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Re-engineering headache
© Martin Watkins, 2013



North Hessary Tor is - it has to be said - a long way from BBC HQ, and as a result was a little prone to lead a life all of its own for many years. When I first "met" NHT in late 1977 it still broadcast 405 line TV (and would do so for another seven years or so).

The link from HQ to NHT for VHF TV was tenuous enough, an operational choice (depending on conditions) of rebroadcasting Wenvoe's channel 5 or relying on the SHF link via Eggardon Hill from Rowridge, that is when that two-way reversible two-stage hop wasn't being used in the back-haul direction for sending material from Plymouth up to London. Chris Youlden has described this elsewhere in detail, but by the time I encountered NHT in 1977 the 405 line signal came from Caradon Hill 625 lines and then via a systems converter, so at least it was reliable.

FM Radio

The FM radio feed situation had also improved, just two years before I first met NHT, at least for Radio 3 which - alone - was by then broadcast in stereo from NHT, due entirely to an historical accident at Wenvoe: this accident stemmed from the fact that in the original plan for Band II there had been no available frequency for Radio 3 at Wenvoe; like other stations it was allocated three frequencies initially, but because of its status as a "two-nations" high-power site the BBC felt it had to broadcast the BBC Welsh Home service as well as the West Home service, and the BBC felt that this dual choice of Home Service region was a more pressing priority than providing the Third Programme; it wasn't until some years later that a frequency was found for a fourth service at Wenvoe on an unusual allocation of 96.8 MHz, so Radio 3 at Wenvoe was on a channel well away from the "normal" BBC sub-bands.

North Hessary Tor transmitter.
Photo Alex Lakey. mb21 Tx Gallery

When the GPO PCM link from Birmingham to Wenvoe went into service in 1974 and brought stereo to Western England, it was realised that - at least for Radio 3 - a stereo off-air receiver could be used at NHT to bring in the stereo Radio 3 signal on 96.8 MHz for rebroadcast on 90.3 MHz to the South-West. This wasn't however possible for Radios 2 or 4: Wenvoe Radio 2 on 89.95 kHz was too close to NHT's Radio 3 on 90.3 MHz for stereo RBL, and Wenvoe's West of England Radio 4 (92.125 MHz) was similarly too close to NHT's Radio 4 on 92.5 MHz.

However, as it was the case that at least Radio 3 in stereo at NHT was technically feasible, the BBC acted to provide it; Radio 2 however continued in mono (RBL of Wenvoe, which was of course possible in mono but not stereo) and Radio 4 line-fed via Plymouth, sounding as though it had passed through several old socks by the time it was broadcast...

End of Day Closedown

The end-of-day closedown arrangements for this off-air feed of Radio 3 from Wenvoe were at times somewhat haphazard. Normally Wenvoe Radio 3 went off air about twenty minutes after Radio 3 closedown, (although the stereo encoders were normally turned off very promptly after closedown so this twenty-minute period of intermittent tone-whoops was in mono) and after Wenvoe switched off NHT would continue with mono carrier and silence for a while. However, on a couple of occasions the loss of Wenvoe caused a switch at NHT to - believe it or not - off-air 647 kHz from Daventry.

As that transmitter had invariably left the air anyway by that time of the night, one might as a result hear a variety of odds and ends rebroadcast by Radio 3 NHT, in other words whatever happened to be receivable on 647 kHz as picked up on Dartmoor; certainly I heard what appeared to be weakly-received Russian language broadcasts on several occasions for minutes at a time before the NHT time-switches came to the rescue and put Radio 3 out of its late-night misery. On another occasion whatever squelch arrangements were in place at NHT's receiver failed to act, and 90.3 MHz rebroadcast receiver "noise" generated by the "idle" RBL unit. However, these technical "treats" were by no means regular occurrences.

Circuit provision

To the BBC's credit, they wanted to provide more stereo from NHT as soon as possible, but were "up against" the complications and Spanish Practices of their complex relationship with the GPO. For reasons too complicated to go into they weren't apparently allowed to operate their own PCM link to NHT straight away, but they could provide a temporary link on the basis that the GPO wouldn't or couldn't, and this is what they proceeed to do. Off-air stereo receivers for Radio 2 and 3, tuned to Wenvoe, were installed at Mendip.

