[Not] Living with 6m/50MHz interference!

A severe case of TVI

This challenge (if that is the right word) started in the time honoured way, a knock on the door while engaged in trying to work DX one afternoon. As far as I could gather I was completely obliterating their television picture and I was stronger than Radio 3 on the FM radio. They didn't know it was me but saw my aerials (they could hardly miss them!). They lived three doors away. Oh dear!

That evening I visited their house and went into a well practiced patter to ascertain why and when I interfered, in all of these sort of cases in the past the problem has been with their equipment rather than anything to do with me but I believe that trying to explain that to anyone suffering interference is very difficult, but not impossible. It is most important to remain friendly and be seen to be as helpful as possible BEFORE any chance bad relations raises its head. I went on a tour of inspection. I was aghast! I had never seen anything quite file it before. By way of a backgrounder, Farnborough is in a weak signal UHF TV area. This means that many houses have aerials for both regions which are somehow combined. Both signals are weak and pre-amplification is really needed to remove snowy pictures. In this particular house they had a London aerial with some sort of masthead pre-amplification on one chimney and a Southern aerial with some sort of masthead pre-amplification on another chimney on the opposite comer of the house.

These two signals were combined and split between the two major living rooms of the house using yards of coax strung on the outside of the house and at least six passive splatters (these were resistive splatters that had, I suppose, at least 3dB attenuation). It was a wonder they got any picture at all! On both TVs they had video recorders, the master one being capable of NICAM. This was connected to an old stereo amplifier that had definitely seen better days. The same sort of wiring and splitting was used on the FM radio aerial as well except that old thin VHF cable was used to connect the chimney mounted dipole. They even complained that they could not listen to stereo signals in one room because of high background noise. No wonder I interfered. The more I thought about it, the more I came to the conclusion that there was no way the interference would go away with just the fitting of simple filters. I went home to think about it all.

There was little choice. On one hand I could not really ignore the problem as it was too severe and I did not want to develop a "no-go" aerial quadrant, on the other, I knew the only,real way of solving the problem was to completely strip and rebuild their aerial systems. I also suspected that the masthead amplifiers were old, possibly wide-band and certainly cross-modulating and this combined with the attenuation of signals caused by extra cabling and splitting made the system prone to interference. There was no choice other than to bite the bullet and rebuild. For several years I have successfully used Labgear masthead amplifiers which have guaranteed cross-modulation specifications and fully filtered inputs. One masthead amplifier, the CM7974, has both the above benefits plus four output ports. The use of this preamplifier with the addition of a simple high-pass filter placed between the aerial and the input of the preamplifier has sorted out EVERY case of TVI I have ever been involved with. In this instance I went back the next day and proposed the following solution for which they were to pay. It was based on the fact that not only would my proposal sort out the interference problems but it would improve the quality of the TV and FM programs as well - and it would cost the following.... This is always a good approach.

I proposed that I would put both TV aerials on one mast combined with a good quality diplexer to prevent ghosting (an improvement). This would feed the above described masthead preamplifier via the homemade high-pass filter. The outputs of the pre-amplifier would then be fed to four rooms (another extra). With regard to the FM aerial, I would place a 5-element beam on the same mast as the TV aerials and feed both living-rooms from a masthead mounted transformer splitter. It is obvious that to do this work would take at least one weekend and cost in excess of 100 pounds. Why should they pay? My argument was that it would significantly improve the quality of their reception. The question to ask was: was there a simpler way?

Was it worth the effort just to operate on 6m? To my mind the answer to my first question was no and the second an emphatic YES. They agreed for me to do the work AND agreed to pay. When I took down the old aerials (which I reused) I discovered the real cause of my problems. The London aerial had a masthead preamplifier which seemed to date from the mid 1960s and was decidedly antique. This amplifier actually had a gain of 10 dB on 50Mhz. The southern aerial had a late 1970s preamplifier that stated on the case "super wide-band Amplifier, VHF gain 18dB". I measured the gain to be 16dB at 50 MHz! As I thought, no amount of filtering before or after the preamplifiers would have sorted the problem out. When I finshed the work, which took a weekend, they were "over the moon" with the improved quality of FM radio reception and the quality of the TV pictures. The wife was pleased because the piles of cables next to the television sets had been removed. Did it solve my problem? Yes and no!

The interference to the television completely disappeared. But I was informed that them was still bad audio interference when they listened to NICAM stereo generated by their new video recorder. Inspection showed that they had this connected to a cira 1965 stereo record player which was completely unscreened and built in a hardboard case. No amount of circuit or input filtering stopped the problem. Again I really new that the only way of preventing this problem was to get them to realise that they needed to buy a more modem amplifier that used a metal case and that had properly filtered inputs. To this end, I lent them an old amplifier of mine on a temporary basis to show them this would solve the problem. As it happened it didn't. I discovered through experiment that my amplifier which was enclosed in a metal case (but again was rather old) was not RF proof. None of the input phone sockets were earthed actually at the inputs but the screens were taken from the input sockets through to the switches on the front panel. Hence the screening was introducing RF into the inside of the cabinet. As an experiment I covered all of the input sockets with aluminium foil on the outside of the case and earthed the foil. The audio interference disappeared.

They appreciated the fact that I had solved the problem and that it also demonstrated to them that a "good" amplifier cured the interference. It so happened that a couple of weeks after lending this amplifier it blew up (a fuse went) and it seemed that through a chance visit to a HI-FI shop they bought an amplifier in a sale that fitted my "specification" of a good amplifier. I must say that I did NOT and WOULD NOT recommend that they go out and buy a new amplifier. As I predicted the use of this amplifier sorted out the audio interference problems and improved the quality of their NICAM audio at the same time. I have one very happy near neighbour! I have discovered to my own satisfaction that the real enemy in most cases of TVI, assuming the transmission quality is above reproach. are old preamplifiers that cross-modulate with so much as a sniff of RF or modem built-to-a-cost wideband VHF and UHF pre-amplifiers. Fitting filters without changing the pre-amplifier (in bad cases of TVI), especially if the preamplifiers are mast mounted, is, in my opinion. a waste of time. Find out what is installed before you start work. Remember, if the preamplifier is mounted at the top of the mast placing a filter on the TV will probably not help much.

Also remember a simple high-pass filter, whether home built or commercial is unlikely to be designed to pass through the DC power needed to power the preamplifier. If you live in a strong signal area you should consider yourself very lucky. For the last few months it has been quiet in Farnborough. I have not had any knocks on the door (I'm touching wood while writing this) and 6in and 4m are quieter than I have ever known. I am looking forward to a profitable F2 season this autumn. I do ask myself whether I would go through this again? I suppose I have to say that them would be little choice if I was not to suffer no-go aerial sectors or unacceptably high noise levels in certain directions. One thing though - Iíll think twice about moving! Chris G3WOS

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