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-   -   DX FM reception. (https://www.vintage-radio.net/forum/showthread.php?t=179271)

SteveCG 23rd Apr 2021 8:30 am

DX FM reception.
 
The high pressure over the UK has finally brought me some long distance FM today. I'm on the West side of the Malvern Hills but today I've been receiving BBC Radio 3 from the East: Wrotham (91.3 Mc/s), Oxford 91.7, and the real prize for me, Tacolneston on 91.9. All in hiss free Stereo with RDS. The aerial is a loft mounted J-Beam 'H' model SBM2 pointed ENE to the Hills. The tuner was a Teac T-H300. In addition I may have been getting a Dutch station on 87.6 Mc/s.

Yesterday evening I detected meteor pings (Lyrid) on 87.5 on a plain dipole mounted to receive from N-S.

paulsherwin 23rd Apr 2021 9:32 am

Re: Dx fm
 
I did hear some co-channel mush yesterday, a sure sign that there's a lift on here. That was with a bog standard 70s portable.

SteveCG 24th Apr 2021 7:12 am

Re: DX FM reception.
 
Some more tropo DX this morning - Wrotham & Oxford again (both BBC Radio 3) - but not quite as strong. Tacolneston was back to its usual v. weak state.

For me, on this particular receiving set-up, Wrotham is usually blotted out by Llandrindod Wells and Oxford is weakish mono.

Hartley118 24th Apr 2021 9:11 am

Re: Dx fm
 
Quote:

Originally Posted by paulsherwin (Post 1367153)
I did hear some co-channel mush yesterday, a sure sign that there's a lift on here. That was with a bog standard 70s portable.

This is bad news for those of us in the East of England who like to enjoy high quality FM stereo. Here in Cambridge, under ‘lift’ conditions, stereo FM is no longer fit for purpose because of co-channel interference. Had the band been as crowded back in the 1960s as it is today, I guess that the current stereo FM system might have been rejected as too vulnerable.

These problems are despite a 4-element external aerial and decent tuner(s), one with selectable IF bandwidth. In particular, there’s a continental station that causes annoying ‘birdies’ on the Radio 2 frequency of 88.9 MHz, forcing me to switch to DAB to silence the background. I guess that the North Sea offers rather little attenuation under these conditions.

Martin

SiriusHardware 24th Apr 2021 10:02 am

Re: DX FM reception.
 
I'm finding this interesting because I keep one eye on the Hepburn Tropo forecast here:-

https://dxinfocentre.com/tropo_nwe.html

..and at the moment the Tropo activity level is nothing special. I'm wondering if some other mode is responsible for this current activity. We've just had the Lyrids meteor shower but any effects due to that would be quite short-term (I would have thought).

SteveCG 24th Apr 2021 4:48 pm

Re: DX FM reception.
 
SiriusHardware,

Well I agree that meteor events (usually of duration about 1 second) would not explain what I've been receiving.

For what it is worth, I set up my Eastward looking system back last Sept/Oct (before it got too cold to want to go up into the loft!) and have been waiting for any settled weather to see if 'interesting' tropo propagation could happen in the Wintertime compared to the known Summertime effects. And the answer has been for me 'not a lot'. I've had Wrotham and Oxford a couple of times but not what I was after - Tacolneston - until now. Last summer I did receive Dutch FM so I knew that this was a possible path (via knife edge diffraction over the Malvern Hills).

I reckon that ducts are sometimes formed at a height where the Tx aerials and the ridge of the Malvern Hills exist in them, hence my reception. Having a very gentle wind flow direction from the Tx to the Hills seems to aid this reception process.

As for satisfactory Stereo reception in Cambridge - oh, that used to be quite a story years ago...

Hartley118 24th Apr 2021 6:04 pm

Re: DX FM reception.
 
Quote:

Originally Posted by SteveCG (Post 1367552)
SiriusHardware,
.....Last summer I did receive Dutch FM so I knew that this was a possible path (via knife edge diffraction over the Malvern Hills).
...........As for satisfactory Stereo reception in Cambridge - oh, that used to be quite a story years ago...

