Living with 6m Interference!
Issue 30 & 31 Six News June 1991 & October 1991

Maybe I have had bad luck with regard to the number of interference problems that I have had to suffer while operating on 6M over the last few years. But I have a shrewd suspicion my situation is little different to many amateurs trying to pursue their hobby while living in heavily built-up areas.

Not only have I sometimes caused interference to neighbouring (and not so neighbouring) television receivers, so called HI-FI and modem telephones but I have suffered from S9+ l0dB wide-band hash that is semi-permanent at several points of the compass, mysterious carriers that usually reside 50.1 10 MHz, plus the usual selection of beeps and whistles that usually derive from unscreened personal computers and the like. This interference has caused me to miss much DX because of an inability to hear weak stations.

As you will read in this article, a considerable amount of time has been devoted to this endeavour over the last two years which could have been better spent but it has proved to be very worthwhile with all major problems cleared up. I have taken a very pro-active approach to each case and I would NOT recommend that other amateurs tackle equivalent problems in a similar way unless they are very confident about their negotiation and technical abilities and understand some of the possible consequences of their actions.


Sometime in the summer of 1989 I realised that the S9+ l0dB hash covering the band that peaked up from the northeast was becoming a real nuisance. Fortunately, it was the type of noise that the noise blanker on my Icom IC-751A could cope with fairly easily. The difficulty was that as soon as any strong station came up nearby on the band the noise blanker became Ineffective and weak signals were completely blotted out I've had many instances of being unable to complete a weak F2 QSO because of the hash suddenly coming up in the middle. The hash was at its worst on wet and windy days which, in my opinion, clearly pointed to be overhead power lines being the culprit. One weekend when 6m was quiet I decided to track down the cause of the interference. I usually tackle this by assembling two elements of a 4 element Jaybeam and bolting it to a short 4-foot pole sticking up through the sun roof of my car. This is connected to my portable 6m receiver.

After about half an hour of driving around I tacked down the noise to a fenced-in enclosure about two miles away from my house which contained a brick building fed by overhead power lines. This turned out to be a water pumping station owned by the local water authority. The noise seem to peak when my aerial was pointing to the building from all four compass points and because of this I was convinced I had located the source of my problems. A letter was immediately sent to the Engineering Director of the local water board which was passed down through several levels of hierarchy (this was tracked day to day through telephone calls). The procedure took two weeks and I eventually managed to arrange to meet the engineer at the pump site one lunchtime. A quarter of an hour's testing, which involved switching the pumps on and off and activating the automatic remote data monitors, proved that the pumping station was not the cause of the interference. I must add however that the engineer was most helpful. It was then that I decided that the interference must be coming from the power lines feeding the station and half the area's houses. After obtaining permission from the farmer owning the land over which the power lines "crossed' it only took a couple of hours to track the interference to an individual pole in the field. Another letter was dispatched to the local electricity board Engineering Director and three weeks followed when the letter was down through several levels of hierarchy yet again. Eventually, when it arrived at the local engineering manager's desk, I found them most helpful and they even dispatched an engineer with an FM radio to see whether they could trace the fault. They couldn't. After two months of inaction and a bit of frustration on my part the engineering manager called me to say that they were switching the line off for maintenance and they would take the opportunity to inspect the identified pole. I only hoped that I had got it right this time.

The offending piece of wire!The offending piece of wire that resonated on 50.110MHz and the burnt through insulation!

The day comes around, I arrive home from work to find a pile of burnt wire on my desk. It seemed a pleased engineering manager had called at my house to show me that on that very pole I selected the insulation on the wire carrying the 240V AC mains had been eaten through and was shorting to the burnt-through copper wire tying it to the insulator. The tie wire consisted of about six turns about 2 inches in diameter which I am sure resonated on or near 50MHz which explained the lack of interference on other frequencies. 6M is now quiet to the northeast. The above efforts took numerous telephone calls over a period of six months but the eventual positive outcome was very satisfying. It was also very interested in locating the problem and wanting to help.

S9+ l5dB Narrow-band Hash

For six months I suffered from intermittent hash centred on, you guessed it - 50.110. It peaked up at 315 degrees but it was so strong it could be heard with the aerial at all points of the compass.

The noise appeared mainly at weekends and at random times. Needless to say when it came on it was totally impossible to operate with the aerial anywhere near the northwest. I yet again mounted the aerial in the car and the family went out on a noise hunt. It took me several trips to track the noise down to a private house 1.5 miles away. Outside of the house the noise was end-stopping my S-meter. I knocked on the door and tried to explain my problem. This was not so easy as the occupier was disbelieving and didn't want anything to do with me. Eventually he agreed that I could telephone him the next time the noise occurred and come to the house to track down the noise. The next time the noise occurred they were out and the next time. One evening at 11 p.m. the noise appeared. I telephone him and it rang for what seemed an eternity before he answered it. As soon as he did the noise went off. It turned out he was in bed. What was I to say? I explained what happened but he was not too pleased and put the telephone down. Within two or three minutes the noise came back up. I decided bravely to call him back again. After a brief conversation he agreed that I could come over to the house and try to clear the problem up. His exact words were "yes come, I’m not going to have this every night!" I drove to the house and went in the gate with my receiver in hand. As soon as I pressed the door bell the noise instantly disappeared! He opened the front door and I tried to explain what happened, but he was obviously incredulous. As I thought it was the doorbell causing the problem we spent 10 minutes fiddling with this to no avail. Then all of a sudden the noise came on and it only took a few minutes to trace it to a cupboard which contained yes, a burglar alarm! The problem seemed to lie with the mains on/off switch which was arcing. Why did the noise disappear when he answered the telephone and I rang the doorbell? I can only suppose the electrical arc was unstable and when a current pulse was generated by switching on a light or operating the bell, the arcing stopped. Why was I getting such strong interference 1.5 miles away only at 50MHz? I can only assume that the wiring near to the alarm on-off switch resonated on 50MHz and the alarm wiring throughout the house was acting as an efficient aerial. The alarm was eventually replaced because I convinced him that it was a potential fire hazard. I have not suffered any interference from that source since. This noise took a lot of courage to sort out and I have crossed my fingers that I don't get a case like that again. The lesson learnt was to be polite but persistent. If I had given up because of the owners attitude I am sure I would still be suffering that bad interference to this day.


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 fmished 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 - I’ll think twice about moving! Chris G3WOS

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