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Old 17th Jun 2021, 5:41 pm   #1
agardiner
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Default MW hum - higher frequencies only - 200Hz

Hello all,

I don't think this has been answered before, so apologies if it has.

When tuning the MW band at home, I have a noticeable hum on higher frequencies. It is completely clear upto around 1200Khz, then any stations above have hum on them. Very noticeable on Smooth at 1251Khz which is one of the stronger stations, and also on my Spitfire when tuned above 1200Khz, but clear below. Clear again above around 1600Khz. It doesn't matter if the radio is mains or battery powered and it is all sets.

So, I have checked around the house for the usual suspects, but I don't think this is mains hum as such anyway, as the frequency of it is too high; it sound around 200Hz. I don't think it is the neighbours house either, as moving into the garden away from all properties does not reduce the hum.

I live in a rural area, and am blessed with very little RF noise. The house is supplied from a pole with a 3 phase transformer. So - first of all - could this be a phase related hum from the mains supply? Also, talking to Tony from 6V6, he wonders if it could be ground radar from Lakenheath air base, which is approx 12 miles from me.

Any thoughts? It is not a problem, just interested in learning what it might be!
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Old 18th Jun 2021, 11:47 am   #2
kellymarie
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Default Re: MW hum - higher frequencies only - 200Hz

Is it a hum or more like a buzz? I have a buzzing noise at various spot frequencys thru the mediumwave there's so much carp out there now who knows where it comes from
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Old 18th Jun 2021, 12:20 pm   #3
agardiner
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Default Re: MW hum - higher frequencies only - 200Hz

More of a hum really, but higher than the usual mains hum.
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Old 18th Jun 2021, 12:21 pm   #4
Andrew2
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Default Re: MW hum - higher frequencies only - 200Hz

It may be a type of 'modulation hum', where a strong local transmission puts a high level of RF on the mains wiring, causing intermodulation between itself and the 100Hz switching frequency in power supplies etc. I get it badly on our local 1458 kHz station, which is only 2 miles from here and puts in a huge signal here.
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Old 18th Jun 2021, 11:44 pm   #5
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Default Re: MW hum - higher frequencies only - 200Hz

Have you tried with a battery operated receiver? By that means you could identify whether the interference was being carried on the supply mains.
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Old 20th Jun 2021, 1:13 pm   #6
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Default Re: MW hum - higher frequencies only - 200Hz

Going on from what Croozer says above,have you got access to a car radio with medium wave? If so, a drive around the local streets may prove revealing.
Some electric fences produce various forms of R.F.I. as they contain step-up invertors, so since you're in a rural spot, that's also a possibility Tony
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Old 21st Jun 2021, 9:45 am   #7
agardiner
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Default Re: MW hum - higher frequencies only - 200Hz

Interesting. Well, yes, have tried on a few battery sets; same problem.

My car does have MW, (and indeed SW) and is the only set that does not appear to manifest the problem.

One interesting thing I noted over the weekend - hum is much worse if I am near the set. If I move away by at least 5 metres then the hum becomes barely audible.
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Old 21st Jun 2021, 10:25 am   #8
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Default Re: MW hum - higher frequencies only - 200Hz

Try moving it from the ground as far as possible ie. put it on the top shelf.
See if it changes.
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Old 21st Jun 2021, 10:40 am   #9
Ian - G4JQT
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Default Re: MW hum - higher frequencies only - 200Hz

Is it audible between stations, or only on stations?

Whether modulation hum (only on stations) or interference being radiated, listen to it on a portable radio and switch off each ruing main in turn - including lighting. Doing it this way enures you don't miss some hidden device somewhere! You may find the culprit or it may be next door...

Ian
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Old 21st Jun 2021, 3:26 pm   #10
agardiner
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Default Re: MW hum - higher frequencies only - 200Hz

Quote:
Originally Posted by Ian - G4JQT View Post
Is it audible between stations, or only on stations?

Whether modulation hum (only on stations) or interference being radiated, listen to it on a portable radio and switch off each ruing main in turn - including lighting. Doing it this way enures you don't miss some hidden device somewhere! You may find the culprit or it may be next door...

Ian
Only on stations. I have tried switching off the main breaker! Don't think it is the neighbour either, as it doesn't go away when I go outside and move away from the properties.
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Old 21st Jun 2021, 10:53 pm   #11
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Arrow Re: MW hum - higher frequencies only - 200Hz

Having read all the posts so far, to my mind, post #4 seems highly relevant

From that - and what you have said, quote: hum is much worse if I am near the set, but if I move away by at least 5 metres then the hum becomes barely audible, unquote, leads me to wonder if it may be that the mains wiring radiation is being picked up by your body and re-radiated. Yes, really weird, I do appreciate - but you do seem to have a very strange fault condition.

However, this may be of assistance:
https://www.vintage-radio.net/forum/...ad.php?t=42507

If it isn't radiation from the mains, then it must be that where you are located there is a strong radiated electro-magnetic field that could be coming from a nearby radio transmitter - or some other source. Perhaps it's coming from a high-voltage street step-down transformer: it's entering your body and re-radiating.
Perhaps, with the aid of a portable transistor radio that shows same problem, a travel around your vicinity - which might be a few miles - might locate said transmission source.

I'll be intrigued to discover the cause - as I expect you will be - and other member here also!

Al.
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Last edited by Skywave; 21st Jun 2021 at 11:16 pm. Reason: General additions, subtractions, etc.
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Old 21st Jun 2021, 11:26 pm   #12
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Arrow Re: MW hum - higher frequencies only - 200Hz

Add to my last post: post #6 is also possibly relevant.
(Ran out of editing time).

