All posts by Steve Donaldson
Below are all of Steve Donaldson's postings, with the most recent are at the bottom of the page.
Paul Dursley: It's good to hear all Backwell multiplexes have returned to good strength and quality.
The channel allocations of Backwell and Wenvoe COMs lend themselves to combining into one the feeds of an aerial on each transmitter. The main six are: Backwell's PSBs: 22, 25 and 28, and Wenvoe's COMs: 39, 42 and 45. Wenvoe's Cardiff local multiplex is 37 and its PSBs are 41, 44 and 47.
ATV currently lists a single UHF-UHF diplexer[1]. It splits at C38 and according to the photo, is a Fringe Electronics model. Fringe doesn't list such a model on its website[2]. The diplexer attenuates channels neighbouring the split, so this may not be the best one here.
UHF-UHF diplexers are thin on the ground. There is an Antiference UD3437 diplexer listed on eBay. If you put the model number into the eBay search bar you should find it. Its lower input actually goes all the way down to 88MHz, so will pass FM broadcasts too. The UHF channels it passes on the two inputs are 21 to 34 and 37 to 68.
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[1] Diplexer Channel 38 (UHF/UHF) A.T.V. Poles, Brackets, Clamps & Aerials
[2] https://fringeelectronics.co.uk/diplexers.html
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Chris.SE: Paul's aerial, which appears on Google Street View as if it may be a 13-element contract, was installed in the days of analogue. As I have already identified, Backwell had a doubling of power in 2019 which I believe suggests it may be able to withstand a little more extra loss.
I went looking for diplexers to see what I could find. ATV is always a good starting point. As with any research into products, the question is whether it can be bettered in terms of price, and in this case in terms of splitting at a more suitable channel too.
The Antiference model on eBay does indeed require F-connectors, and these can also be picked up on eBay for a few pounds delivered. This is the best I could find in terms of price which comes in at a little under 20 including delivery and F-connectors from another seller. The seller of the diplexer is open to offers, so it is always worth seeing if he/she is will give a discount.
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CHRIS JACKSON: As StevensOnln1 says, there is the posting above yours that Heathfield has engineering works. The 'Transmitter engineering' information above is sourced from this page on the Freeview site:
Planned engineering works | Freeview
Another possible cause to consider is interference by a nearby mobile phone base station operating on a frequency previously used for TV. Restore TV is the organisation charged with alleviating such issues and will send out a free filter. A lookup of your postcode on the Restore TV says postcards have been sent there, meaning there could be an issue: https://restoretv.uk/post…ure/
While we cannot say for certain where the base station is, I see there is the water tower with antennas on overlooking you. I suggest you make enquires with Restore TV to receive a filter.
If you have a rooftop amplifier then any filter will need to go before this, meaning it will need to be on the roof. Tell Restore TV you have a rooftop amplifier and they should send an engineer to fit a filter on the roof for you.
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CHRIS JACKSON: The Rye relay transmitter broadcasts on UHF channels 40, 43 and 46 which are the same as the three COM channels of Heathfield. Rye is vertical polarity, while Heathfield is horizontal.
Thus, should you experience poor reception on these channels, a possible suspect is interference from Rye. Your aerial is obviously horizontal, but it might still pick up the vertical signal from Rye to some degree.
The COM channels are COM4 (SDN), COM5 (Arq A) and COM6 (Arq B). For a list of programme channels carried on each of these and the other multiplexes, see the Freeview list:
Channel listings for Industry Professionals | Freeview
The three PSB channels of Heathfield, PSB1 (BBC A), PSB2 (D3&4) and PSB3 (BBC B) are not shared with Rye.
The COM channels of Heathfield and the three Rye channels have been the same since 16 October 2019.
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margaret: The channel numbering varies by platform. What platform are you watching on?
ITV2 is on channel 118 on Sky and channel 113 on Freesat. On Freeview (terrestrial TV) it is on channel 6.
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M Wickham, Bob Reeve: It sounds like the issue could be related to the work at Rowridge. The multiplex (PSB2) which carries ITV1, Channel 4 and others is received off-air from Rowridge by Patcham. The two other multiplexes are the responsibility of the BBC and arrive at the transmitter over fibre.
A single digital TV broadcast signal known as a multiplex or 'mux' carries multiple programme channels. This is unlike with analogue where each programme channel was transmitted on a signal of its own. Thus, loss of a single multiplex will result in loss of multiple programme channels. It is a case of working out which multiplex or multiplexes are missing.
Patcham transmits three multiplexes, the three PSBs, or public service broadcasters.
