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Old 11th Oct 2011, 9:34 pm   #1
dave walsh
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Default "Off The Shelf" LW Transmitters

An item on today's PM [R4] was complementary about the BBC's preservation of it's vintage Marconi LW gear [as still in use] and made it clear that there is ready access to an international replacement product [ie a Transistorised LW TX] available since the early 80's! This was the sort of info I was looking for from the previous thread [closed due to an OT shift]. I'm wondering how these things compare with the valve predecessors? I hope this is within bounds even if it does confirm that a shortage of valves can't be the real issue. Failing that perhaps Mods can make it an "info only" posting?
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Old 11th Oct 2011, 9:42 pm   #2
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Default Re: "Off The Shelf" LW Transmitters

There is no such thing as an off the shelf 500kW LW transmitter. There are relatively few big LW transmitters in the world and each one is made to order.

The existing Marconi Droitwich transmitter isn't particularly old, having been installed in the mid 80s. The RTE Clarkstown transmitter near Dublin is about 5 years newer and is solid state.

There was an interview with a transmission engineer on PM over the weekend. Apparently the BBC (actually, probably Arqiva) have 10 final output valves for the transmitter in careful storage. These could easily last 25 years.
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Old 11th Oct 2011, 9:55 pm   #3
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Default Re: "Off The Shelf" LW Transmitters

Our latest baby is a RIZ (Radio Industry Zagreb) 250kW HF analogue / digital transmitter; one of several installed at Skelton, Woofferton and (analogue only) on Ascension Island.

I'm pretty sure RIZ could turn out an LF set to order (as, probably, could Nautel or Harris or Continental or Transradio). And yes, although brand-spanking digital new, it has a valve in it! A TH558. All S/S up to the valve, though. I won't get the thread embroiled in contentious political issues (for that would be quite wrong), but, suffice to say, I'm of the opinion that the valve factor at Droitwich is specious.
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Old 11th Oct 2011, 10:19 pm   #4
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Default Re: "Off The Shelf" LW Transmitters

There is no doubt that the BBC could continue LW broadcasting indefinitely if sufficient funds were available, so there's no point in discussing the matter. People who are upset will achieve more by writing to their MP rather than grumbling here.

This thread is about LW transmission technology, not BBC policy decisions. Please stay on topic.
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Old 12th Oct 2011, 2:48 am   #5
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Default Re: "Off The Shelf" LW Transmitters

Hi Paul. I was describing what I heard to the best of my ability. "Jens Seberg" from Radio Denmark, was invited, by the presenter, to say that the BBC was behind the times but he ignored this and instead, he expressed his admiration for skillful BBC Engineers and their work on the Marconi TX! He felt that the UK LW technical requirement was, overall, very similar to that in Denmark and Germany [where they make a "Trans Radio" solid state TX unit]. He also said that Canada produced a "Nautal" Tx unit which is the one that Radio Denmark are currently using but there was no explanation why they went overseas?
Worth a listen on I-Player!
Apologies if some of my spelling is incorrect. 25 years? Well the manager said 10 at least......so halve that!
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Old 12th Oct 2011, 7:30 am   #6
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Default Re: "Off The Shelf" LW Transmitters

Any new transmitter would require a considerable amount of capital outlay to commission, and it might take quite a while for the savings rendered through more efficient technology, less involved maintenance (or 'different' maintenance, at least) to pay for those savings. Unless different budgets were involved, which I doubt is the case.
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Old 12th Oct 2011, 8:53 am   #7
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Default Re: "Off The Shelf" LW Transmitters

Sticking purely to the technical here, handling 500kW with semiconductors is, I believe fairly easy these days. I say that, not because I know much about LW transmitters, but because multi-megawatt invertors are now fairly common for DC power transmission.

Its been realised that using AC (I mean your standard 50Hz stuff) for long distance power transmission is a bad idea because you lose a lot of energy due to the capacitance of the wires to ground. DC doesn't have this problem, and only cable resistance comes into play in this case.

Such DC power transmission has been put forward as a solution to extracting lots of solar energy from North African deserts and feeding it around Europe. In fact there is a project already up and running to make this happen (Desertec).

All this is a round about way of suggesting that a mere half megawatt on 200kHz is a fairly easy matter these days.......


