[RFI] CATV Leakage? Horizonal Osc Harmonics

Roger (K8RI) k8ri at rogerhalstead.com
Mon May 11 10:36:47 PDT 2009

Jim Brown wrote:
> On Mon, 11 May 2009 08:05:35 -0700, Michael Tope wrote:
>> If the comcast system is leaking, you should be able to pick-up leakage 
>> in the VHF band. 
> Yes. The picture carrier of an analog TV signal on cable channel 18 is 
> 145.25 MHz.  
> There are three principal mechanisms by which the carriers Larry is 
> hearing are radiated. 
> 1) Directly by the TV set's internal wiring because it is poorly 
> shielded.
> 2) Common mode onto wiring connected to it.
> 3) Differential mode as a signal on the CATV coax.  
On that is very common is some one with cable and an outside antenna not 
wanting to bother with an A/B switch. They just wire the to cables 
together at the TV, or sometimes use a combiner. This invariably ends up 
with the TV antenna radiating  a number of signals, but not usually down 
in the low frequencies.

Bad connectors and the missing A/B switch will give a strong signal on 
145.25 for analog ch 18 as already mentioned. The missing AB switch may 
even give free channels to the neighbors, or let them watch someone's 
"home movies"<:-)) However this doesn't sound like typical cable 
problems.  It sounds more like an inverter, or battery charger.  OTOH 
RFI is very good at coming up with new and unexpected ways of keeping 
life interesting.

It's rare to hear the horizontal oscillator except on some of the older 
TV sets and then IIRC rarely go above the broadcast band and they 
usually are relatively narrow and very short range. At least I don't 
remember them being broad.


> I strongly suggest that you find one of the VHF/UHF talkies that 
> includes a receiver for the HF bands and do a bit of poking around your 
> neighborhood.  An HF transceiver in your car working as a mobile 
> receiver can also be a useful probe. 
> Differential signals leaked from CATV systems occur at bad splices. Tune 
> a 2M talkie to 145.25 and run around the neighborhood looking for that 
> Ch 18 pix carrier.  I found one of my own very old crimped cables was 
> leaky. I replaced it with a cable having a much better connector reduced 
> the leakage by 10 dB.
> 73,
> Jim K9YC  
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