by Dave Clemmons, K1VUT
Television interference is quite possibly the secret dread of most amateur radio operators. There are some good sources of information available to help with this problem, and several of them are:
RFI Pamphlet - available from the ARRL Technical Department Secretary for a SASE (225 Main St., Newington, CT 06111).
"RFI: How to Find It and Fix it" by the ARRL. Check the QST advertisements for this and other publications by the League.
For those of you with Internet access, there is a user group dedicated to TVI/RFI. To subscribe, send a single line message (just the word SUBSCRIBE) to email@example.com
Rather than make this a technically oriented article, Id like to share a few experiences Ive had, with the expectation that they are not uncommon, and that they might therefore be helpful in pointing others in the direction of solving similar problems.
My earliest experience with TVI was caused by a groundloop effect. I had no problem with TVI, but I decided one day to ground my xcvr, outboard vfo, and amplifier together and run the main ground wire off one of them. (Prior to that, I had simply grounded the xcvr, and let the coax braid handle the rest.)
The result of my efforts was instant TVI! Boy, was I ever puzzled.
Although the solution was obvious (replace the new ground system with the old one),it wasnt until I had talked to a few people that I found out what happened. A groundloop can be created when the length of the grounding wires between all the radios happens to be resonant on some particular frequency. Probably I had created a loop which was resonant somewhere around 56 Mhz, where the VHF TV frequencies start.
A couple other times I had TVI complaints from a neighbor whose house is only 80 or 90 feet from my antennas. I would normally hope to find the cure for any TVI for which I am responsible, but even more so in this case because these people are such good neighbors. I tried changing low pass filters on my rig, as well as adding a high pass filter on their TV. I checked and cleaned the connection on the ground rod and at the radio. Finally I started visually checking my PL-259 coax connectors. Several that looked suspicious were replaced and eventually the problem was solved when I accidentally replaced the culprit.
I suspect that poor solder connections on PL-259s are a common cause of TVI.The number of these connectors in use at my station is somewhere on the order of 60, so I have learned over the years to try to pay attention to proper soldering techniques. One thing which makes it easier is to use silver-teflon connectors instead of the cheaper nickel ones. It is all too easy to try to save a few dimes on connectors after spending some hundreds of dollars on the rig or antennas, but the reward for such frugality may be many frustrating hours spent chasing down a case of TVI. You also need to use the correct size soldering iron. Many of my earlier connectors were installed with a small pencil type soldering iron. Unfortunately, it would not quite get hot enough to solder the coax braid to the connector. I solved this problem when I finally decided to spend the extra money and buy a bigger soldering gun.Now I have no problems with the soldering of the braid.
Recently, I have had problems with TVI on my own TV, but evidently not on my neighbors. Furthermore, when I replaced my TV with my daughters, using the same TV stand, there was also no problem. (Both TVs are 8-10 years old.)
At this time I have been unable to cure the problem completely, but I have made significant progress by using a high pass filter on the TV, yet another low pass filter on my radio, and ferrite beads on the coax line right just before the antenna switch in my shack. My feeling is that another string of ferrite beads outside by the lightning disconnect box may completely solve the problem.
My best experience with high and low pass filters has been with a company called ICE (Industrial Communication Engineers). They have an outstanding reputation for well engineered products, and will take the time on the telephone to make sure you are buying what you need. For example, do you know that your TVI may be caused by the house wiring or TV AC line acting as an antenna? You can check this out by disconnecting the antenna and seeing if you still have TVI. Although the picture will be very weak, it will be obvious if the TVI is still present. A different type of filter is required if your problem is related to ac lines. (I did have an initial experience of having horrible reception with the ICE high pass filter, but it turned out that the 75 to 300 ohm adapter I used - bought in the local Radio Shack and manufactured in BY land - was labeled backwards!) Besides high and low pass filters, ICE also manufactures band-pass filters, an ideal addition to Field Day stations. The folks at ICE can be reached at (800)423-2666.Next time I plan to elaborate on the ferrite beads I mentioned, as well as on telephone interference, stereo interference, and a humorous case of RFI lighting up standard incandescent bulbs.