A four-channel ANALOGUE wideband SHF link was provided from Mendip to Stockland Hill and from Stockland Hill on to NHT, that latter path nearly grazing ground level on the Eastern slopes of Dartmoor and being quite long at over 40 miles. And just before Christmas 1977 the new link came into service, amidst BBC claims of improved quality for Radio 3 in stereo and the benefit of Radio 2 newly in stereo.

Reception issues

But when I first heard the broadcast results of this new link early in the New Year of 1978 it quickly became apparent that it wasn't working terribly well at times. There was often the impression of one or more high pitched whistles and general hiss present - low-level admittedly but audible nevertheless, and from about March 1978 things suddenly got much worse, with an intense "video" like buzz which would come up from nowhere, completely blot out the programme broadcast from NHT for about 10 seconds, and then subside again, leaving the by now somewhat less low level hiss and whistling. This intense buzzing might happen two or three times a day - sometimes more.

After this had happened several times over several days I became seriously interested, and quickly was able to ascertain that the buzz on Radio 3 was being rebroadcast by the MW relay of Radio 3 at Exeter - I drove out to Pearce's Hill and checked on the car radio. Realising that beyond doubt the problem wasn't anything to do with my equipment, in April 1978 I wrote (well one did in those days) to EID and reported the problem. I had a somewhat pompous letter back, saying that the problems I had noted were "curiously unexplained" and conveying the slight impression that I was talking through the back of my hat. But I persevered, wrote another letter, and this time was rewarded with an admission that a problem had indeed been found, namely that falling ice at NHT had damaged the microwave dish receiving the incoming analogue radio feed, and that a new dish was on order. The buzz I could hear on occasion was presumably interference from the BBC 1 feed from Plymouth via NHT to Stockland Hill, which used the same dishes as the analogue radio feed but firing in the opposite direction.

Commercial transmission

Well it took until early June before the problem was finally resolved, and I can only guess that the following events happened on the very day the dishes were swapped. At about four o'clock one sunny afternoon I turned on Radio 3, to be assaulted by "popular music" and - heaven forfend on Radio 3 - a Disc Jockey. Surprise turned to incredulity when a number of advertisments were inserted into the programme. By this time I was speechless, but pulling myself together I ascertained that the commercials appeared to be - in some cases - for the benefit of Plymouth-based businesses. And at that point the explanation became obvious - in order to maintain programme during engineering work on the damaged dish at NHT, the old off-air RBL from Wenvoe had been pressed back into service. But someone had contrived to mistune the receiver - and NHT was re-broadcasting Plymouth Sound on 96.0 MHz, rather than the wanted Radio 3 signal on 96.8 MHz. Well I daresay it's an easy mistake to make.

I attempted (from a callbox, we didn't of course have mobile phones back in that dim and distant era) to telephone Broadcasting House, and was eventually put through to what turned out not to be EID but a drama studio, where a remarkably camp man listened patiently to what I had to say and then said that unfortunately he had a studio-full of thesbians waiting to record and so I'd better try phoning again. I didn't bother.

The situation lasted an hour or so - from what I remember the stereo encoder was "on" but Plymouth Sound was actually being broadcast in mono. This wrong programme presumably was rebroadcast by Redruth as well, certainly the Okehampton relay was carrying it.

After all that though the link worked perfectly, and when the BBC were satisfied that they had fulfilled the condition of "temporary set-up" as far as the GPO were concerned they completed the last leg of the link (Wenvoe to Mendip) and replaced the four-channel analogue with proper PCM, at which point (summer 1980) Radio 4 joined the stereo party at NHT.

And finally...

It's often forgotten now, but a similar four-channel analogue system was used from 1976 to 1980 to feed Tacolneston with stereo Radios 2 and 3. In this case off-air receivers were installed at Sudbury to pick up Wrotham, and a single hop SHF link brought the signal to Tacolneston. I can only assume that the BBC didn't have the same adventures in East Anglia as they did in the South West.

Years later I was able to look in the NHT files at EID in Henry Wood House - there was mention of the problems of the dish but no mention of the hour-long commercial experiment. I simply can't imagine why not - but then as I said NHT is an AWFULLY long way from London...

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