There are many areas or pieces of equipment where this interference can enter your system. Let me mention a few that I have heard of: (I am sure that there are many more.) Bad, loose, corroded or improper size cable connectors; improperly grounded cable systems; poorly-shielded (cheap) patch cords between cable boxes, VCRs, TVs and the like; poor 300- 75 Ohm cable transformers; any twin lead wire; distribution amplifiers (either in your house or in the cable company distribution system); splitters; A/B Switches; video games; TVs; FM broadcast radios. In fact, anything that is connected to your cable antenna system, cable ground, or A/C source to any equipment that is connected to any part of your cable system, or any combination of them all can be the culprit!! Well, that kind of narrows it down! Are we having fun yet?
But do not despair yet. With the right amount of perseverance and a little luck, you might be able to solve the problem. First of all, let's focus on the simple and most obvious solutions. Sometimes, we as amateurs, as soon as we think of RFI, we think of either low pass filters, high pass filters, baluns, SWR, proper grounds or the like, when in fact the #1 influencing factor is our fundamental frequency radiating out of our best radiator, the antenna! Let's use the least amount of power necessary to accomplish the communication. Try to avoid putting our antennas or transmission lines right next to cable installations.
Maybe you will be lucky and find a leaky connection with your H/T as described in Part 2! If this is the fact, then eliminate that and you are all set! If not, we have a little detective work to do using the process of elimination.
In an ideal typical cable installation, the cable will be brought in from the pole, parallel to the ground with the shortest run possible, connected to a lightning arrestor which is connected to a good low resistance earth ground. This ground should be exclusively for the cable system, although more often than not, the earth ground is shared with the telephone system. This in itself is not really a problem, as long as the cable ground is connected directly to the ground source and not some kind of terminal block in the telephone system. The cable then should continue downward to ground level and run under the house to the appropriate rooms. All runs of coax up the walls and in the attic or ceilings should be avoided if at all possible. In many locations this is not possible. The people in apartments with multiple units and distribution systems are really at a disadvantage because of the amount of cable run all over the place. All of this excess cable can act as a long wire antenna system for both differential and commonmode interference.
(Continued in Part 5)