Ferrite beads are another solution I've used in an attempt to cure certain types of RFI. Many commercial antennas come with a choke (or instructions on how to make your own coax coil choke). The idea behind the use of chokes is to prevent any of the radiated signal from traveling down the outside of the coax shield. If some of the signal does travel down the outside of the coax, the result may be SWR degradation (i.e., multiple nulls and maximums within the transmission band) or the coax may act as an antenna radiating harmonics which might otherwise be suppressed (i.e., if the coax has a length resonant for that harmonic). Besides using these types of chokes, ferrite beads which fit over the coax may be used for the same purpose. (Ferrite comes in many types, and the proper type will depend on the frequency of transmission.)
In attempting to eliminate TVI from 20 meter transmissions, I found some success using a combination of ICE low pass and high pass filters, and 8 ferrite beads (type 43) on the coax near my shack antenna switch. Thinking I might completely solve the problem by adding more chokes near the antenna, I climbed the tower and placed 8 more as close to the antenna feed point as I could reach (about 3 feet). Much to my surprise, the problem was severely magnified! Not knowing why, but certainly knowing what to do, I immediately removed the new set of chokes. A few months later I discovered the probable explanation while reading "Lew McCoy on Antennas" published by CQ Communications, Inc. In all likelihood, decreasing the length of the long coax run by 3 feet made the remaining length resonant on one of the TV frequencies. One of Lew's suggestions is to experiment in changing the coax length - something I plan to do soon.
Other RFI experiences include telephone reception of my SSB signal (only 100 watts output) on both our and our neighbor's telephones. Further investigation showed that his 5 watt SSB CB signal was also received on both telephones, though a good deal weaker as would be expected. There are two probable causes for such a problem, either the telephone wires running through the house are acting as an antenna, or the circuitry in the phone itself is to blame. We found that the K-COM filters advertised in QST entirely removed his signal, and greatly reduced mine. This indicates that our problem was one of the first type mentioned. About the same time, we also experienced signal pickup in our stereos. Often this is caused when the speaker leads act as an antenna, and usually it can be solved by wrapping the leads several times through a ferrite core. When we were ready to experiment with this solution, we found that there was no more stereo RFI problem. I don't know why the problem disappeared, but if "it ain't broke, don't fix it" seemed like a reasonable approach at the time.
Last, I recount an unusual experience from the late 1970's. At the time, we lived in a 90 year old Victorian house which had "knob and tube" wiring (the two sides of the 110 volt lines ran separately through the house, spaced about a foot apart). One has to crawl carefully around such wires, as it easy to cross a couple of them with your body. Even so, I managed to hang up dipoles for 20, 15, and 10 meters in the attic. One night I was operating and my eldest son (about 6 years old, and who was supposed to be asleep in bed on the second floor), came down and told me his lights kept going on and off. Kids can be innovative at bed time, so I just told him to stop imagining things and get back to bed. A few minutes later, my wife came down cellar to champion his cause. It seemed that my CW signals were causing the incandescent bulbs on the second floor to glow orange in time with the dits and dahs, even though all the light switches were off. It made sense when I realized that my antennas were parallel to and about 4 feet above the main run of attic wires. A short time later, I replaced all the attic "knob and tube" wires with romex, thus removing a perfectly good excuse for my kids to stay up late at night.
All in all, I guess the lesson to be learned is that TVI/RFI has many causes. Even if your TVI/RFI problems are eliminated, they may very well return if another antenna is added to your collection, or if a PL-259 solder joint goes bad from corrosion. My advice? Keep familiar with the published material on this subject so that you are prepared for future problems. This is a great hobby, but is much more enjoyable if we aren't aggravating our family or neighbors with unwanted signal reception in their electronic appliances.
Next time I hope to write about my favorite aspect of the hobby. (If you don't know what it is, you'll just have to remain in suspense!) After that, perhaps I will detail the 40 meter antenna I hope to have built for and tested during Field Day.
73, Dave Clemons K1VUT