RF on earth? - UK Vintage Radio Repair and Restoration Discussion Forum

ITAM805 · 16th Sep 2013, 6:12 pm

Hi folks

after fixing an amp, which amongst other problems had a burnt out 10R in the Zobel network, I turned it on and noticed there was some noise on the o/p, which was going into an 8R dummy load. On closer inspection it looked to be RF.

So I quickly turned it off thinking it still had a fault, but noticed the RF was still there! I thought perhaps it was residual voltage left in the PSU caps, it's a 400 watter, but no it persisted after a minute or so. Only when I unplugged the mains did it stop? So I stuck the scope probe on the the earth pin on the wall socket and was surprised to see about a 100mV of RFI sinewave!

I switched off my laptop and router so it's wasn't that and we have nothing else plugged in that could cause this. I'm not clever enough to work out the frequency but the scope is set to 1uS trigger and 20mV sensitivity. Holding the signal probe just produced the familiar mains hum but no RF and shorting the probes flat lines the trace

What (on earth!) would be causing this?

G8HQP Dave · 16th Sep 2013, 7:30 pm

Almost certainly a nearby AM transmitter, or a combination of them. If you look closely at the scope you will see the trace varying. Tuning around on MW you will probably find a station whose programme material varies in the same way.

Nickthedentist · 16th Sep 2013, 7:50 pm

I make that about 1500KHz which is within the MW band.

Could be wrong - I'm tired!

N.

ITAM805 · 16th Sep 2013, 8:06 pm

Thanks for the replies guys.

Dave, I was going to say that it looked like it was amplitude modulated by voice! I'm unsure of any MW TX nearby here though we're only a couple of miles from the Dover TV twig and lots of French radio channels can be heard here.

Our mains wiring is tacked to the wall under the eaves about 20' up, so is it possible that it's acting as MW aerial? I get an inordinate amount of QRM here, making MW/SW all but pointless during the most of the day and I've wondered if the mains cable positioning - not that there's anything I can do about it

paulsherwin · 16th Sep 2013, 8:20 pm

There's a substantial BBC R5 transmitter just outside Folkestone.

http://tx.mb21.co.uk/gallery/gallerypage.php?txid=1459

ITAM805 · 16th Sep 2013, 8:30 pm

Quote:

Originally Posted by paulsherwin

There's a substantial BBC R5 transmitter just outside Folkestone.

http://tx.mb21.co.uk/gallery/gallerypage.php?txid=1459

Thanks Paul, yes I just found that, although if what Nick says is correct then the frequency is too low? However I did find this...

paulsherwin · 16th Sep 2013, 8:40 pm

BFBS MW transmitters tend to be very low power and are unlikely to cause problems unless they're next door to you. I think this one is only 1W - it just covers the barracks for the Gurkhas there.

ITAM805 · 16th Sep 2013, 8:47 pm

1 watt, cripes! I can receive it here but we're about 2 1/2 miles from Shorncliffe so it's not that is it.

I think you're probably right Paul with the R5 site which is just over a mile from here, but out of sight as it's atop of the cliffs and we're a couple of hundred feet lower down. I had often wondered what the array was?

Ian - G4JQT · 16th Sep 2013, 9:07 pm

Tune thru MW and see which stations' modulation match the scope trace.

I've had similar apparent parasitic oscillations on the scope, but turned out to be the nasty low-energy lamp nearby!

Ian

ITAM805 · 16th Sep 2013, 9:18 pm

Quote:

Originally Posted by Ian - G4JQT

Tune thru MW and see which stations' modulation match the scope trace.

I've had similar apparent parasitic oscillations on the scope, but turned out to be the nasty low-energy lamp nearby!

Ian

I'll give it a go Ian!

ITAM805 · 16th Sep 2013, 10:48 pm

Well I think I found the culprit - or at least one of them

I found a strong signal that correlates on 1377kHz - France Info. Now this emanates out of Lille according to that webpage, which is a 100 miles from here. But... they have an FM TX just across the channel from here, in fact you can see it with the naked eye on a clear day.

So I wonder if their AM signal is coming from there as well?

Fezziwig · 19th Sep 2013, 6:40 pm

I seem to remember a Medium Waver sited at Sutton Coldfield and it got into all sorts the electronics in the main building. They had to shift it in the end,

Restoration73 · 20th Sep 2013, 10:54 am

I once worked in a 12 storey building that acted as a quarter wave vertical aerial on
medium wave - we had to put rf decoupling on the amplifier experiment boards to
get rid of it.
RF breakthrough from the old Rugby time signal station on 16kHz (GBR) appears on
quite a few famous records made at the Manor Studios, Oxon, e.g. Tubular Bells.

ITAM805 · 20th Sep 2013, 2:46 pm

Thanks for the replies chaps.

I know RF has some strange qualities but I'm puzzled, how an earth can carry such a signal, why isn't it grounded?

cmjones01 · 20th Sep 2013, 2:52 pm

Because the earth wire has some inductance, so at radio frequencies, it isn't really grounded. If you think about it, a long-wire radio aerial for a crystal set is also 'grounded' via the receiver it's connected to and the crystal set's earth, but it still carries a fair old radio-frequency signal - enough for a crystal diode to detect.

