Reply With QuoteJust out of interest, if you swap your PSU for a battery, is the noise still there?
Noise reduction on 80 Mtrs
I've been involved in radio for nearly 50 years and it never ceases to amaze and confound me.
As you can imagine living in the smoke I get my fair share of noise, you just learn to live with it. Today I was messing around on 80 Mtrs where the noise level has always been about S 9, it has never stopped me using the band and I've had some very good contacts on there, in spite of the noise.
I had better explain the antenna system, it is a vertical loop with a circumference of about 34Mtrs and roughly square and fed with 6 Mtrs of open wire line into a H/B balanced matching unit in the shack. Today I was listening to a QSO on SSB and decided to change the taps on the matching unit because it was originally tuned up on the CW end of the band. I grabbed one of the legs of the line and suddenly the noise stopped, and I don't mean by a little bit, this was a whopping 4 S points, from S 9 down to S 5and the signal dropped just a little, which soon came back when I adjusted the matching unit to compensate with me hanging on the feeder, which is insulated I might add.
The noise is like a buzzing, like a slow running motor, I don't think it's PLT noise because I've heard recordings of that and it doesn't sound like it, the amplitude of the noise is quite variable and the pulse width is variable.
I have now put a small wide spaced capacitor from one leg of the feeder to ground and adjusted it for minimum noise, at the moment I'm at a loss to explain why the drastic loss of noise just by placing and itsy bitsy capacitor in one leg of the line. I have got an RF choke that bleeds off any static that might be on the line as the RF side of the tuner is completely isolated from the electrical ground system.
I'm not complaining, I'm just curious what cause such a drastic reduction of noise. The only thing I can think of is the feeder system is grossly out of balance on 80 and by adding the capacitor balances the system just enough to cancel out the noise that is probably being picked up on the line, any ideas?
Just out of interest, if you swap your PSU for a battery, is the noise still there?
Yes I did, that is the first thing I thought of as well, anyway, it is a linear PSU and not one of those nasty switchy things. I did try a portable radio as well, and it seems to be strongest near the main incoming cable from the road, I did try knocking off the power briefly, but it is still there, so it must be something getting into the mains somewhere. I've also got a loop antenna, but I can't seem to null the signal to any degree to get a bearing, that's what make me think the noise is widespread.
It seems to be mainly on 80 and 60 Mtrs, even Top Band seems OK considering the location here.
By the way, the capacitor is only 50pf.
Still, I'm not too concerned with the noise as I have a way of nulling it out, it's just strange that the little cap has such a dramatic affect.
You're probably catching a lobe of a bigger harmonic on the power line, a HiFi mains "cleaner" is probably the way to go.
I built a kit many years ago, which handles all my shack power, it was well worth the 30 quid or thereabouts.
Maybe "leaking" some of the noise from various locations on your mains supply would have some cumulative effect?
The trouble is Dem, we live in a terraced house so the neighbours mains supply is only a few feet away from me, so I can probably clean up my supply without too much trouble, I can't do anything about the neighbours.
I dug out my spectrum analyser and had a look. It isn't very clear but you can see the grass just below and above the 75M band. I still can quite puzzle out what it is, it seems to only affect a small part of the spectrum between 3.520 and 3.680MHz where it tails off till you reach the 60Mtr band then it disappears by the time I tune to 6MHz.
Analogue TV timebase? (I forget the frequency now).
ADSL? nasty broadband over telephone twisted pair technology.
Could even be a cheap preamp on something, gone bad and radiating, but they tend to be quite broad.
I'd get the DF gear out 8)
OK, I've come to the conclusion that this noise is probably coming from a TV. It isn't there in the morning but always appears in the evening, it could be one of these crappy plasma TV's. When I looking into changing my TV a couple of years ago the local TV shop were trying to get me to buy one of these pieces of junk until I explained why I didn't want one and the reason why, he admitted that there was a problem with them emitting interference, but it looks like someone in the local area has got one.
I don't intend going around knocking on every door trying to explain, most of them don't understand English, so it would be an uphill struggle.
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