All You Want To Know About CATV Interference  By:  Chuck Manetta  N4YJC

Interference To CATV Channels From Amateur VHF Operation
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Part #1: Overview And Channels Effected

The problem exists because the cable-television spectrum covers a 
wide frequency range.  It starts down in the low end of the VHF 
range and continues on through the UHF range.  This spectrum, as 
you know, also includes a number of amateur bands!  In theory, 
the cable system is supposed to be a "closed" system.  By 
"closed", I mean entirely enclosed in shielded cable.  No cable 
leaks out, and no amateur radio leaks in!  But in reality we find 
that this is not true.  There are many areas of improper 
shielding, loose connectors, substandard coax and even the 
possibilities of RFI or EMI (Electro-Magnetic-Interference) 
radiating through the cabinets of your TVs or being introduced 
into your TVs by the cable coax shielding itself!

As amateurs, before we start any investigation into the causes of 
a case of cable TVI, we should first make sure that our shack is 
clean.  Make sure your equipment is properly grounded.  Make sure 
that your TNC is not over-deviating your radio.  Check that your 
transmission lines are in proper condition. If necessary, apply 
the needed cures to your own cable-connected television system. 
This demonstrates that the cures used do work and that they cause 
no harm.

The television channel is 6 Mhz wide!  Any signal transmitted in 
this 6 Mhz spectrum will cause noticeable interference to a 
television signal! Even a signal that is 40 dB weaker than the 
television signal will result in perceptible interference.

There are a number of cable channels that we as amateurs can have 
problems with.  These cable channels roughly, if not exactly, 
correspond with these Amateur Bands:

Cable  2  [ 2]   (55.55 Mhz)    =     6 Meters 
Cable 17  [ D]   (139.25 Mhz)   =     2 Meters 
Cable 18  [ E]   (145.25 Mhz)   =     2 Meters 
Cable 23  [ J]   (217.25 Mhz)   =  1.25 Meters 
Cable 24  [ K]   (223.25 Mhz)   =  1.25 Meters 
Cable 64  [UU]   (421.25 Mhz)   =    70 Centimeters 
Cable 65  [VV]   (427.25 Mhz)   =    70 Centimeters 
Cable 66  [WW]   (433.25 Mhz)   =    70 Centimeters 
Cable 67  [XX]   (439.25 Mhz)   =    70 Centimeters 
Cable 68  [YY]   (445.25 Mhz)   =    70 Centimeters 
Cable 69  [ZZ]   (451.25 Mhz)   =    70 Centimeters

Be advised that there are no stipulated rules as to what channel 
number designator has to be on what frequency.  (This is supposed 
to be a closed system!)  These channels and frequencies may vary 
from one cable area to the next, although most cable systems use 
one of three "standard" systems. (The nice thing about standards 
is that there are so many to pick from!)

This interference can go both ways!  In the majority of the cases 
reported, the interference was caused to the cable by the 
amateur.  But there have been instances where there have been 
1large enough leaks from the cable system to interfere with the 
amateur!  From what I have seen, these instances have been 
created by either an improperly terminated cable ends 
(unconnected!), or by customer-installed "illegal" hookups. 
Rarely, the problem can be caused by a bad cable distribution 
amplifier or shield break somewhere on the pole cable.

As an example, I was receiving a S9 carrier on 145.250 on my 
packet radio.  My radio was an Icom 28H and I had an 11 element 
vertical beam at 60 feet.  I reported this to my cable company.  
A few days later they sent out their Engineer and DF Vehicle.  I 
brought him up in my shack and showed him the interference.  We 
used my beam antenna to determine that the interference was 
coming from a beam heading of 315 degrees (NW). We went to our 
vehicles and found on that heading, the offending equipment 500 
yards away!  It was an improperly terminated connector inside 
someone's house!  An illegal hook up!  So you can imagine what a 
bad distribution amplifier outside your window will do!  Most of 
these leaks can be easily located with your HT or a hand held 
scanner. These are a few frequencies to listen for:

Cable Channel 17 (D) --- Audio at 143.75 Mhz. 
Cable Channel 18 (E) --- Video at 145.25 Mhz. 
Cable Channel 18 (E) --- Audio at 149.75 Mhz. 
Cable Channel 23 (J) --- Audio at 221.75 Mhz. 
Cable Channel 23 (J) --- Video at 223.25 Mhz.