I guess that Dutch FM may well account for my birdie problem. I can cope with a bit of pink noise hiss, but a twittering birdie is just too distracting.

As for the story of stereo reception in Cambridge, we feel very much part of that. When we moved to this “Little place in the Fens” in the 1970s from the civilised metropolis of Manchester. FM reception seemed just as relatively primitive as the local rail service and road system.

Far from the clean stereo Holme Moss signal we’d previously enjoyed, the Cambridge FM service (from the top of a gasometer) was strictly mono. So I’ve always been something of an FM DX enthusiast because the only available stereo signal then was from Wrotham, which had to scramble over the Chiltern Hills. (That of course was a similar problem to the one that led Pye of Cambridge to develop such expertise in fringe area TV reception). Tacolneston was an alternative, but that too was only mono, and its audio bandwidth sounded to be limited to around 7kHz.

Today, everything’s better now that high tech Cambridge has arrived in the 21st century, with fast electric trains, a motorway, and a nearby international airport. We also have a local stereo FM transmitter - on the mountain top in Madingley. But it’s pretty weedy and, here in South Cambridge, it does need a 4-element roof mounted aerial array to get what I’d call noise-free stereo. But then stereo multiplex FM always did seem to me outrageously greedy for RF input volts.

The trouble is that my high gain aerial is just too good at receiving Continental signals to interfere with the local service!

Martin

G6Tanuki 24th Apr 2021 6:43 pm

Re: DX FM reception.
 
I too have noted 'burblies' this afternoon when listening to Janice Long's show on Greatest Hits Radio [107.7MHz, Swindon Tx] on my Roberts R707.

The enhancement hasn't, alas, extended as far as 144MHz.

Synchrodyne 25th Apr 2021 12:24 am

Re: Dx fm
 
Quote:

Originally Posted by Hartley118 (Post 1367426)
Had the band been as crowded back in the 1960s as it is today, I guess that the current stereo FM system might have been rejected as too vulnerable.

An intriguing thought, although back in the 1960s I don’t think that there were any alternative systems for FM stereo that would have been materially better than the chosen Zenith-GE system. Maybe the “problems” could be moved around somewhat, but not eliminated. Other approaches suggested during the 1950s and 1960s included FM and SSB subcarriers, but these voided the interleaving that Zenith-GE offered, meaning that baseband modulation depth was reduced, in turn requiring higher transmitter powers and/or more transmitters to compensate. Whether the Russian polar modulation system, developed for ±50 Hz deviation and upper Band I transmission frequencies, offered any greater resistance to co-channel and adjacent channel interference I don’t know.

I think that basic problem was that the mono FM system, with a deviation ratio of 5 referred to a 15 kHz upper modulation frequency, was quite robust, but that robustness disappeared once subcarriers with upper sideband frequencies above 30 kHz were added, the deviation ratio then dropping towards 2. Not only that, although the subcarrier modulation itself was subject to pre- and de-emphasis, the subcarrier as a whole was not. And part of the robustness of the mono system was its pre- and de-emphasis (originally 100 µs in the USA, dropping to 75 µs in 1945, with 50 µs I think developed in both Russia and by the BBC around 1944-45).

Companding of the subcarrier was a possibility, although that adversely affected interleaving. It was actually tried with the FMX system, but that was a failure. DBX companding was used with the American Zenith-DBX (MTS) TV stereo sound system, but in that case the overall deviation was increased so that the baseband was unaffected, and interleaving was not required. (As an aside, the DBX effects were not completely inaudible.)

From a reception (of the Zenith-GE system) viewpoint, using only the lower sideband of the subcarrier might help, and I recall that that was actually done by one of the Japanese hi-fi makers (although I forget which). Evidently the idea did not catch on, which suggests that the typically realizable improvement did not justify the complication, at least absent a dedicated IC. (The FMX IC, with quadrature demodulation of the subcarrier, might have been a reasonable starting point.).


Cheers,

stuie319 25th Apr 2021 1:14 am

Re: DX FM reception.
 
