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Photos by Bill Wright Page last updated: 2014-01-07
Strathyre Stirling
NGR: NN559171 Maps: Google  Bing (Ord Surv) Site Height:       Structure Height: 17m
Digital TV: BBC A: 27 D3&4: 24 BBC B: 21
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Comments: Strathyre is a relay of Black Hill (via Strathyre Link)

May 2005

Strathyre is a small village in a deep glen between Callendar and Lochearn. Driving through on my way to Fort William I noticed the transmitter, and wondered how it obtained its incoming signals since it is in the bottom of a fairly deep and narrow glen, miles from anywhere.

My interest aroused, I took an unscheduled left turn to the fury of the bus driver behind (who was much too close anyway so it was his own damned fault) and found myself parked on a very narrow road surrounded by Highland-posh houses. Curtains twitched. (We do look a bit like gypsies it has to be said.) Climbing down from the cab with my camera I advised Hil "If anyone asks tell them we'll only be here a month or two, and the others are coming soon."

The arrangements for TV reception at Strathyre are more than averagely elaborate. On high ground just above the tree line of the Strathyre Forest is Strathyre Link. This receives Black Hill and retransmits on the same channels and with the same polarisation, which I believe makes it an 'active deflector'. The transmit power is pathetically low at 0.5W. Maybe the link is solar and/or wind powered, making higher transmit powers difficult to sustain. From the village I think I could see a solar power array up there near the link mast, but even for mb21 I didn't feel like staggering all the way up the mountain just to confirm it. Low transmit power would also help avoid feedback. Re-transmitting the RF as received is presumably another way of saving power, since the active equipment need consist of nothing much except amplifiers, I suppose. If the quoted figure of 0.5W is ERP (in other words, if it takes into account the gain of the transmitting aerials) the actual RF on the coax feeding the aerial must be pretty miniscule.

Unfortunately this half a watt of RF seems barely adequate for the purpose, hence the elaborate stacked and bayed yagis used to receive the signal down in the glen. I parked next to the mast in the village and found that a log periodic with a masthead amplifier produced a very snowy picture from the link - quite inadequate for rebroadcast, but the output of Strathyre is clean, so the elaborate receive array is obviously doing its job well. All the more surprising then, that it appears to be off beam!

Strathyre went into service in the mid 1990's. DSO was on the 8th and 22nd June 2011.

This shot taken from behind the Rx array, with the link Tx in the distance, seems to show the yagis pointing somewhere to the right of the link Tx. Of course the actual angular discrepancy is exaggerated by the camera, and it is really very small - much too small to matter if only one yagi had been used. But with yagis this far apart and phased together gain would be greatly reduced in the direction of the link. That's assuming that the feeders to each yagi are exactly the same length. And it's also assuming that the aerials are precisely 'side by side'. And maybe the link's transmit aerial is actually some distance from the tower, to prevent feedback, and the receive array in the village is in fact pointing at it with great exactitude...

Here's another minor puzzle. How come the receive array seems to have been bolted to the tower so that it's exactly square onto two of the legs? The left hand aerials are exactly the same distance forwards as the right hand ones, suggesting that no phasing adjustment was necessary to maximise gain. Coincidence or good planning?

Of course, the four receive aerials might not be connected together in phase at all. Maybe the top two or the bottom two provide back up. Or maybe the left hand pair is stacked in order to reduce pick up from the transmit aerials and the car park below, with right hand pair providing back up. Or maybe it's the other way round. Maybe the Holy Grail is in a safe deposit box at Paddington Station and Osama bin Laden is driving a taxi in Bradford.

In any case the Strathyre receive array must struggle to provide a clean signal given the very low field strength provided by the measly 0.5W link transmitter. Since it's mounted not very far above the junior school car park I wonder if the whole village suffers impulse interference on school open nights!

If only my old legs had been willing to carry me up the mountain I could have provided answers to some of these questions. In the absence of enthusiastic legs all I can offer is this wild speculation. Better information is needed here. Anyone?

Strathyre Link

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