Interference on 198kHz, BBC Radio 4

[ukcol]

The work on my Murphy A40C is going well, but its performance on Radio 4 Long Wave is spoilt by a particularly annoying type on interference.

I have recorded it but the forum doesn't support uploading or linking to mp3 files so I will attempt to describe it.
It sounds digital possibly about a 1kHz square wave but there is no chatter that suggests it is carrying information. It is likely that the ear is not sensitive to the type of modulation used perhaps.

The A40C uses a low I.F. (119kHz) so I thought that perhaps the interference was caused by an image frequency. I disabled the local oscillator and provided a local oscillator from a signal generator, first at 317kHz and then at 79kHz. The interference was the same on both frequencies so it was not an image problem.

I then listen to R4 on my Bush DAC90A which has a 465kHz I.F. (why didn't I do that in the first place ) and the interference was also present but at a much reduced level.

I did the same thing with a Decca TP22 which is a battery transistor portable. This set has a 472kHz I.F. and the interference, although present, was at an even lower level then the Bush.

The Bush has a frame aerial and the Decca has a ferrite one; this led me to think that the better performance of the Bush and the Decca was down to the aerials being direction, so I rotated the Decca to see if I could null out the interference and keep R4 strong but R4 and the interference dropped at the same angle of rotation.

I walked toward my house from the workshop with the Decca and then near to my neighbours houses but there was no increase in the interference.

When I returned to the workshop and put the Decca down on the bench there was a significant increase in the interference. There was nothing switched on except for the lighting and I turned the lights off and there was no change.

However when I unplugged an extension lead which was trailed across the bench the interference dropped considerably. The equipment connected to the extension lead was switched off so perhaps the interference is being carried on the mains and radiated from the wiring, extension leads etc?

Does anyone recognise my description of the type of interference?

I use a simply wire aerial for 1930s and 40s mains radios. Is there a more sophisticated type of AM aerial installation I could use that might provide a solution to this problem?

Colin M

[hkn102l]

I have "Suffered" the same symptoms and think that it emanates from the You View box which uses my ring main to distribute "FreeviewTV service. Of course, the use of energy saving lights is a well known source as well as PCs or laptops!!
cheers
John

[Nuvistor]

Belling Lee (I think) produced an aerial that had a matching transformer at each end of a screen down lead. I don't know more about it but we had one at home in the early 50's when I were a lad. Dad was convinced it helped, I cannot remember if it did but the info at the time stated it was made to help reduce local interference picked up near the house. Perhaps it was a loaded dipole of some sort.

[Nuvistor]

Just found a thread on this forum.
https://www.vintage-radio.net/forum/...ad.php?t=38209

[emeritus]

Both Belling Lee and Aerialite were offering this sort of thing circa 1952: see attached PDFs of contemporary literature.

No constuctional details are given of the Aerialite version.

While the BL leaflet does provide a circuit, the catalogue reproduction has very poor resolution; in the enlarged hi res scan it can just be made out that electrostatic screening was provided for the set-side transformer. Interestingly enough, the leaflet indicates that the special screened balanced feeder (no indication of characteristic impedance stated) that had been specially designed for these transformers was being withdrawn, and would only be available while stocks lasted. Different models were made for different frequency bands. The patent that covers BL's design, GB 479118, indicates that the design dates from about 1936, and gives more detailed technical information, although not the characteristic impedance of the feeder.

[Glowing Bits!]

When setting up sets, or at least finding faults with them, I have to turn off the UPS in the cupboard, followed by putting the mains off to it as it creates lots of interference, especially on LW.

Colin, does the extension lead have a neon? If so, this may be worth disconnecting it to see if it helps anything.

[ukcol]

Originally Posted by Glowing Bits!

Colin, does the extension lead have a neon?

No it doesn't. I should make it clear that the interference has quite a high frequency (audio) sound and is tunable. It is only present on R4.

Thanks to all for the info above I have saved the files and shall have a read later.

I am going to try and see if I can see anything on the mains with a scope. I shall make up the filter below. C1 will be a class X capacitor which with R1 will form a "safety" filter. C2 and R2 will form a high pass filter to pass the signal to the CRO. I will work out suitable values for the components.

I will let you know what, if anything, I see.

Colin M

[Fezziwig]

Can't you just listen on FM or DAB?

How long can 198 last with the valve shortage?? It must cost a fortune in electricity as well. You can't even buy LW sets easily these days and many car radios don't have it.

[ukcol]

Originally Posted by Fezziwig

Can't you just listen on FM or DAB?

I can and I do.

For me the whole point of restoring a vintage radio is to use it. R4 L.W. is one of the few stations remaining on A.M. worth listening to (IMHO).

I also have a mini transmitter transmitting at the LF end of the MW band that I use with various sources.

If I can solve this problem I would like to listen to R4 on L.W. without interference. If I can't solve it, it won't be the end of the world.

Colin M

[turretslug]

I'm, er, less than convinced by the 198kHz "valve" story- it sounds like the sort of thing that would be cooked up by deceitful businessmen in order to sow derision amongst an uninformed public- "why are you using this antiquated and obsolete service when you could have wonderful, crystal-clear DAB". Etc. Etc.

Nonetheless, it's an expensive-to-run and interference-prone service, and the proliferation of switching and signalling devices (SMPSUs, motor drives, PLT and so on) means it won't improve in the future. I think individual interference solutions will be down to perserverance and ingenuity- and be prepared for some surprising culprits- and, often, resignation that there's not much that can be done about it.

