Anything which transmits radio frequency signals seems to have the potential to create television interference (TVI). Well, pretty much anything that I can think of, if it is positioned in the wrong place.
There are plenty of things to consider when looking into these cases. Why has the interference started? What has changed? If you can answer those questions you stand a chance of solving the problem.
Fundamentally of course the television has to be capable of dealing with RF signals on frequencies other than the ones it is intended to receive. The manufacturers claim that this is true. These days they are generally pretty good at this, but you cannot rule out this as the root of the problem. And then, even if the TV is OK in this respect, possibly some wideband distribution amplifier is not good a keeping out signals from outside the TV band. I would suggest, look for a wideband distribution amplifier as your first possible culprit.
So, when I got a report of TVI coming from a neighbour I immediately wondered what had changed. Oddly, nothing significant had changed on my side for a few years. True, I had got stuck when my knee failed at a time when I was in the middle of moving antennas about. Now though all those antennas are back where they had always been. Even my 28MHz vertical, which I use on 24MHz as well, is back where it had been several years ago.
If my antennas had not changed, and the neighbour said that her TV setup had not changed for a couple of years, then I was a bit stumped.
This seemed like an odd one, but in the end the same thing solved it as usually solves such things - a braid breaker.
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| Garex HPF1 HPF and braid breaker similar to the one used |
I first used a braid breaker for this purpose within weeks of first being licensed all those years ago. I think I still have that one somewhere ...
Anyway, at first it did not seem too obvious what was wrong and it was too soon to jump to conclusions.
As usual, the neighbour had come to me with certainty about the interference, but no definite dates or times. This is not her fault as it is not her duty to keep a logbook. She had contacted her TV antenna installer who thought it might be due to telephone interference. This type of interference came about when part of the "Freeview" terrestrial TV frequency allocation was given over to mobile phone usage. That sounded a bit unlikely to me.
I had in my mind an incident which happened a couple of years earlier. The same neighbour had come to me as she had knocked the preset channels off the television in her bedroom. She asked me to retune the channels. This proved to be very difficult. When I checked it the signal level at the antenna socket in the bedroom was at a very low level, to the point that tarnishing of the socket was preventing any useful signal reaching the TV. By cleaning the plugs and sockets I managed to get the thing tuned, but it was marginal. At the time I suggested to the neighbour that she call her antenna installer and get him to check out why the signal was so weak.
From this visit I had gained the impression that there were three televisions in the house (bedroom, kitchen and lounge), which implied that there would be a distribution amplifier somewhere. The antenna was probably in the attic space, a common arrangement around here. The main TV was connected to a "BT box", which is an internet connected entertainment console, plus a DVD player. There were plenty of coax and internet leads around to pick up rogue signals, though over the years I have added ferrites anywhere I could, as much to keep them from radiating noise than picking it up.
TV in this area is served by an infill repeater on the nearby headland overlooking Eyemouth. The field strength from the repeater is high, but this repeater offers very few programme options. Several villagers have erected tall masts to point at the main transmitter (Selkirk). Although we are outside the Selkirk service area they still get a reasonable signal, though when I tried this some years ago it could drop out at times. Faced with this problem, most of the other villagers have installed "Freesat" satellite TV, or have subscriptions to Sky TV. We have Freesat.
Unusually, this neighbour uses the local Freeview UHF repeater, which is within sight of her antenna. It seemed odd to me back then that some time ago she was getting such a weak signal on her TVs, and indeed, was only now getting TVI.
I sought advice from Mike, GM3PPE, who kindly offered to help. It is impossible for one amateur to use their equipment and monitor TVI at the same time. Mike also agreed to come over and do some tests. We pondered over what could be happening as we went round with various pieces of test equipment, a jar full of ferrites, and a box of braid breakers. The main TV was getting a strong signal now (something had changed but at first I did not know what). We checked things out and went up to look at the antenna. In the attic space we found a mains powered distribution amplifier with four outputs. The LED light on this was on, but beside it was another Labgear amplifier, also with four outputs, which was disconnected.
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| TV distribution preamp similar to the one involved |
Now we started the tests. I left Mike with a PMR hand portable and I took a second handie back into the shack. I then transmitted on all the frequencies I normally use, beaming in various directions and using different power levels. Mike watched the television in the lounge which had been affected by TV and reported back.