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Old 12th Oct 2011, 9:40 am   #8
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Default Re: "Off The Shelf" LW Transmitters

Quote:
Originally Posted by trh01uk View Post
Sticking purely to the technical here, handling 500kW with semiconductors is, I believe fairly easy these days.
It has been easy for some time. You get some money, go to a transmitter manufacturer and arrange a contract. There are solid state 250kW HF/800kW MF transmitters on A'Seela (BBC) and Thomcast 250kW solid state HF transmitters at Nakhon Sawan, Thailand (BBC).

Quote:
Originally Posted by trh01uk View Post
All this is a round about way of suggesting that a mere half megawatt on 200kHz is a fairly easy matter these days.......
It has to be understood that the thing that drives the technical development of high power transmitters are efficiency and reliability, which, in turn, is governed by the likelihood of selling such transmitters. No broadcaster in their right mind will purchase a transmitter that isn't as efficient as it possibly could be in today's accountability-savvy world. So, you see, the politics of broadcasting and the the technical development of high power broadcast equipment is inextricably linked. The two greatest outlays a broadcaster faces are staffing (maintenance and operation) and energy. Both have to be kept as low as is acceptable in a capitalist arena.
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Old 12th Oct 2011, 11:01 am   #9
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Default Re: "Off The Shelf" LW Transmitters

Quote:
Originally Posted by dave walsh View Post
25 years? Well the manager said 10 at least......so halve that!
I know we have members with experience of working at Droitwich, so maybe they would like to comment on the typical life of one of the final output valves. I would expect it to be pretty long.
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Old 12th Oct 2011, 12:54 pm   #10
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Default Re: "Off The Shelf" LW Transmitters

Quote:
Originally Posted by paulsherwin View Post
There is no such thing as an off the shelf 500kW LW transmitter. There are relatively few big LW transmitters in the world and each one is made to order.
I can well believe that. Nobody's going to have a LW transmitter sitting in stock, to fulfil the next order which may come along, in the same way that 1kΩ 0.5W resistors are kept in stock.

However, I'd be surprised if there weren't a few off-the-shelf designs, ready to be built to specific order (no doubt with some customisation along the way for power, frequency, supply, aerial loading, etc). Following posts almost confirmed this, and it's good to see that hybrid designs are still current, with a thumping big valve PA. Quite probably, obsolescence issues arise between one build and the next, but again, over the course of a year that's likely to be the case for the materials to make the off-the-shelf 1kΩ resistor too!
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Old 12th Oct 2011, 5:31 pm   #11
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Default Re: "Off The Shelf" LW Transmitters

I laos heard the article on PM, and thought the interviewee was trying not to say that the 10 valves they have left will last an awful long time, that isn't the real reason for closing.

Seems daft to me. You can make a 198kHz radio with fencing wire, aliminium foil, plastic sheet and a diode, so come the emergency with no electricity you can at least communicate. Not so with FM, DAB, internet or anything else.
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Old 12th Oct 2011, 6:10 pm   #12
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Default Re: "Off The Shelf" LW Transmitters

Quote:
Originally Posted by woodchips View Post

Seems daft to me. You can make a 198kHz radio with fencing wire, aliminium foil, plastic sheet and a diode, so come the emergency with no electricity you can at least communicate. Not so with FM, DAB, internet or anything else.
Seems daft to me, too, but the ethos of Civil Defence and the need to galvanise the UK populace at a moment's notice cheaply and efficiently, using equipment that might be housed in a biscuit tin under the stairs has evaporated steadily since the Berlin Wall came down and the siren system and ROC (UKWMO) were disbanded in the early 1990s. There won't be any putting it back, either.

LW will eventually go. But it's a generation thing, like getting shot of Post Office and the corner shop. And DAB is a relatively new phoenomenon, yet to achieve the tried-and-trusted method of promulgation by AM radio. I don't expect my grandchildren will be listening to LW, anyway, but I'd like to see it stay a while yet.
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Old 12th Oct 2011, 8:35 pm   #13
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Default Re: "Off The Shelf" LW Transmitters

This thread has drifted onto BBC policy again, despite my reminder in post #4, so the mods have decided to decommission it.
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