Hi,

Travelling back from Sheffield to Manchester on Friday afternoon over the Woodhead pass, I discovered Radio 4 was suddenly replaced by a very strong Italian station. In the old days this would have me rushing home to switch on the D100 convertor, or my old Thorn 1400 set to 625 VHF and searching for European or Russian TV signals. Alas this is no longer possible. It hadn't occurred to me to listen on 2 metres. Whilst I do possess a two echo ticket, I've never ever actually used it

Best

Stu

Radio Wrangler 25th Apr 2021 1:26 am

Re: DX FM reception.
 
FM enjoys a signal to noise advantage over AM "The FM advantage", where the occupied bandwidth of the modulated signal is unnecessarily large.

You can view this as the FM modulation process produces multiple pairs of sidebands for each component of the baseband. With wideband modulation from wide deviation, these can be seen as many duplicate transmissions of the baseband.

The FM process puts all these components back together and convolves the baseband back into existence. In this process the noise floor of the occupied bandwidth is also convolved, leading to a stronger amount of noise in the resulting baseband. The difference is that the modulation sidebands correlate with each other as they convolve, and add as voltage, while the noise components, being random, cannot correlate with each other and add as power. So although the noise gets stronger, the baseband gets increased by a greater amount. FM can be viewed as one sort of spread spectrum modulation, complete with a spreading gain.

With stereo done as a difference signal as DSB on a high frequency carrier added to the baseband, the difference signal is shifted high in the baseband spectrum. Even with the relatively small increase in channel bandwidth, the difference information gets a lower modulation index, fewer almost-duplicate FM sideband components to add together in the demod process. So the difference signal components suffer lack the same S/N ratio benefits the lower frequency baseband (sum signal) components get. This is why stereo signals, when the stereo decoder comes on-line have significantly worse S/N than you'd expect from what there was in mono. To get back to what you would have had, you need to collect a stronger signal so coverage areas shrink, or more aluminium needs to go up in the sky.

The zenith pilot-tone stereo system inevitably had to involve a compromise, and the compromise's result in terms of a fast drop in audio S/N as the decoder switches in is clearly obvious, but the mechanism for producing that loss of S/N is not obvious to people not versed in modulation theory. Most people see it as something which just 'IS', and wonder if it can be bettered, without realising that it's mathematically inevitable. To counter it we would need either more transmitter power or a lot more bandwidth. The old 'no such thing as a free lunch' business.

Of course, some skilled and dedicated firms did manage to make tuners/decoders which were very noticeably worse than theoretical....

David

SteveCG 25th Apr 2021 4:12 pm

Re: DX FM reception.
 
... not forgetting Ray Dolby's 1970's foray into Stereo FM S/N improvement using Dolby 'B', albeit not quite mono receiver compatible ...

P.S. Hartley118 - you could have added Peterborough to your list of mono transmitters back then.

bikerhifinut 25th Apr 2021 9:09 pm

Re: DX FM reception.
 
Quote:

Originally Posted by stuie319 (Post 1367709)
Hi,

Travelling back from Sheffield to Manchester on Friday afternoon over the Woodhead pass, I discovered Radio 4 was suddenly replaced by a very strong Italian station. In the old days this would have me rushing home to switch on the D100 convertor, or my old Thorn 1400 set to 625 VHF and searching for European or Russian TV signals. Alas this is no longer possible. It hadn't occurred to me to listen on 2 metres. Whilst I do possess a two echo ticket, I've never ever actually used it

Best

Stu

This morning we had our regular eden valley "lockdown" net on 2m using theGB3EV repeater and we had some pretty good propagation with one of our members working portable with a rucksack mounted set and another on an electric mountain pushbike. Both were readable in locations we'd expect difficulties in the lakes.
Myself I was having issues from a home made dipole but once a quick and dirty fix was made with 6 foot of bamboo beanpole to get the vertical dipole away from the house structure both reception and my maximum 10W emission were going 5 9 to most in the net.

So perhaps you might have some fun with a Ł25 baofeng handheld and a tenner or less spent on an alternative whip antenna as afetr todays net I scrounged a n aftermarket whip antenna that turned my toy baofeng into a usable device.