[ukcol]

These are my observations using the filter described.

As you can see there is a sine wave which looks slightly ragged due to either noise and/or modulation. The CRO timebase is set to 2uS/DIV and the Y-amp to 20 mV/DIV.

I count 9 cycles in 8 divisions which is 9 in 16uS.

t = 16/9 or 1.77.. uS
frequency f = 1/t
= 1/ 1.77.. x 10(-6)S
= 562500Hz or 562.5 kHz

The amplitude is 20mV x 2 peak to peak
or approx 14mV RMS.


__________________________________


Although I think this signal is the source of the interference, I am not sure how the frequency is interfering with R4 on 198kHz since there is no direct harmonic relationship. The fundamental or a harmonic of the local oscillator or the I.F may be involved but this will have to work for all 3 sets with their different IFs.


BTW I check the calibration of the CRO on the 2uS/DIV range using a generator and a frequency counter.

Colin M

Originally Posted by hkn102l

You View box which uses my ring main to distribute "FreeviewTV

And this also destroys shortwave listening for any neighbours. See http://www.ban-plt.co.uk/ I have suffered from this in the past but OFCOM got them removed for me.

[hkn102l]

Hi Merlinmaxwell: so what did Ofcom do for you?? BT are still selling the product so surely it can't contravene any "Rules"?
Cheers
John

[turretslug]

Btw, just been reading up about the Murphy A40C- what a marvellous beast! Hope the interference problem gets resolved, it would be frustrating if LW was hobbled by it on this set.

[dsergeant]

It will NOT be powerline interference from your Youview adaptors. PLT uses 2-28MHz, nothing down in longwave. Most likely it is radiation from a switch mode power supply local to you. One other possibility I saw yesterday is inductive charging systems for mobile phones and similar, which use frequencies around longwave (the system was designed in the USA which has no LW broadcasting). And of course it could be TV line output interference, there are still plenty of those CRT sets around pumping out stuff every 15.625 kHz.
Dave

[ukcol]

Yes the A40C is a nice bit of kit. The audio quality is remarkable due in no small part to the loudspeaker design.

I have just been having a look at the link provided by merlinmaxwell in post 12 and Power Line Technology looks like a good candidate for suspicion. I think a high aerial feeding a signal to the set by some screened or low impedance means may be the answer.

Colin M

[Keith]

Hi Colin,

As has already been suggested it is most probably a switch mode PSU, possibly even in your own property. Worth going round the house switching off everything you can find that might be the culprit - set-top boxes are often to blame.

I had a similar problem but that turned out to be a newly installed electricity meter which was singing away at ~100kHz plus harmonics. I was about to complain to NPower when a nearby lightning strike dealt with the problem for me. Obviously the design had failed to incorporate line transient protection as well as emc filtering! I'm happy to report that the replacement meter (differenet manufacturer) was totally quiet, RF wise.

Vintage sets fed with unbalanced aerials (i.e. single length of wire) are always more vulnerable than a portable with a ferrite rod. You could try a dipole type aerial fed through a balanced ATU. You could do this by connecting the dipole ends across a parallel tuned circuit (e.g. LW coil/variable-capacitor from a scrap set) and connect to the Murphy via an isolated link winding (a few turns close to the tuned coil). Tune the capacitor to peak the R4 signal.

Good luck,

[Hybrid tellies]

I suspect a nearby nasty switch mode psu possibly from a set top box or a wireless router. Plasma tv's throw out high levels of interference especially as they age. LCD sets are generally quieter except for some of the cheaper models. I am running a 15 year old Philips CRT TV which is very quiet across the LW band but produces a hash like noise across the top of the MW band.
In our house LW is clear except in part of the living room where there is something from next door causing a loud buzzing humming noise across the entire LW band and most of the MW band. I am afraid it is very difficult to trace interference from outside of your house but best of luck.

[ukcol]

Originally Posted by Hybrid tellies

I am afraid it is very difficult to trace interference from outside of your house but best of luck.

Yes, I'm afraid you're right and I've already eliminated my own house from the problem.

______________

Hi Keith

I'm pretty sure its not a SMPS because it tunes in at the R4 frequency with the rest of LW pretty quiet and if you look at the CRO display in post 11 it is a sine wave.

As it doesn't appear to be coming from anything within my property the solution is going to be in the aerial system, possibly along the lines you suggest.

Colin M

[majoconz]

Had a similar problem here in the late 90's - a constant whistle that would change pitch occasionally or sometimes disappear on our local equivalent of your R4, the "National Programme" on 675kHz. A neighbour had the same problem. I wandered around with a portable and found a good loud signal outside on the phone line and by the main power board. I took my Kenwood R2000 out to the front drive and powered it from a car 12v supply and a short length of wire lying near the phone line. In USB mode I tuned down from 675kHz looking for a similar signal - found plenty of harmonics on frequencies all multiples of 75kHz. Phoned the Post Office Radio Inspector's office and they came round in a few days to establish that I wasn't dreaming it, but they found the signal too. A couple of days later the signal disappeared, the man from the PO came back to report that they had found a stereo in a house about 200metres away that had a faulty power supply - a cracked ferrite cored transformer that was making nice square waves on or about 75kHz.

I guess that to make the signal audible, it needed a carrier to beat with. I don't begrudge paying my Ham licence fees now!