Absolute Radio 1197kHz.

[hannahs radios]

I noticed up until yesterday that absolute radio on 1197 kc/s had a slight hollow sound to it down to there being a delay in the audio chain to different tx sites now I know this is nothing new but yesterday and today there is about a quarter of a second delay the audio from the nearest tx to me now sounds more solid but the delay is really offputing any one else notice this? I'm in Weymouth so I here the Bournemouth relay and presumably the Torquay relay also there is some severe phase distortion at times

[paulsherwin]

1215kHz has always been a lousy network right back to the launch of Radio 1 in 1967, when it was cobbled together from various old Light Programme infill relays. 1197 is just a frequency to provide infill for the infills. The 1197 infill here in Oxford is awful so I wouldn't be surprised if yours is the same.

1215 at least used to be a synchronised nework in the BBC days, but now I think each transmitter uses the most convenient source, which will be either satellite or internet. This can cause significant echo effects if you can hear more than one transmitter.

[G4XWDJim]

I live about ten miles as the crow flies from Droitwich and with a couple of feet of wire as an aerial I can hear the second harmonic of Absolute on 2430 and the third on 3645 very strongly. I thought once that it must be a problem with my receiver but I've heard it on many different receivers and all the time with just short aerials to minimise the signal strength. On my full aerial it wipes everything out. I've just listened to it now on a comms receiver with a first IF of about 70 mHz so I know it's not going to be a receiver problem.

A friend in Droitwich suffers the same problem but another friend in Kinver doesn't.

Their transmitter engineers advanced the 'rusty bolt' theory but where is the rust bolt. On their mast or somewhere near me.

I walked along my road with a portable receiver tuned to the third harmonic and at every telephone pole the signal strength increased.

The only time in the last 7 years that this has not been a problem was when we had a substantial snowfall perhaps shorting out the 'rusty bolt'.

I'd be curious to know if others here notice a similar problem.

Jim

[Herald1360]

I wonder if there's a connection here between 1197 and 198?

Maybe the "rusty bolt" is something in the telephone network being overloaded by 198 pickup?

Kinver is further from Wychbold than yourself or Droitwich.

[Ian - G4JQT]

I've known BBC HF stations put out significant second (and third) harmonics but it was after transmitter work and the output filters had been left unconnected or the wrong ones fitted in error.

The rusty bolt theory sounds like a bit of a red herring in this case as the spurious signal is so strong.

Maybe an email to some technical department (clutching at straws here I know - even if you find the contact details!) may get a response?

Ian

[G4XWDJim]

I've mostly lost interest in amateur radio and have been QRT for some time now so it isn't too much of a problem for me - except for being annoyed about it on the principle that it shouldn't be happening. A healthy degree in apathy though stops me taking it any farther.

Jim

[Phil G4SPZ]

Jim and I live not far apart, at similar elevations above sea level. I can see the Droitwich masts from here, yet I can hear nothing on the second or third harmonics of 1,215kHz.

One clue may be the presence or absence of overhead telephone lines near your QTH, Jim. For what it's worth, all telephone lines here run underground.

[Hybrid tellies]

Originally Posted by hannahs radios

I noticed up until yesterday that absolute radio on 1197 kc/s had a slight hollow sound to it down to there being a delay in the audio chain to different tx sites now I know this is nothing new but yesterday and today there is about a quarter of a second delay the audio from the nearest tx to me now sounds more solid but the delay is really offputing any one else notice this? I'm in Weymouth so I here the Bournemouth relay and presumably the Torquay relay also there is some severe phase distortion at times

I would imagine that you would get co channel interference between Bournemouth and Torbay especially as the latter is a nice straight sea path. Looking at the coverage map the best signal for you would be 1215Khz from Washford. Try using the directional property of the frame or ferrite rod aerial to null out the Droitwich or Brookmans Park signal.

[G4XWDJim]

Phil, the many miles of phone lines must pick up and re-radiate all sorts of signals. Within their system BT must have thousands of non linear junctions that do many strange things to radio signals. It's interesting to hear that that you pick up no apparent harmonics which is a plus point for the transmitter.

But why Absolute radio? I suppose if I think about it perhaps BT's lines re-radiate many other signals but I've only been alerted to 3645 because it's in a band where I used to operate.

I'd like to find the answer but it's all a bit academic now.

