Other SOTA sites: SOTAwatch | SOTA Home | Database | Summits | Video | Photos | Shop | Mapping | FAQs | Facebook | Contact SOTA

Combatting Urban Noise

The noise level on my doublet has steadily increased and is now over S9 on the lower bands. I have been looking into what I can do about it and have got to the point where I can experiment.

I am blessed with a long garden, 50 metres long and 9 metres wide, with high trees at the far end. Beyond my boundary is the car park for a club before the rear gardens of the next row of houses. I reasoned that if I moved the antenna to the lower end of the garden and used coax with a choke each end I would minimise the noise from my row of houses and would be too far from the next row to have much trouble with them. I purchased a 20 metre long wire antenna terminating in a 9:1 unun and ran it from the mid point of the garden to the trees at the far end. Since it was a proof of concept exercise I used cheap flexible coax that I had to hand, winding nine turns on a toroid near the unun and nine turns on a toroid adjacent to the ATU. This is a long run of coax, about 50 metres, so the losses are bound to be significant.

The result from the new antenna was remarkable. On 20 metres the noise fell from S8 to about S1, and the band was full of stations that had been hidden by noise earlier in the day! There were a few S points of noise on 80, 60 and 40 but I could cope with it - I had my ham radio back and was jubilant. The one fly in the ointment was that at 18:00 local time a severe noise source started up and continued until 01:30 before vanishing. This appears to come from the club building adjacent to the far end of the antenna, but I will need to confirm that before taking any action.

The next step is to replace the long run of cheap coax with something much less lossy, though I doubt that my toroidal chokes will be possible with better coax! In the future I may try an 80 metre OCF dipole, it may be noisier as the near end will be closer to the house but it will be more efficient on the lower bands.

Brian

Sounds like they put the lights on - I had a similar problem with the hotel next door to me at my previous qth except it started at 0700 and went on to 0100 (yes 18 hours). Turned out to be cheap & nasty LED GU10 bulbs. I forked out and replaced them myself with the LED bulbs I used in the shop - result!

I have just moved into my house in Chatteris Brian, with a higher home noise floor then I am used to.
Strangely 80m has the lowest noise floor, Using a flat top dipole, fed with open wire 450R ribbon. 40m is one
of the nosier bands of the three.

I suspect the reason (sadly) that you have a lower floor is because of the increased attenuation with the additional coax and the 9:1 impedance transformation.

My strategy so far was to drive around with some wire wrapped around the roof rack of the car, I did find a noise source that I believed to be coming from a particular house. I borrowed a battery spectrum analyser last night to try and record the noise with a short whip antenna, unfortunately it did not reappear.

I suspect, possibly that the noise is radiating from a street lamp not so far away. If it is the case it will be somewhat easier I hope to deal with then a neighbour with noisy electronics.

Don’t ever be fooled by the witch stories that state coax is quieter then open wire for feeding aerials, its complete nonsense based on a lack of understanding of EM fields.

My noise issue isn’t as bad, but thats not to say I won’t try to get it as low as I possibly can. Its very hard even to make sure the house that you purchase is in a low noise area, apart from setting up the usual portable station.

Jonathan

Excellent news Brian. Having a large garden is a distinct advantage. Some of us are not so fortunate. :frowning:

Yes, I agree that this can be a significant source of noise. Recently our local authority changed the lighting from sodium to LED and the noise now goes up around 2 S points on the LF bands when they come on. Not as though the new lighting is any use as the lights are spaced so far apart that all they do is serve to define the curve in the road. Many of our neighbours won’t walk out after dark any more as they don’t feel there is enough illumination to be safe.

Experimenting with antenna combinations to get the best signal relative to the noise background is essential nowadays. In my pocket handkerchief garden, even which way around I connect my 40m dipole has an effect… maybe because of the electricity sub-station at the bottom of the garden, something which I didn’t notice when we moved in. I am seriously considering going remote on receive if I can find a suitable nearby location and a friendly farmer.

73, Gerald G4OIG

Interesting experiment, Brian.

If the overall effect on receive is beneficial eg signals slightly attenuated, noise greatly attenuated, then another option would be to use that antenna on receive, and keep the transmitting antenna closer to the house.
Tell the neighbours that all the wire is a new soft fruit cage :wink:

73
Adrian
G4AZS

Brian, have you thought about building a noise mixer. i.e. a small antenna designed to pickup noise and a phasing box so you can add the local pickup out of phase with the main antenna and reduce the noise. You need a transmit relay to remove the phasing system from the signal path on transmit of course.

That sounds like a cracking DSP project for someone a lot more clever than me :smile:

Probably easier than finding the tuning capacitors and inductors nowadays for an external box. But really you wan to add the out of phase signal before you’ve gone through the 1st RF stages and swamped them with an S9+60 noise signal.

I don’t think so, Jonathan, partly because the attenuation of the noise is considerably greater than I would expect from the coax, and partly because of the A/B comparison between the two antennas - admittedly there was perhaps an hour separating the use of the two antennas and there could have been a big jump in propagation in that time, but in the week since the changeover the impression remains that I have a significant improvement in S/N ratio.

I’m reminded of a friend who moved to the country a few decades ago to get a low noise environment. He was driven wild by electric fences!

I may be wrong, but I think that method only deals with one noise source at a time, whereas I have to deal with multiple sources.

Brian

That is a useful solution that some adopt.

The other great alternative is the small receiving loop antenna “Wellbrook Loop” to give its commercial name. It won’t do much for far field noise, but for near field they are brilliant.

Of course all of these solutions add additional complexity to your R/T/R routines.

Jonathan

Multiple extra antennas with multiple phasing units. So basically you’ll be building a phased array and steering multiple nulls where you want them.

Use it to reduce the strongest.
Or make a noise phaser with several noise inputs.

When I put a balun on my home dipole, the noise floor dropped by 6 dB.

A balun that rejects local common-mode noise will only work on a balanced antenna, like a dipole. Pretty hard to get rid of noise on an end-fed wire.

My balun is from Balun Designs, their “QRP” model (250 W). It is better at rejecting common-mode noise than simple wound-coax chokes.

http://www.balundesigns.com/

Isn’t possible to ask to Ofcom to check ?
CT Ofcom (ANACOM) check those complains and enforce the resolution.

73 de Pedro, CT1DBS