[Keefbc]

I've been having some interference problems since the DSO and wondered if anyone could comment on the nature of the interference.

Originally, the problem was seen with an old CRT screen TV and a Freeview set top box, but that was all chenged to a new set up to try and resolve the issue.

The set up is now a brand new Panasonic LED TV with a Freeview HD tuner. The aerial has recently been checked/realigned and new double screened cable fitted - no masthead amplifier or other amplifier is used. A masthead UHF bandpass filter is fitted. All Freeview muxes broadcast from Belmont (Ch 22, 25, 28, 30, 53, 60) give a good signal at the TV and the normal picture and sound quality are outstanding.

At various times, which can last for hours, and seems to be random during the day and night, interference is seen with picture and sound break up and sometimes the 'no signal' display on the TV.

If I tune a portable FM radio off station (to say 90.00MHz), the interference can be heard as a cyclic burst of noise that occurs at a frequency of just over 1 Hz - I have recorded the audio and the main noise burst lasts for approximately 0.2 seconds in each second.

When the interference stops on the TV, the noise signal on the FM radio disappears and the picture and sound return to normal.

I have tried to report this problem to the BBC, as they have taken over from Ofcom for RFI reports, but they seem to be disinterested, even though they recently reported that the case was to be escalated.

Can anyone shed some light?

[Keefbc]

Some additional information:

When the interference is breaking up the sound and picture, muxes 53 and 60 are OK - it only occurs on channels broadcast on muxes 22, 25, 28 and 30.

[kjhskj75]

Ask you neighbours if they have similar problems. This may help localize the source. Or you could just go wandering the neighbourhood with your radio.

Do you live in a mostly residential area, or is there industry locally ? Does rather sound as if it's some kind of machinery.

[Rodders53]

Where you live is very relevant.

Was the portable radio on batteries or mains power? {Interference can be radiated through the ether, but is often conducted via the mains cables}.

Electric fence? Brush motor device? Faulty insulator on overhead power cable? Street light starting and failing to light (sometimes)?

When the problem is happening, you'll need to switch off everything in your home and unplug all but the TV direct connected to the aerial on the only ring main powered to 'prove' (as best you can) that it isn't anything in your home causing it. {Have you done that already}.

If it is from outside your home your near neighbours (or causes I suggested above) are the next suspects - but they should also be affected by it?

[Keefbc]

Thanks for the questions.

It is a residential area - no industrial area even close.

Radio was on batteries. Tried walking it around the estate - it does appear to get stronger outside and fade out away from my house, but difficult to locate a proper direction or pattern - I suspect the walls/houses are good absorbers, so it's not obvious - really need to fly the radio above the roof tops!

Unplugging everything has been tried and the house is clean. As I said before the 1Hz cyclic interference can happen anytime during the day or night and can go on for hours, so its hard to think of a device that is used or operated like that? e.g. I assume street lights would normally be night time. Does anyone know how amateur radio interference would manifest itself if a transmitter wasn't well filtered? I have located what looks like a dipole antenna in the line of fire to the transmitter at about 500m away?

Neighbours in the same aerial line of fire to the transmitter have some issues but none as bad as mine. Another neighbour only 50 metres off my transmitter line has no problems at all.

All Sky dish neighbours have no problems.

Even the aerial installer is baffled - he's not seen anything like this in his many years of experience.

He measured the RFI and it appears to be very strong close to the fundamental frequencies of channels 22, 25, 28 and 30.

[Chris Muriel]

One longshot for you...
Could be a radio telemeter - used for off peak (white meter) electricity.
The old ones include a long wave receiver (radio 2) + microcontroller etc.
About 15 years ago I had the local leccy board instrumetnation guys round after I traced interference on the amateur 144 MHz band to these; they found 2 different manufacturers created bad RFI in the VHF (and perhaps UHF) region.
They then swapped out the meters in 3 premises in my road.
I was able to find out eventaully that the units had been tested for susceptability but not for radiation !

[kjhskj75]

Any faulty street lights in the vicinity ? You might be able to use the antenna on the radio as a direction finding device (I don't know how this would work, but someone on the forum might). Given that this is affecting a wide range of frequencies, electrical faults rather than illegal/faulty radio transmitters seem the most likely.

[Keefbc]

Thanks for the comments.

Street lights seem to be fine and all switch off during the day. Just wondered if the frequency and energy required could be achieved by a 50Hz street light but I'm open to similar examples of interference to TV signals?

I'll try and check out the radio telemeter issue - pretty tricky but worthy of further investigation.

The 1 Hz interference frequency must be a clue to someone who has seen something similar in the past?

[Keefbc]

Here's the audio mp3 file recorded from the FM radio tuned to 90.00MHz

[Gavtech]

Originally Posted by Keefbc

Here's the audio mp3 file recorded from the FM radio tuned to 90.00MHz

Is there any chance you can get that to the BBC? [... by which I mean get an appropriate person prepared to receive it. ]

They can be a very helpful organisation - if you are lucky enough to get that right person.

The presence or absence of this signal definitely correlates to the UHF interference you suffer?