PSB1 - BBC A - BBC standard definition TV and BBC radio
PSB2 - D3&4 - ITV1, Channel 4, Channel 5 and some ITV1 and Channel 4 sister channels
PSB3 - BBC B - All HD channels, BBC and non-BBC
Freeview has published a list of programme channels and which multiplex they are carried on here:
Channel listings for Industry Professionals | Freeview
Rather than resetting and carrying out automatic tuning, use the manual tuning facility to scan the appropriate UHF channel for the required multiplex from Patcham. Automatic tuning takes time and looks through all the channels. Manual tuning focuses in on only the actual channels that are used and which are required. Channel numbers for Patcham are:
PSB1 - C29
PSB2 - C31
PSB3 - C37
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Dennis Anthony: This begs the question: are you sure?
At your location there look to be two potential transmitters: Heathfield and Hastings (Bohemia). Heathfield is horizontally polarised and Hastings is vertically polarised. It sounds like your aerial might have been changed from the former to the latter.
UHF channel numbers for each are as follows:
- Heathfield -
PSB1 - BBC One - C41
PSB2 - ITV1 - C44
PSB3 - BBC One HD - C47
COM4 - QVC - C40
COM5 - Sky Mix - C43
COM6 - Quest - C46
- Hastings -
PSB1 - BBC One - C25
PSB2 - ITV1 - C28
PSB3 - BBC One HD - C22
COM4 - QVC - C23
COM5 - Sky Mix - C26
COM6 - Quest - C30
Check which you are tuned to by viewing the signal information/strength screen on each of the above programme channels.
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Paul Evans: While you may have line-of-sight to the transmitting antenna at the top of the mast, the Malvern Hills are well within the Fresnel zone, meaning they could potentially cause reception difficulty. This isn't to say it will.
The radius of the Fresnel zone varies according to frequency, the lower the frequency the longer the wavelength and the bigger the radius. It is largest half way between the two antennas, getting smaller the closer to the two antennas.
The radius of the Fresnel zone as the signal passes over the Malvern Hills is in the region of 82m to 87m. However, line-of-sight at this point is only around 20m above ground at its closest point, this judging by the following terrain plot:
Terrain between ( m a.g.l.) and (antenna m a.g.l.) - Optimising UK DTT Freeview and Radio aerial location
If the reserve antenna is in use, then line-of-sight runs even closer to the ground over the peak, as this is lower down the mast.
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Cliff Tubb: I have looked at the Ofcom data giving technical parameters of broadcast transmitters and concur with Chris.SE that St Thomas is unlikely to be of help to you.
Rather than positioning your aerial generally facing St Thomas and Holcombe Down (roughly westwards), your best bet may be in the direction of Holcombe Down, Budleigh Salterton and Stockland Hill (roughly from about north east to south south west).
Ofcom data gives includes detail on how much signal each transmitter puts out in each direction at 10-degree intervals. I have taken the location of All Saints Church as the sample for my analysis.
The St Thomas DAB transmitter (6.7 miles away, 325 degrees) is not in any way omnidirectional. You are on the side on which it puts out very very little. Looking at photos of the transmitter, this would appear to be explained by the fact that the DAB antennas look to be on just one side with the tower acting to block the signal behind them. You are on the side where the tower shadows the signal coming from the antenna. Plus, there is high ground in the way.
Holcombe Down (5.2 miles away at 195 degrees) is not omnidirectional either, but in your direction you are getting nearly the full radiated power of 500W on all three multiplexes it broadcasts.
For Budleigh Salterton (5.3 miles away, at 94 degrees), you aren't on the side where the signal is the strongest, but there is line-of-sight, meaning no higher ground in the way which is the case with Holcombe Down. The transmitter's effective radiated power is 600W for both multiplexes (BBC National and Devon), but in your direction it is in the region of 44W and 48W.
Stockland Hill is further away but more powerful.
There are different multiplexes and not all transmitters carry all of them. Holcombe Down, Budleigh Salterton and Stockland Hill carry the BBC National (12B) and Devon local (11C) multiplexes.
The other two multiplexes are the two national commercial ones, D1 National (11D) and SDL National (11A). SDL National is the newer of the two and has fewer transmitters across the country providing less coverage. In your area, SDL is only available from St Thomas, so you aren't likely to be able to pick it up. Even Stockland Hill, as a main TV transmitter, doesn't have it.
This leaves just D1 National, which is broadcast by Holcombe Down and Stockland Hill. Thus, if you find you can pick it up the most likely transmitters it is coming from is/are Holcombe Down and/or Stockland Hill -- Budleigh Salterton is no help for this one.
