[RFI] CATV Leakage? Horizonal Osc Harmonics

Michael Tope W4EF at dellroy.com
Mon May 11 08:05:35 PDT 2009

larryW8UJ wrote:

>I've been dealing for years with strong RFI on 160 and 80 that appears every 15,734 KHz - It's hard to get an exact center measurement of each harmonic as the signal peak is broad. 
>I decided to get into solving the problem, thus the post.
>I've uncoupled and unplugged (from AC) all the TVs,VCRs, DVD equipment as well as any cellfone or cordless fone chargers. I also found the RFI in the AM broadcast band around 1510 on the dial - this using a battery powered AM portable. I can't track down the RFI source with the portable to any nearby pole.
>[In the past I managed to null the RFI with a MFJ bridge circuit designed for the purpose (used a short sense antenna along with the lead from the dipole).]
>We aren't on the CATV system and have no CATV drops to the home. Since interference does abate for periods (seems to happen on the half-hour time-wise, I have speculated that there's a bad termination on a CATV port somewhere and perhaps a neighbor's set is being turned off after their viewing is completed. 
>Or, I've wondered if the CATV provider uses chargers in those DA's that one sees hanging on the poles here and there. Needless to say, so far no response from Comcast.
>Enough already - any ideas or experiences? Thanks,
>Larry Kozal W8UJ
>RFI mailing list
>RFI at contesting.com

If the comcast system is leaking, you should be able to pick-up leakage 
in the VHF band. I don't have a channel chart in front of me, but at 
least on CATV channel falls right in the middle of the 2 meter band. If 
the leakage is bad enough, you might be able to pick it up using a 
portable TV. Per FCC rules, CATV systems have to do annual CLI 
measurements (cumulative leakge index) - at least that's what the rules 
were back in 92' - 93' when I worked in that industry, so they have a 
vested interest in keeping their systems clean. I suspect that if 
Comcast doesn't respond, mention of getting the FCC involved may get 
their attention.

Most old style picture tube type NTSC televisions emit gawd aweful junk 
in the HF bands (15.7 KHz comb spacing) at their receiver input (connect 
the coax input of your TV to a spectrum analyzer or your HF rig if you 
don't have a spectrum analyzer - you will be amazed). If the CATV system 
uses a low-band return path (5-30 MHz), they sometimes have to put 
high-pass filters on the TV inputs since the cumulative impact of this 
junk from hundreds of TV's being summed together can lower the SNR of 
their return path.

73, Mike W4EF................

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