Mini-Mod MW 'Micro Transmitter'

[Terry]

... you can't have the mini-mod in one room and the radio in another ...

David, this was the part of your post that I was addressing ...

[Ian - g4jqt]

I designed a higher power crystal-controlled  MF 'modulator' that appeared in the Autumn 2000 BVWS Bulletin and published it under a pseudonym. That certainly has a far greater range and could be tuned up into a variety of antennas! The circuit isn't much more complex than the MiniMod.

The advantage of crystal control is that it can easily be fine-tuned up to within a few Hz of an MF channel, removing (almost) any co-channel heterodyne. The disadvantage is that a MW frequency crystal can cost up to £30 to have made!

Of course there are various alternatives using synthesizers or mixing, but I think it's debatable whether it's worth the complexity for radiating a few milliwatts.

(There is or was a commercially available 'pantry transmitter' that used a synthesizer circuit first published by the Medium Wave Alliance...)

Anyway, there are many, many designs for these devices, all giving decent results.

Ian

[Terry]

Of course there are various alternatives using synthesizers or mixing, but I think it's debatable whether it's worth the complexity for radiating a few milliwatts ...

I was fascinated by the WARC-79 decision to move most of the MW channels by 1kHz so that they are all multiples of the 9kHz channel spacing. LW channels moved 2kHz for the same reason.

This was said to have been done to ease the design requirements of digital receivers but, as the IF frequency is not a multiple of 9kHz, then the local oscillator frequency can't be either! Therefore, I fail to understand the reasoning behind the argument ...

However, in this instance it might help as any handy crystal which is a multiple of 9kHz can be divided down to 9kHz with another preset divider controlling the output frequency.

Might be an interesting experiment for someone!

[Ian - g4jqt]

Drifting temporarily off topic...

Intermediate frequencies varied from make to make, and often had changes for certain locations to overcome heterodynes.

But modern AM sets (if you can still buy them!) standardised on 455 kHz across the globe.

455 x 2 = 910. When Radio Five started on 909, there were listener complaints about a 'tunable whistle'. Presumably because the station was very widely listened to on a variety of sets.

The joggling onto 9kHz spacing on MW (actually all multiples of 9, e.g. 891, 900 kHz, 909, etc.) was intended to not only make synthesized set design easier, but to get the few European stations to get onto the correct channel; there always had been a few that had odd 1 kHz offsets to the main channel causing hets. It took some months to get a few Portuguese and North African stations on the right channel - where they'd never bothered before. Then there was (and still is?) Albania on approximately 1215 kHz!

The MW shifts were done almost entirely over one night, but LW took longer, done in two or three stages - I can't remember now. Although there are fewer LW stations, they usually run more power, and a 1 or 2 kHz shift is a lot to adjust the antenna for at these frequencies!

There is an inconstancy to the LW 9 kHz spacing; two stations on either side of the old East/West German border had to split a channel. That's why digital long-wave tuners don't jump in 9 kHz steps. I'm sure someone can come up with the names and locations of these two stations, but I expect one if not both of them have now closed.

Then there's the whole area of what frequency accuracy is necessary, internationally and domestic synchronised groups like 693, 909, etc. But that's drifting way off topic...

Ian

[Terry]

... modern AM sets (if you can still buy them!) standardised on 455 kHz across the globe.

455 x 2 = 910. When Radio Five started on 909, there were listener complaints about a 'tunable whistle'. Presumably because the station was very widely listened to on a variety of sets ...

Sorry, Ian, but you are wrong there on two counts! 

Firstly, 455kHz is the US standard IF and was, therefore, adopted for all the sets made in the far east, initially for the US market. For some reason, nobody seems to have ever persuaded these manufacturers to move the IF to the European 470kHz for their exports aimed in this direction!

It has not, as far as I am aware, been adopted for use in Europe or 'globally standardised' as you put  it

Secondly, this problem first became apparent long before the arrival of Radio Five when what our American friends call 'vest pocket radios' first landed on these shores in 1962!

455 x 2 = 910, as you say, and was 2kHz above the 908kHz transmissions (pre WARC-79) of the Home Service! The resultant whistle tended to go unnoticed in those days, though, as the vast majority of the users of such sets were youngsters only interested in the Light Programme and Radio Luxembourg plus, later, Radio Caroline et al ...  

[Ian - g4jqt]

Hi Terry,

OK, wrong when I said "globally standardised", since as far as I know, no international agreement came to this decision. But for the past 30 years or so, any new MW radio I ever bought, or saw someone buy, came from the Far East, with fixed ceramic filters at 455 kHz. Even those without ceramic filters had an IF of 455 kHz, so we got it by default.

Nevertheless I may be wrong if there are still moderately-priced MW radios assembled in Europe, or especially for Europe with a non 455 kHz IF.

I wonder what the IF is in modern analogue Roberts Radios - assuming they're still using some sort of superhet circuit?

In old service sheets I've seen reference to some British manufactures changing the IF when new stations appeared at or near twice the IF, so indeed, this problem was known about.

I believe planning FM use had to take into account 10.7 MHz IF and local oscillator radiation, but expect that's now a thing of the distant past!

Ian

[Old Sparky]

One of the two stations on LW is R. Europe #1, which broadcasts from Saarlouis on 185Khz,instead of the 'correct' frequency of 180Khz, the other, in Oranienburg,Eastern Germany, is/was on 178hz, and may not still be on the air. Some of the ex-DDR Stations became part of MDR (MittelDeutscheRundfunk) after reunification.

[AlanBeckett]

There's a Wireless & Electrical Trader Sheet for 'Old' Sets.

Alan


  if-values.pdf (Size: 675.61 KB / Downloads: 23)

[Yorkie]

That's a very useful doc Alan - what a shambolic state of affairs though, given the wide range of I frequencies back in the 30s/40s!

It would be rather nice (well I think so anyway), to have a  design for a micro-transmitter that uses valves, along the lines of those ubiquitous Chinese 'Little Bear' valved headphone/pre-amp amplifiers.

A bit like this sort of thing. They run in 12V and I assume they have a tripler or some such thing to raise the voltage for the HT:

http://www.ebay.co.uk/itm/like/291244381244?adgroupid=&hlpht=true&hlpv=2&rlsatarget=&adtype=pla&ff3=1&lpid=122&poi=&ul_noapp=true&limghlpsr=true&ff19=0&device=c&chn=ps&campaignid=&crdt=0&ff12=67&ff11=ICEP3.0.0-L&ff14=122&viphx=1&ops=true&ff13=80

Yes, I know they're not to everyone's liking, but I think they look quite dinky.

[Joe]

I've got a couple of designs for valve pantry transmitters, one by our own MurphyV310, but have never built them as I don't trust them as regards harmonics.  I'm no expert — FAR from it — but I can't see anything that looks vaguely like a low-pass filter on them.

There was also a fearsomely complicated one using EF91s in the Bulletin a few years ago, though again no discernible LPF.

Maybe I'm paranoid but my understanding is that harmonics can travel further than the fundamental so for all that the later may not be getting far enough to be a nuisance and/or attract attention the former could well be.

Luckily most of the old wirelesses I actually use have got Gram sockets on so a DAB signal can be shoved up those.

— Joe