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Saturday 13 April 2024 7:31PM
Paul Dursley: Considering what I have found, which follows below, I'm not surprised the filter has had no effect. I believe this is an issue with reception of Mendip by the Backwell transmitter and needs reporting to the BBC.
The signal path between the Backwell transmitter and Mendip runs close to the ground for around a mile where it goes over the top of Barrow Hill. The signal path is the line through free space between the transmitting aerial of Mendip and the receiving aerial of Backwell. I think this is why there are two receive aerials on the Backwell transmitter.
I suspect the reason you lost reception of PSB3 (C22) and at the same time had lower quality on PSB2 (C28) is because Mendip was broadcasting from its reserve antenna. This is lower down the mast than the main which is at the top. I think that PSB3 came back because Mendip returned to transmitting from its main antenna.
The three Mendip channels for the PSBs are C32 (PSB1), C34 (PSB2) and C35 (PSB3). If your issue is in fact a reception issue for the Backwell transmitter then when you lost PSB3 it was C35 that was worst impacted with C34 less so (you had lower quality on PSB2). If this was because Mendip was on its reserve antenna and now it is on its main, then the quality issue on C22 may in fact be a quality issue on C35 as received by the Backwell transmitter.
Thus, it looks like the objects in the signal path between Mendip and Backwell are acting to the detriment of reception in the frequency range of C35 (around 586MHz). With the lower height of the reserve antenna the obstruction in the path increased, and when this happened the impact on reception on C35 frequencies became more pronounced, spilling over into the adjacent channel, C34.
- Terrain Plots -
I have set out the aerial heights below. Here is a terrain plot with Backwell on the left and Mendip on the right:
Terrain between ( m a.g.l.) and (antenna m a.g.l.) - Optimising UK DTT Freeview and Radio aerial location
The plotter uses six-figure OS grid references, meaning the locations of the two transmitters are set to the 100m interval closest to where they are. Backwell is set a few tens of metres away from its true location where the ground height is a little higher, so I have compensated by reducing the height of the antenna. For the height of the Mendip antenna I have entered that of the lower/bottom of the reserve, so we can see worst-case scenario, with Mendip transmitting from its lowest antenna.
The yellow line denotes the line-of-sight and the pink line is the outer of the first Fresnel zone. The Fresnel zone is the area between the pink line and yellow line. Anything in this area can have the potential to affect reception. This isn't to say that it will, but we can see that the ground and objects on it are in the Fresnel zone for around a mile, from somewhere about 2.6 miles out to maybe 3.7 miles out from Backwell. This corresponds roughly to a line over Barrow Hill from about The Conygar (around Water Catch Farm) through to the high ground of Yewtree Farm, which is to the south of Freemans Quarry and north of the A38.
The point then is that the lowering in height of the Mendip antenna (by way of switching from main to reserve) increases the level of obstruction in general present in the Fresnel zone (the Fresnel zone having lowered also). Increasing its height by returning to the main reduces the obstruction.
This is a plot from the Backwell receive to the bottom of the Mendip main antenna:
Terrain between ( m a.g.l.) and (antenna m a.g.l.) - Optimising UK DTT Freeview and Radio aerial location
- Aerial Heights -
Ofcom publishes the average aerial heights of transmitting antennas (main, not reserve). I referred to planning application diagrams for the height of the Backwell receive, and Mendip main and reserve antennas height spans (source reference notes in square brackets):
-- Backwell --
Site height: 96m
Receive aerial height above ground: 23m [1]
-- Mendip --
Site height: 303m
Main antenna height: from 282.3m to 293.7m [2]
Reserve antenna height: from 245.7m to 257.2m [2]
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The Mendip source [2] was published in 2013 and therefore with the previous Mendip antennas, as they were subsequently replaced for 700MHz Clearance. However, the current ones are in the same place, so the figures will be still relevant.
As can be seen in photos[3], between the main TV and reserve TV aerials are DAB and FM broadcast antennas. I checked the height of these other antennas and they are between the main TV and reserve TV heights quoted above, so those figures look to be right. The average height of the main TV is given as 288m, which ties in with the above.
We can see from this that the Mendip reserve antenna is 36.6m lower, from the bottom of the main to the bottom of the reserve.
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[1] Planning application 15/P/1969/F with North Somerset council, 'Proposed Elevation' document.
[2] Planning application 2013/1080 with Somerset Council, East District (former Mendip District), 'Existing SE Elevation' document.
[3] mb21 - The Transmission Gallery