Chris

Fezziwig · 20th Sep 2013, 3:20 pm

All to do with wavelength. If an earth wire is a significant fraction, even just 1/10 of a wavelength, at the frequency in question it no long behaves as a simple earth wire. You might make things worse by earthing the far end.

16th Sep 2013, 6:12 pm	#1
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	RF on earth? Hi folks after fixing an amp, which amongst other problems had a burnt out 10R in the Zobel network, I turned it on and noticed there was some noise on the o/p, which was going into an 8R dummy load. On closer inspection it looked to be RF. So I quickly turned it off thinking it still had a fault, but noticed the RF was still there! I thought perhaps it was residual voltage left in the PSU caps, it's a 400 watter, but no it persisted after a minute or so. Only when I unplugged the mains did it stop? So I stuck the scope probe on the the earth pin on the wall socket and was surprised to see about a 100mV of RFI sinewave! I switched off my laptop and router so it's wasn't that and we have nothing else plugged in that could cause this. I'm not clever enough to work out the frequency but the scope is set to 1uS trigger and 20mV sensitivity. Holding the signal probe just produced the familiar mains hum but no RF and shorting the probes flat lines the trace What (on earth!) would be causing this? Attached Thumbnails

16th Sep 2013, 7:30 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: RF on earth? Almost certainly a nearby AM transmitter, or a combination of them. If you look closely at the scope you will see the trace varying. Tuning around on MW you will probably find a station whose programme material varies in the same way.

16th Sep 2013, 7:50 pm	#3
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,865	Re: RF on earth? I make that about 1500KHz which is within the MW band. Could be wrong - I'm tired! N.

16th Sep 2013, 8:06 pm	#4
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: RF on earth? Thanks for the replies guys. Dave, I was going to say that it looked like it was amplitude modulated by voice! I'm unsure of any MW TX nearby here though we're only a couple of miles from the Dover TV twig and lots of French radio channels can be heard here. Our mains wiring is tacked to the wall under the eaves about 20' up, so is it possible that it's acting as MW aerial? I get an inordinate amount of QRM here, making MW/SW all but pointless during the most of the day and I've wondered if the mains cable positioning - not that there's anything I can do about it

16th Sep 2013, 8:20 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: RF on earth? There's a substantial BBC R5 transmitter just outside Folkestone. http://tx.mb21.co.uk/gallery/gallerypage.php?txid=1459

16th Sep 2013, 8:40 pm	#7
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: RF on earth? BFBS MW transmitters tend to be very low power and are unlikely to cause problems unless they're next door to you. I think this one is only 1W - it just covers the barracks for the Gurkhas there.

16th Sep 2013, 8:47 pm	#8
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: RF on earth? 1 watt, cripes! I can receive it here but we're about 2 1/2 miles from Shorncliffe so it's not that is it. I think you're probably right Paul with the R5 site which is just over a mile from here, but out of sight as it's atop of the cliffs and we're a couple of hundred feet lower down. I had often wondered what the array was? Last edited by ITAM805; 16th Sep 2013 at 8:58 pm.

16th Sep 2013, 9:07 pm	#9
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,326	Re: RF on earth? Tune thru MW and see which stations' modulation match the scope trace. I've had similar apparent parasitic oscillations on the scope, but turned out to be the nasty low-energy lamp nearby! Ian

16th Sep 2013, 10:48 pm	#11
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: RF on earth? Well I think I found the culprit - or at least one of them I found a strong signal that correlates on 1377kHz - France Info. Now this emanates out of Lille according to that webpage, which is a 100 miles from here. But... they have an FM TX just across the channel from here, in fact you can see it with the naked eye on a clear day. So I wonder if their AM signal is coming from there as well? Attached Thumbnails

19th Sep 2013, 6:40 pm	#12
Fezziwig Retired Dormant Member Join Date: Sep 2013 Location: Flitshire, UK. Posts: 35	Re: RF on earth? I seem to remember a Medium Waver sited at Sutton Coldfield and it got into all sorts the electronics in the main building. They had to shift it in the end,

20th Sep 2013, 10:54 am	#13
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: RF on earth? I once worked in a 12 storey building that acted as a quarter wave vertical aerial on medium wave - we had to put rf decoupling on the amplifier experiment boards to get rid of it. RF breakthrough from the old Rugby time signal station on 16kHz (GBR) appears on quite a few famous records made at the Manor Studios, Oxon, e.g. Tubular Bells.

20th Sep 2013, 2:46 pm	#14
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: RF on earth? Thanks for the replies chaps. I know RF has some strange qualities but I'm puzzled, how an earth can carry such a signal, why isn't it grounded?

20th Sep 2013, 2:52 pm	#15
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,681	Re: RF on earth? Because the earth wire has some inductance, so at radio frequencies, it isn't really grounded. If you think about it, a long-wire radio aerial for a crystal set is also 'grounded' via the receiver it's connected to and the crystal set's earth, but it still carries a fair old radio-frequency signal - enough for a crystal diode to detect. Chris __________________ What's going on in the workshop? http://martin-jones.com/

20th Sep 2013, 3:20 pm	#16
Fezziwig Retired Dormant Member Join Date: Sep 2013 Location: Flitshire, UK. Posts: 35	Re: RF on earth? All to do with wavelength. If an earth wire is a significant fraction, even just 1/10 of a wavelength, at the frequency in question it no long behaves as a simple earth wire. You might make things worse by earthing the far end.