but they
haven't said anything to me personally about it.  The fact that
multiple neighbors are having interference on CATV-connected TVs leads
me to believe the problem is that my signal is getting into the cable
system, but I don' t know for sure.

I have sent e-mail to the Missouri section manager hoping he can refer
me to some TVI experts in Kansas City but haven't heard back yet.  I
do have the ARRL's RFI book and have tried to follow all the
troubleshooting steps in it, to no avail.  The neighbors haven't
gotten upset (yet) so I'm hoping to resolve the problems before things
get ugly.  

What should be my next step??  I plan to call Industrial
Communications Engineers today to order a high-pass filter,
common-mode choke and AC line filter to add to my trouble-shooting
arsenal.  Any suggestions would be appreciated!

-Marty KI0LO

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