It's nature suggests a rotary component... a motor perhaps.
But any time day or night for several hours... seems too long for fridges or freezers.
A pump? .. but for what???

[A1944]

Originally Posted by Keefbc

Here's the audio mp3 file recorded from the FM radio tuned to 90.00MHz

The sound of that definitely rules out any sort of voice communications radio transmitter (Amateur, taxi, CB etc.). It sounds like some kind of electrical arcing, perhaps from a malfunctioning thermostat on something (maybe central heating, fridge, freezer). It sounds like something switching on and off with the click being the switch-on and the buzz the switch-off.

I have observed a street light causing a problem like this, but it was more like a long drawn out sound for about 10 seconds, repeating every couple of minutes (that is not to say that lights don't have other faults). This one showed no visible sign of a problem, the light was perfectly steady.

[Rodders53]

How are you in contact with the Beeb's Radio Investigations bods? If you have a direct e-mail send it to them. If a 'phone number call and point them to this thread to download and play it. (I know some of the people concerned and it may well help them).

To me it sounds like something sparking while trying to strike, failing and trying again... e.g. fluorescent lights or discharge lamps (as in street lights). Check lofts and sheds for the former. Check the street lights aren't glowing dim red rather than fully lit at night.

A sparking source of interference has frequencies from zero to the speed of light (well almost) and can be radiated and conducted through the mains at the same time. The AM band on a portable has a highly directional aerial inside and may aid tracing it?

Have you asked any of your neighbours to try switching off everything (at their consumer unit) when the problem is occurring? {Naturally you'd do the same so they can witness it's not your house causing it}

[noiseboy72]

My best guess would be your 3 way valve on your central heating system. This uses a motor and microswitch to set its position. I had a similar problem with my system when the valve stuck. The motor would run backwards and forwards against the microswitch at a similar rate.

[Chris Muriel]

Having heard the MP3 file, definitely not a radio telemeter.
Something seems to have an on/off or perhaps rotary cycle as mentioned above; alternatively could be from a digital communications or telemetry device (tetra, energy companies digital telemetry, ham radio using one of the modern digital modes etc.)

[Keefbc]

Thanks for all the additional useful comments.

Some neighbours are more cooperative than others but I will try to get things switched off and report back. Just a note that this did start before the need for the central heating to be on, but its still worth checking on again.

I'll also try and get the BBC more interested - all contact has been by phone so far and they have tried to throw me back to Ofcom, who threw me back to the BBC, ping pong style.

I like the 'trying to strike' suggestion - the interference is so regular that maybe it is a spark initiated by a control circuit. The noise seems to be very broadband. The 1 second rotary motion is something I'd already thought about but can't think what might be in the vicinity - there is an army base and helicopter landing pad a few miles away, but in the opposite direction to the Belmont transmitter.

If anyone has looked at the audio file trace there are actually two signals each second - a very short burst of about 0.02 seconds a silent time of just over 0.1 seconds and then the longer burst of about 0.2 seconds. Not sure if that is an extra clue for someone?

Just to reiterate - the radio audio interference definitely correlates to the TV interference and you can see the 1 second cycle effect on the signal quality and signal strength on the TV signal diagnostic screen.

If I use an AM radio to track the interference, how do I determine which direction the internal AM radio aerial is pointing?

Thanks again!

[Keefbc]

Additional information.

See attached for a jpg of the audio trace - 2 cycles are included.

[A1944]

Originally Posted by Keefbc

If I use an AM radio to track the interference, how do I determine which direction the internal AM radio aerial is pointing?

Usually the ferrite rod aerial is positioned along the top of the radio.

The aerial has its minmum pickup off the ends, therefore if you rotate the radio until you are getting the least signal and then one end of the radio should be pointing towards the source, so that will give you two possible directions.

Hopefully, moving to somewhere else, you can repeat the exercise and get a second direction from there. Where the two lines cross should give a rough indication of the source.

Trouble is that, if it is within the building, the signals could be confused by re-radiation from metal objects like wiring.

[grahamlthompson]

Originally Posted by Keefbc

Some neighbours are more cooperative than others but I will try to get things switched off and report back. Just a note that this did start before the need for the central heating to be on, but its still worth checking on again.

!

The boiler and diverter valve will be used throughout the summer for hot water.

And yes the noise sounds like a rf source close in frequency to the the affected uhf channels modulated by a low frequency square wave with unequal on and off periods. It doesn't sound like any interference I have ever heard from a rotary device. Most odd.

As to directionality you may be better off locating the direction of the source using a gash TV aerial and the TV. TV antenna are inherently directional, the more elements the narrower the forward gain beam. The reflector vastly reduces the rear pickup.

[Gavtech]

On a trace, I measure these as slightly faster than 1 Hz:

12 cycles in 11.5 seconds.

[Keefbc]

I see the point about hot water in the summer . . . would the diverter valve continue to try and operate even after the water was to temperature? Obviously, I've switched everything off in my house to try and isolate the source but not at all the neighbours yet.

Yes, the signal is slightly faster than 1Hz - 1.066Hz? Does anyone know what sort of speeds a rotarty radar system spins at?