As soon as the tests started the surprises began. Having found the distribution amplifier I had guessed (correctly as it turned out) that the amp was being affected by my signals and not the television directly. I was wrong however in assuming that it was suffering from overload. I had feared that the electromagnetic field I was creating was simply overwhelming the amplifier circuit boards, as sometimes happens. Not in this case.
Mike quickly reported that I was indeed causing TVI on some bands (4m and 2m, which surprised me), but lowering the output power by 3dB stopped it entirely. Likewise, moving the beam antennas 10 degrees or so away from the TV antenna also stopped the interference. Rather than totally swamping the amp this looked like there were simply unwanted frequencies getting into it and being amplified, probably arriving down the antenna coax.
With swamping, where the signal is getting directly onto the boards, quite small signals can cause big problems by desensitising the amplifier, and there tends to be a "cliff edge" effect whereby only by reducing the signal to a very low point does the interference stop. I was still thinking about the time in the past when I found low signal levels, and I thought any desensitisation would cause problems. Here, though, the interference weakened very quickly, and seemed to be stopped entirely at about -3dB lower signal at my end. This should be fixed fairly easily, I thought.
Next stage was to try a filter in the antenna lead between the antenna and the amplifier. I had various devices in my box, including 50, 70 and 432MHz notch filers, and several general purpose braid breakers. At this stage Mike's wise approach came into play. He studied the data sheets for the various braid breakers to check which would introduce the minimum attenuation to the wanted TV signal. My approach was a bit gung-ho, so I would have just tried something. Mike rightly wanted to select the best one for the purpose. The best one of the bunch turned out to claim an insertion loss of better than 2dB "at UHF". Other details were rejection on the inner of better than 60dB at 30MHz and below, and better than 25dB at 30MHz and below on the outer. Given that 3dB seemed likely to do the trick we tried this.
The choice was made on the basis that although the HPF1 filter I brought had greater rejection of the unwanted signal, it did not quote a insertion loss, whereas the HPFS had a quoted figure for loss at TV frequencies. Based on the tests we felt that we did not need more rejection of lower frequencies. Of course, I use frequencies up to 432MHz (I doubt if 1296 is an issue) so the attenuation will be less as the frequency rises, but it seemed like a useful first step before trying the notch filters. In the end we did not need to try anything else.
I fitted the braid breaker (the HPFS, not the similar HPF1 in the photo above) and returned to the shack to do the tests again. Mike reported no TVI. Not on any band, at any power, beaming in any direction. Mike then moved on to the television on the bedroom and achieved a similar result. So what had happened to suddenly cause this TVI?
The homeowner told us that the antenna installer had installed the current distribution amplifier a couple of years ago. The old one, which I guess had failed and was the cause of the weak signals I had noted back then, was a neat Labgear. Like most Labgear amps, no doubt it was well screened. The new one produced a stronger signal for her but was wide open to interference. So why did this not prove to be a problem immediately when the amp was replaced? Well, probably because this was when I had my antennas moved while I was recovering from my knee operation. At that time they were displaced, but lately I have put them all back where they had been originally.
When the TVI was reported I could not work out why, when everything was where it always had been, there was suddenly a problem. But the new preamp had been there for a while and it could not have coped with my original layout as the old preamp could.
Incidentally, we never did see the TV antenna involved. Once we found the preamp there was no need to climb any further. We simply connected the braid breaker into the input of the amplifier. Checking the antenna would have involved a lot of attic crawling which was not needed. Clearly there was enough signal getting down the coax, wherever it was coming from.
Also, there turned out to be a fourth output from the preamp, feeding a TV in another bedroom. We just did not know what we were going to find that day. As it turned out, there was a proper preamp, and it worked quite well once it had a filter added to the input. I attached a label to the filter "Do not remove" - we can but hope.
Initially I had feared that, as in some other houses, we would find a non-amplified "aerial splitter". These "dumb" splitters introduce 3dB loss per output. Some antenna installers fit them to avoid installing mains wiring and possibly later to rule out the possibility of preamp circuit failures. It is true that the earlier preamp had failed, so perhaps the point is arguable, but for me a properly installed preamp should always produce the best results.
So that seems to be that. It is impossible to check every outcome at different power levels and beam directions but several weeks have now passed and no further complaints have been received. I have checked and the neighbour seems quite happy with things as they are now.
I could not have solved this problem without the help of GM3PPE. His advice was very valuable, and he carried out the tests at the other end. He tactfully discussed the issue with the neighbour and provided a very useful expert for her to rely on. Thanks Mike.
Now, what is next?
73 Jim
GM4FVM
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