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 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.
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