Al.
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Old 22nd Jun 2021, 1:03 pm   #13
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Default Re: MW hum - higher frequencies only - 200Hz

Quote:
Originally Posted by Skywave View Post
If it isn't radiation from the mains, then it must be that where you are located there is a strong radiated electro-magnetic field that could be coming from a nearby radio transmitter - or some other source. Perhaps it's coming from a high-voltage street step-down transformer: it's entering your body and re-radiating.
Perhaps, with the aid of a portable transistor radio that shows same problem, a travel around your vicinity - which might be a few miles - might locate said transmission source.


Al.
Thanks for your comments. So - yes - I will take a portable and go for a wander, weather permitting, this weekend. I do wonder if it is indeed a step down transformer being as my street is supplied from a nearby 3 phase transformer on overhead 11000 lines. Perhaps a combination of the high voltage and the fact the properties will be on different phases is causing the modulation hum.

Clearly the two factors that make this interesting are 1) the higher frequency of the noise, not the usual 50Hz, and 2) the fact that it is only audible on the higher part of the MW band.

Will update next week when I have gone for a walk.
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Old 22nd Jun 2021, 2:03 pm   #14
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Arrow Re: MW hum - higher frequencies only - 200Hz

But why 200 Hz? The fact that 200 Hz is a harmonic of 50 Hz sounds significant. (Pun not intended! ) However, the fact that it is only detectable on the higher 'ends' of MW is intriguing. Perhaps there are two sources! An H.V. transformer and an R.F. device that is radiating a strong enough signal to overload your receiver - which takes me back to my last post . . . .
And then the rather complex matter of intermodulation could be a factor.

Like many other members here, I've had - and tried to help with - diagnosing and fixing strange faults in the past - but this one is probably the most puzzling I have ever read!
Methinks your best tool is sheer persistence!
Good luck!

Al.
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Old 23rd Jun 2021, 8:50 am   #15
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Default Re: MW hum - higher frequencies only - 200Hz

I get the same effect where I live. I think it is a mains modulation problem as it only affects stronger signals.
Its bad on the two strongest local signals 882Khz and 909Khz. Its also very bad on the stronger signals on the 6Mhz and 7Mhz short wave bands. It effects radios running on batteries and is the same around the house and reduces in the back garden.
I have done some detective work and found it is worst towards the front of the house and seem to peak at the end of the drive where even 198Khz is affected.

I am wondering if its a UG cable under the pavement and finding its way into the house via the mains wiring.
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Old 24th Jun 2021, 12:40 pm   #16
agardiner
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Default Re: MW hum - higher frequencies only - 200Hz

OK - I have done some more tests, walking all around the area with my PL330, trying both the ferrite bar and telescopic antennas. Here are my findings...

1) The noise is worse with the rod antenna. This suggests magnetic coupling and further re-enforces the notion that this is mains modultation.
2) Although 'very' present in the garden, away from the properties, the noise quickly diminishes as I move further away from the residential area. There is no noise when standing directly under the 3 phase, 11Kv transformer.
3) I agree with Simon in that the although only on the higher part of the band, the noise is only actually a problem with stronger signals. 1251Khz for Smooth which is a good signal, my pantry tx when tuned high in the band etc. When pulling in Libra AM from much further away, it is clean.
4) Perhaps most interesting of all - the noise is everywhere in the home with 1 exception... My fuse box is on the landing along with the meter. It is just as noisy all around the fuse box, BUT, move the radio above the meter so that it is across the incoming mains tails, and the noise is completely gone.

So I have to agree with others here, that it is indeed mains modulation hum interacting with strong RF signals, and the ring mains are clearly forming some kind of radiating loop. The 200Hz must just be a harmonic. I have tried adding plug(s) with 100nF capacitors in them, but it makes no difference. Nor does isolating the main breaker.

No matter - I have moved the pantry TX to a lower frequency and the rest of the band is clear. Just interesting!
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Old 25th Jun 2021, 2:07 pm   #17
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Default Re: MW hum - higher frequencies only - 200Hz

Just a couple of ideas.

This problem - the 200 Hz radiation - could be caused from a building - or buildings near to you - in which there is a device - or devices - that use a switched-mode PSU. Such power supplies are notorious for distorting the waveform of the 230-v. supply and produce lots of harmonics of 50 Hz, usually predominately, but not exclusively, the 3rd harmonic. The cabling to such buildings will almost certainly be underground and possibly radiating those harmonics, therefore making any attempts to locate the source very difficult. The last sentence in post #15 states much the same.

As for the 200 Hz not detectable near the incoming 'tails', but detectable near the fuse box / consumer unit might be due to the tails being close to each other and thus acting as a balanced short line - so phase cancellation is taking place - whereas in the CU, the wiring in it is not so balanced.

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Old 25th Jun 2021, 3:28 pm   #18
agardiner
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Default Re: MW hum - higher frequencies only - 200Hz

Thanks for the ideas. It certainly isn't a switch mode in my house, but cannot rule out the neighbour of course. However - I live on a rural estate with very few properties and only one immediate neighbour.

What it could be therefore, is the estate has a working farm, and their repair/maintenance workshops are on the same power lines and only a few hundred metres away. They also have 3 phase supply to them and plenty of air conditioning units, motors etc. I think this is probably the most likely cause of the interference, and then it is being brought in on the 'balanced' power lines and radiated from the ring main!

Fortunate that it is only on the top of the MW band. Lower down is nice and clear, and being rural, no problem at all with SW.
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Old 26th Jun 2021, 3:17 pm   #19
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Default Re: MW hum - higher frequencies only - 200Hz

Hmm, yes: a 3-phase motor could well be the source. Bearing in mind all that electrical machinery they are using, does cause me wonder if they have power factor correction installed - which, of course, may be quite irrelevant!

Al.
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