DX RX The Doctor of DX prescribes the answers to your problems, low and high, near and far. A how to of what for, by Mike(aka Billy)Graham, NE1V

Many newcomers to ham radio are at a loss as to where to start when trying to pinpoint the source of TVI and how to cure it. For that matter, so are a lot of SSO's (Sexy Senior Ops). It behooves us to tackle this thing in a calm and intelligent fashion. But where do we start.

First thing is to be friendly…..very friendly….with your neighbor. No need creating any additional animosity. While you may not, indeed, be the culprit, it is best to immediately begin checking every possibility. Be a good ham diplomat. And don't forget that the club and the ARRL have technical resources to assist you. Contact them to get the ball rolling. One piece of advice….do not become too accommodating to your neighbor…..they may make improper demands, such as ceasing all operating when they watch TV.

Start your investigation by checking to ensure that your own equipment is operating cleanly. The ARRL suggests that if you have a home TV operating without TVI while you are transmitting, this can be a powerful persuader that the source of the problem is certainly not the amateur station. The FCC sees it that way also. A high-pass filter, ac-line filter and common-mode choke should be installed on your TV sets, whether you need them or not. Check your transmitter to ensure it is operating properly and is properly grounded and terminated to the load via a commercially-available low- pass filter. Check to ensure that your linear and any tuners are tuned and operating properly, and are properly terminated, shielded and grounded. Don't overdrive your final stages for that extra watt or two. Use good tune-up and operating procedures. And recheck all grounds. A good RF ground has always been considered the first step in preventing unwanted TVI.

If the TV set in question is being interfered with on all channels, not just harmonically- related frequencies, it is a very good bet that the problem is one of "fundamental overloading", which occurs when certain TV manufacturers did not design the TV so as to reject high-level signals that are a part of its normal environment. Other possibilities are harmonics or mixing products (InterModulation Distortions, or IMDs). The problem is most likely fundamental overloading, but check all possibilities. Check to see what all is connected to the TV. Are there unterminated splitters? All splitter output ports not in use should be terminated with a 75-ohm resistor. Is the feedline or coax damaged, cracked, old, or are there signs of rust on terminals, etc.? If the TV system consists of multiple components, such as video games, VCRs, remote tuners, CATV converters, and long cable runs, start by eliminating everything but the basic TV and a short feed line to the antenna system. Reconnect accessories one at a time and recheck for interference as you proceed. The TV itself may be old, misaligned, have broken or frayed shields, etc. Don't fix it yourself…..the owner is responsible for that. Check up on the roof to ensure that external feed lines and antennas are in good condition. If the owner is using 300-ohm twinlead, the problem can sometimes be eliminated by installing 75- ohm coax and matching transformers in the system. Again, this is the owner's responsibility. Sometimes owners use TV preamplifiers; these are notoriously susceptible to strong RF fields. Remove the preamp to ascertain if the interference is eliminated or reduced.

If the installation is on a CATV system, check the condition of the cable and coax connectors, especially if they happen to be the crimp-on type. Shields must be tightly crimped….a floating shield acts as an antenna! Check for illegal hookups of more than one TV per decoder box or other "jury rigged" arrays. Do not, under any circumstances, do any mods to a CATV system yourself. That is the responsibility of the local CATV company. Do not, under any circumstances, open the back of any set or make any adjustments. Internal problems should be left to qualified service personnel. (Editor’s note: Cable channels 18 and 24 occupy the same spectrum as the 2 meter and 220 MHz ham bands. This isn’t a case of fundamental overload or intermod. The TV set is literally tuned to your signal. But since the CATV system is a “closed” system, there shouldn’t be any interference, right? Right. Actually, in most cases the “Cable system” is RF tight. It is the lack of any shielding on the TV set or the cable converter box that allows your 2m or 220 MHz signal to get into the RF path.)

Last, but not necessarily least, be sure you check with the Technical Resources Committee or the ARRL to ensure you have checked out all possibilities and have done everything you can, as the responsible amateur radio operator, to clean up the problem. Also check the ARRL handbook and QST for articles on construction of high pass, low pass and ac line filters. Let's hope that you don't have the problems described, but if you do, USE THOSE RESOURCES.