CABLE TV LEAKAGE

Following are some comments about signal leakage from cable TV systems, written by Bob Cooper 6/6/00 and reprinted here with his permission.  Copyright reserved. 


CATV systems typically utilise the 54 - some upper frequency region(330, 550, 750, 850 MHz), without breaks, to distribute TV channels. Some may also utilise the 88 - 108 MHz band for FM signals but that has been going down in popularity especially in major markets. Bandwidth is 
precious and 20 MHz in the 88 - 108 region can support three additional TV channels (part of "Z", Z+1, Z+2 and part of S1) which may be more valuable than FM radio.

Cable amplifies their hardline coax trunk and feeder lines at intervals varying from every 600 feet out to every 1,200 feet - amplifiers are placed as a function of (a) cumulative cable losses, (b) signal 
splits along the trunk or feeder after an amplifier, and "tap" or "directional coupler loading" - how many tap-off points with each contributing some loss to the line level. And of course the size of the trunk or feeder cable - 1" is great stuff (except to install!) because losses are minimal, older systems still using .412 (that's 412/inch) have greater losses - and there are other sizes in between.

FCC requirements say radiation must not exceed a few microvolts per metre at a distance of ten metres from the lines, equipment. "Cuckoo" and other pieces of test equipment modulate a carrier (such as at 108 or 133 MHz) with an (FM) warble. The carrier is carried throughout the system and the service trucks have a mobile receiver tuned to that frequency. If they hear their inside-of-cable plant "warble" they know they have a leak. The FCC has fined systems tens of thousands of dollars for not repairing leaks, the concern being potential interference to air and safety communications.

Radiation occurs because one (or more) of three conditions exist: (1) There is a break/crack in the line which can occur when a line is flexed (hair line cracks usually occur where mounting hardware exerts pressure on the aluminum hardline), (2) A connector is loose, (3) there is a resonant line section someplace.

Hairline cracks most often happen at connectors, going into or out of an amplifier, a line tap (on feeder lines), a directional coupler or splitter (feeder or trunk lines). The crack acts like a "slot antenna" and RF energy, resonant with the crack, literally leaks or radiates away from the line. 
The physical length of the crack determines the frequency(ies) that radiate just as the physical length of a dipole antenna determines its' resonant frequency. Most cracked line radiation occurs above 100 MHz because hairline cracks long enough for low-band TV channels are so big they cause other system problems and are quickly spotted (such as, moisture ingress into the line).

A loose connector typically radiates low band signals,seldom at frequencies above 200 MHz. "Loose" means the connector's outer nut or shield is not tight, you could turn it with very modest hand pressure if you touched it. Chances are 50-50 that if the outer nut is not tight, the "sleeve" portion of the connector was not properly "inserted" or shoved in at the time of installation. That can turn the sleeve into a "pickup antenna" on the inside, coupling energy back out through the loose nut which in turn acts like a short, stubby antenna radiating signal. If there are chunks of other wire such as strand 
mounting hardware at the same location, everything becomes a radiator, even the case of the amplifier. A loose connector on the input to an amplifier has only marginal signal to radiate, but on the output side, the signal levels are high enough to be very noticeable at distances of up to a mile. The same loose nut problem can occur on a passive piece of equipment "down the line" following the amplifier location, but with each passing foot of cable away from the amplifier, the signal level goes down which means the radiated energy is also lower.

A resonant line section can be very difficult to pinpoint. Two examples.  Cable TV amplifiers require power and every couple of miles of plant there is a pole mounted / ground mounted humoungous looking power supply. It takes 117/240VAC and turns it into 30, 60, or 90 VAC which is then diplexed
onto the coaxial cable to run throughout the plant to be a power source for the solid state amplifiers. The amplifiers run on DC, not AC, but sending DC down coaxial lines is a no-no. Inside of each amplifier is an AC to DC conversion device, creating typically 30 volts DC or less to run the amplifier ICs and bi-polars. Connecting these humungous power supplies to the cable plant is done through a length of hardline that feeds into a power diplexer ("Inserter" in the trade). It is not uncommon for these pieces of hardline to be self-resonant at some (low band) TV channel. So if we have a power inserter co-located with an amplifier and a power supply, that chunk of hardline if not properly grounded and bypassed becomes a radiator (antenna) for whatever frequencies are approximated by the length of the hardline between the power supply and the diplexer. Another "resonant line" problem occurs when they use in line subscriber traps to block certain channels. If the trap screws directly onto the directional coupler or tap, no problem. But if the installer sticks a length of RG6 or RG59 (shudder)
between the coupler/tap and the trap, that line length becomes resonant on some frequency (again, length equals resonance just like a dipole).  

Yes, there are other causes of radiation but mercifully I have used up my space here - and then some. Radiation is very easily traced - a handheld or smaller battery operated TV set, an FM radio with "TV audio" frequency coverage and a whip antenna (or rubber duckie) and your two feet are all that is required. Walk along in the suspected area, as close to directly below (or above if a buried plant with pedestals every so often) the lines and look for the TV-FM signals. The further from your DXing antenna it is located, the stronger it will be when you do find it and therefore the easier for you to identify. Then you can call cable TV and say, "pole number 123 at Third and Elm is radiating between 54 and 82 megahertz - please fix it, now!" Oh yes - when someone connects their TV antenna to the back of their TV set while also having cable TV connected - you have a ready made radiating system. It is difficult (but not impossible) for a consumer to do this with F-connector equipped TV sets, but the older sets with 300 ohm input require a matching transformer to connect to the terminal screws.
And given that situation, having BOTH the TV aerial (on the roof) and the cable matching transformer "in parallel" to the set back 300 ohm screws is not all that uncommon - the TV antenna reradiates the cable channels.

Regards, Bob Cooper

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