The amatuer bands right now are in a state of revival due mainly to the restrictions i guess but if it carries some momentum then who knows.

73

Andy M7ELN (soon to be 2E0*** I hope, and beyond if i learn enough)

SiriusHardware 26th Apr 2021 7:36 am

Re: DX FM reception.
 
Quote:

my maximum 10W emission were going 5 9 to most in the net
A minor clarification Andy - when you are working through a repeater, the signal strength reading the other users see is the strength of the repeater's transmitter, not yours, so any signal strength figures offered to you when you are working through a repeater will be a little bit misleading.

However, it sounds as though your aerial upgrade allowed you to hit the repeater input with a signal sufficiently strong as to be noise-free so it worked, and that kind of experimentation is what the hobby is all about.

Hartley118 26th Apr 2021 8:58 am

Re: DX FM reception.
 
Quote:

Originally Posted by SteveCG (Post 1367878)
... not forgetting Ray Dolby's 1970's foray into Stereo FM S/N improvement using Dolby 'B', albeit not quite mono receiver compatible ...

P.S. Hartley118 - you could have added Peterborough to your list of mono transmitters back then.

Clearly I wasn’t alone! Ray Dolby also lived in Cambridge and wanted to rectify the weaknesses of FM multiplex stereo.

As I remember it, the Peterborough transmitter in the 1970s was a rather low power relay of the Tacolneston service which was of poor quality anyway.

Today, Peterborough, which was the last transmitter to go stereo (in 1985), gives a reasonable signal here and is a useful standby when the Cambridge reception is marred by birdies. I sometimes wonder whether the birdies are actually retransmitted by the Cambridge booster as an integral part of its service.

Martin

SteveCG 26th Apr 2021 9:50 am

Re: DX FM reception.
 
Receiving satisfactory Stereo FM in the 70's occupied many an academic mind in your neck of the woods. There was also Capitol Radio to be had - provided you were a DXer!

Hartley118 26th Apr 2021 10:22 am

Re: DX FM reception.
 
Quote:

Originally Posted by SteveCG (Post 1368056)
Receiving satisfactory Stereo FM in the 70's occupied many an academic mind in your neck of the woods. There was also Capitol Radio to be had - provided you were a DXer!

Yes, we had a massive J-Beam array on the Neve building in Melbourn near Cambridge purely for the purpose of receiving Capital Radio for the factory PA. Mono only of course.

Martin

bikerhifinut 26th Apr 2021 7:25 pm

Re: DX FM reception.
 
Quote:

Originally Posted by SiriusHardware (Post 1368022)
Quote:

my maximum 10W emission were going 5 9 to most in the net
A minor clarification Andy - when you are working through a repeater, the signal strength reading the other users see is the strength of the repeater's transmitter, not yours, so any signal strength figures offered to you when you are working through a repeater will be a little bit misleading.

However, it sounds as though your aerial upgrade allowed you to hit the repeater input with a signal sufficiently strong as to be noise-free so it worked, and that kind of experimentation is what the hobby is all about.

You are quite correct and I stand corrected on my statement!
What I should have said was that one of our members makes a point of listening in on the relays receive frequency 600kHz downband and then passes on signal reports from his QTH which is about 15 miles away but over a quite steep hill from us so when I got those reports I knew I was making progress!
Andy.

SteveCG 27th Apr 2021 8:38 am

Re: DX FM reception.
 
Given the time of year, I'll be listening out more for SpE propagation - principally in the 87.5 to 87.9 Mc/s part of the spectrum which is the only segment that is normally clear for me.

BTW I wonder whether Dolby B applied only to the stereo sub-carrier component might have assisted fringe area reception?

SiriusHardware 27th Apr 2021 8:45 am

Re: DX FM reception.
 
I keep an occasional eye on 10 / 11 metres which is normally a good early warning band for Sporadic-E, then if it's very lively there I look at 6m and 4m. SP-E Activity on 10/11 normally starts to be a more or less daily occurrence from mid-May onwards and continues until late summer.


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