Thus far I've assumed a radiated signal coming down the aerial lead - the Aerial Engineer certainly measured strong airbourne signal peaks in the UHF band (487.85, 501.25 and 592.80) - does anyone have a view on mains filtering - is it worth trying and what to try?

[Keefbc]

Just to be clear - the Aerial Engineer UHF peaks were measured during a period of interference.

[grahamlthompson]

Originally Posted by Keefbc

I see the point about hot water in the summer . . . would the diverter valve continue to try and operate even after the water was to temperature? Obviously, I've switched everything off in my house to try and isolate the source but not at all the neighbours yet.

?

A diverter valve (assuming you have one). Unpowered it sits in the central position sending water to both the radiators and the hot water coil in the hot water storage tank. If the tank thermostat calls for heat and the Central heating thermostat is not (or Central heating is off), the valve motors to only feed the tank coil. Either of the thermostats fires up the boiler and pump. If the reverse is true the valve goes all the other way to feed only the radiator loop. If both call for heat the valve centralises.

Some systems use two single port valves individually controlled by the appropriate thermostats. In more complex systems the radiator circuits may be zoned by using more valves and thermostats.

It sounds a bit like the radio signals you get from a pulsar which when first heard was thought be from an extra terrestrial intelligence. Not seen any little green men around have you ?

The radar idea is pretty cool, you don't live near an airport especially on it's landing flight path do you. They use high power approach radar for final approach and landing.

If it's airborne rf picked up by your aerial, mains filtering is not going to help. Tetra filtering may help though (have you a police tetra mast close by)

http://www.tetrafiltering.co.uk/

[A1944]

Although it is very regular, that signal doesn't sound much like radar to me, at least not the sort I used to hear around Gatwick (the 50cm radar between TV Bands IV and V closed years ago).

As for direction finding, the advantages of using an AM portable, compared with a TV aerial, are that, as long as the inferference is audible on the medium wave band, the ferrite rod aerials are far more directional than TV aerials, usually giving quite a good sharp null compared with rather a broad maximum, and they are much more portable for wandering around the neighbourhood trying to hunt for the source.

[Keefbc]

Thanks again for all your suggestions.

There is a UHF bandpass filter fitted at the masthead - Vision V25-101 (460 to 870 MHz), so I assume that would take care of any tetra mast?

As for mains filtering, it is the only thing that I haven't tried. My thinking is that if this is very strong interference then it may be coming down the mains, as well as being airbourne, and reducing at least one of the sources may help the TV?

I've tried to tune into the interference on the AM radio band without much success unfortunately - off station, it's very strong and obvious on the FM band - there is so much other interference on the AM band that it's difficult to distinguish the cyclic signal I'm expecting to hear.

[A1944]

Originally Posted by Keefbc

I've tried to tune into the interference on the AM radio band without much success unfortunately - off station, it's very strong and obvious on the FM band - there is so much other interference on the AM band that it's difficult to distinguish the cyclic signal I'm expecting to hear.

Did you try rotating the radio while you were checking for it? If it happened to be aligned right on one of the two nulls, it could disappear completely.

[Keefbc]

Yes, rotated the radio through 180 degrees for all frequencies through the whole AM band - disappointed not to pick up some sign of it - also kept checking the FM band throughout to check that it was still there.

Nice sunny, bright, cold day here - interference all last night from about 3pm - still there this morning at 7am and still going now at 10.20am . . . . . so frustrating not being able to pin it down.

I guess most people would give up and go to Rupert's Sky dish or FreeSat

[Rodders53]

That Vision filter is http://www.dastv.co.uk/shop/diplexer...tra-Filter.php so it should stop Tetra!

As it doesn't affect AM radio then I don't think it is a sparking device ... I'm now more inclined to suspect a 'hooting head amp' - where an amplifying device and/or its power supply is in a some form of self-oscillation / feedback loop.

My advice is still to ask all your neighbours - especially those also afflicted - to go down the 'isolate all at the consumer unit' route when it's happening.

And get on to the BBC again (and get your affected neighbours to call them, too). As I understand it, the BBC can call on Ofcom agents to trace and enforce the shut-down of the offending item (the BBC have no powers to do that, they're just acting as a technical 'filter' to screen out receiving equipment issues).

Mains filtering won't help in my honest opinion - unless the FM radio noise gets significantly worse when plugged into the mains... batteries are the best mains filter you could possibly get, if you see what I mean!

[Keefbc]

Thanks for that.

It looks as if I need to convince 'all' my neighbours to switch off and see if we can track it down. If not Ofcom and their detection equipment should be able to track it down with relative ease.

I did drive around the area in my car and tried to pick up the edges of the interference using the FM radio tuned to 90.00 MHz. If I plot an approximate circle of where the signal seems to fade/drop off, it certainly points back to a source point within the housing estate.

I'll keep this post updated with any developments for those that come after me.

[Gavtech]

Originally Posted by Keefbc

I'll keep this post updated with any developments for those that come after me.

Please do.

We want to know what happens here... and help in any way we can.

[A1944]

Keefbc:

Any progress yet?