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THE TRANSMISSION GALLERY
TUMMEL BRIDGE
| Photos by John P Martin | Page last updated: 2021-09-29 |
| Tummel Bridge is a relay, with Kenmore as its programme source. It was on the update-list because of the lack of a photograph of the RX aerial(s). So I trundled up there and took some new photos but was also unable to find an RX aerial, and assumed that the programme source was via one of the many microwave dishes, one of which points towards Kenmore. It was then suggested that Tummel Bridge might be a split site with the RX aerials located remote from the TX site. Indeed the OS map does show another tower very close-by although nothing is visible either from the TX site or from the forestry track which leads to it. So I went back again with my drone and bingo! |
| The drone found the RX site which is about 30 metres higher than the TX site and behind it and to the left, looking from the forestry track. It is otherwise invisible from the TX site and the reciprocity theorem applies! If there was ever a path up to the RX site, there is no evidence of it now: the ground being covered in bracken, heather, peat, rocks and birch trees. |
| After a scramble, the RX site looms into view. |
| Composite view of the entire mast. The RX aerial is a vertical stack of four 18-element Yagis. Vertical stacking makes the array fairly tolerant of left-right movement but less tolerant of up/down tilt. I would have expected a horizontal bay for receive purposes but I'm sure there are good reasons for this configuration. The Yagis point south to Kenmore as does the microwave dish, of which more later. The Yagi feeds combine in a splitter tube and the output coax disappears underground down to the TX site. PS: the apparent discontinuity of some of the boom support poles is due to the stitching together of several photos to make this composite. The boom supports are fine. |
| Detail of the driven element of one of the Yagis. I think it is Jaybeam's old "parabeam" design. |
| Splitter tube combines the outputs of the four Yagis into one, thicker, coax which then disappears down the rabbit-hole to the TX site. I'm sure Honest Ron Aerial Installers would have insisted on a Labgear mast-head pre-amp! |
| Arqiva inspection plate. |
| Before we go back down the hill to the TX site, a word about the dish. It also points towards Kenmore. A flexible waveguide exits the dish at the rear ... |
| ... the waveguide goes into an NEC PASOLINK NEO which is an up/down converter from SHF to UHF for data communications... |
| The IF coax cable also disappears down the rabbit hole to the TX site. It's obviously phantom powered as there is no power supply to the RX site. The box is dated 2007. It may have nothing whatsoever to do with the RBL, of course, but one does wonder if it could act as backup should the off-air reception from Kenmore fail - or vice versa. |
| Back down the hill to the forestry track and this is the first view of the TX site as approached from the east. |
| A little closer and the 4-tier crossed log TX aerials come into view. |
| Composite stitch of the tower as seen from the west. At the top, the usual triangular space-diversity array of three stacks of four folded dipoles for AirWave TETRA emergency services communications. Below that some microwave dishes and then the UHF TX log array. We don't mention the GSM panels. |
| The obtuse angle between the stacked logs gives two more-or-less distinct lobes to the radiation pattern: one points west, the other south-east, thus covering the Tummel Valley in both directions. |
| Splitter tubes provide feeds to the eight logs from a common uplead. |
| There may have been another log below the current stack at some time in the past, judging by the dangling N-type. |
| The logs still carry the manufacturer's label. CSA Limited was, I think, the umbrella company for what was Jaybeam Amphenol. Amphenol Procom still manufacture this aerial (a BBC design) and the current price is £828.00 +VAT each. [Ed]. I've added a link at the foot of this page to the BBC RD report of the design of these UHF Log Periodics. The polythene dielectric between the two booms is to prevent ice forming between them. It also slows the velocity of propagation, making the aerial shorter than an air spaced equivalent. |
| The cabin village at the base of the TX tower. There is no Arqiva sign on the cabin which houses the RBL equipment. Other cabins bear the names of the 'usual suspect' mobile network operators ... |
| The backup generator looks fairly new. The label states it was manufactured in 2017 in Spain. |
| And a final parting shot of the TX site. |
A UHF ruggedised log-periodic aerial (BBC RD 1973)
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