Jim

[Phil G4SPZ]

Good point, Jim. Have you tried listening for the harmonics of other strong MW broadcasters, such as Radio Five Live on 693 or TalkSport on 1053? Both are transmitted from Droitwich. Although as you point out, their second and third (or higher, for that matter) harmonics fall outside any amateur band, so you'd be unlikely to notice them in your normal pattern of listening.

Just trying to establish the frequency response of your local Rusty Bolt, you understand! Out of interest, are your local BT lines carried overhead?

[michamoo]

I have something similar to that happening here also. Absolute radio here is also on 1197khz from a relay which is about 10 miles away at Cambridge. Now apart from receiving it on medium wave it is also possible to receive it at the far end of the Long wave band also. This is the case on all the radios I have here and is not a problem just a bit odd really.

[Phil G4SPZ]

I think this is simple 'image reception'.

In a superhet receiver, the IF stage responds to the sum OR the difference between the local oscillator and the signal frequency. Usually, the local oscillator operates above the wanted frequency, so for example in a receiver with an IF of 465kHz, the image is 930kHz above the frequency to which the set is tuned.

Doing these sums backwards shows, for a typical superhet, that the image of a frequency in the long wave band of around 267kHz is indeed 1197kHz, and a strong signal such as your local Absolute Radio relay on that frequency will "appear" to be received in the long wave band.

Jim's problem is not image reception, as he picks up the spurious signal on a receiver with a 70MHz first IF; this receiver's 'image' will be in the VHF region.

[Ian - G4JQT]

Originally Posted by michamoo

Now apart from receiving it on medium wave it is also possible to receive it at the far end of the Long wave band also. This is the case on all the radios I have here and is not a problem just a bit odd really.

An image in the receiver.
2x465=930
1197-930=267 kHz

Or thereabouts depending upon the receiver IF. Quite likely to appear in all your domestic radios.

I'd guess the 1197 kHz signal is pretty strong?

Ian

[Nuvistor]

Hi Ian,
Would the MW LO not be on the high freq side of the wanted station so image would be 1197+930=2127?

Frank

[michamoo]

Yes we have perfect reception of Absolute on 1197khz day and night here even though the TX at Cambridge is only listed as 200 Watts. The main 1215khz signal is awful though from Brookmans park all the time with distortion and always was right back to when it was used for Radio one and then Radio three.

[Phil G4SPZ]

Originally Posted by Nuvistor

...Would the MW LO not be on the high freq side of the wanted station so image would be 1197+930=2127?

Yes indeed Frank, and if there happened to be a strong signal on 2127kHz you might hear it competing or heterodyning with Absolute Radio, but when the radio is tuned to 267kHz on the Long Wave band, 267+930 = 1197 too.

Most domestic sets have the LO operating on the high side of the signal. This is due simply to the range of capacitance swing available from mechanical variable capacitors - don't forget that the superhet was invented, by Edwin Armstrong, not long after the end of the First World War! The rule is that the "image" is twice the IF away from the wanted signal; if the LO is above the wanted signal, the image will be higher, and if the LO is below the wanted signal, the image will be lower.

[Nuvistor]

Hi Phil,
Of course it would, time for me to call it a night I think and I will try again tomorrow.
Frank

[ianj]

Nice reception here in cheltenham of 1197khz. No problems.

[G4XWDJim]

Originally Posted by Phil G4SPZ

Good point, Jim. Have you tried listening for the harmonics of other strong MW broadcasters, such as Radio Five Live on 693 or TalkSport on 1053?

are your local BT lines carried overhead?

I have now this morning tried listening to harmonics of those other frequencies at the nearby telephone poles with their overhead wiring using my little Sony ICF 7600 portable receiver. Interestingly non of the regular strong stations show a harmonic, only Absolute radio. They are all strong but clean.

A true harmonic surely can only be generated in the transmitter. You can see the masts and don't detect a harmonic which suggests there aren't any. So where is this apparent 'third harmonic' coming from. It seems pretty clear now that it's being generated within BT's phone system but how?

An even bigger question - what can be done about it.

Jim

[Phil G4SPZ]

The BT overhead wire network is quite elderly and, by now, probably contains a myriad of redundant, faulty or disconnected conductors. It seems that there's a length of wire about 120 metres long somewhere that forms a resonant half-wavelength at 1,214kHz with a rusty bolt detector at one end. This will generate a high RF voltage at its ends. The same wire will also be resonant at the third harmonic and will re-radiate effectively.

As for how to cure it, I've no suggestions, but it could be worth having a word with one of our retired GPO engineer acquaintances...