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: Something which I forgot to mention is that PSB3 (C22) uses the DVB-T2 standard whereas the other two are DVB-T.
If the manual tuning screen allows selection of DVB-T or DVB-T2 then select the latter for PSB3.
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Paul Dursley: If you are now receiving BBC One West HD then you must be getting it from Backwell or Mendip -- you cannot be getting it from Wenvoe.
Does the signal information/strength screen tell you what UHF channel or frequency it is tuned to for BBC One HD?
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Paul Dursley: I thought you had successfully tuned in the BBC West HD channels as in your posting at 8:54 PM you said you selected the West region and that that allowed them to come through.
Reception of Mendip at the Backwell transmitter appears not to be as ideal as Arqiva (the company responsible for the transmitter) would like it to be. I say this because the receiving antenna arrangement consists of two aerials, one mounted above the other. Usually relays have just one.
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Anonymous : To receive BBC Three Counties Radio over the air I think DAB will be your only option. The signal for the Herts Beds Bucks local multiplex, on which it is carried, is much stronger where you are:
https://www.arqiva.com/me…-dab
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Paul Dursley: That C22 still has lower quality appears might perhaps imply the issue is still present, just not as bad as it was. With that in mind, it will be interesting to fit the filter and see if the quality for C22 goes up to 10/10 with it fitted and down to its current reading without.
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Paul Dursley: Bristol Kings Weston is co-channel with Backford and it is on nearly the same bearing for you.
During the 700MHz Clearance, UHF channels 49 to 60 were cleared of TV broadcasts, with all transmitters on those channels having to move down. Consequently, with fewer broadcast channels to go around, some transmitters ended up on the same channel when previously they were not.
In your area, the change happened on 5th June 2019 (except for one of Kings Weston's channels which changed two weeks later).
Backwell used channels 22, 25 and 28 before the change. The COMs of Kings Weston and the PSBs of Ubley changed.
Bristol Kings Weston COMs: 53, 57*, 60- changed to 22, 25*, 28
Backwell: No channel changes. Power up from 18W to 37W.
Ubley: Power up from 15W to 25W. 21+, 24, 27 changed to 22, 25, 28
* Kings Weston C57 changed to C25 on 19/06/19.
Kings Weston is on a bearing of 37 degrees at 7 miles out. Backwell is on a bearing of 31 degrees at 2 miles out.
Like Backwell, Kings Weston is vertically polarised. Kings Weston is 200W, much of which is coming in your direction, the radiation pattern being generally to the south from the tower (see the page on this site for Kings Weston).
I think the power increases at Backwell and Ubley were an effort to counter co-channel interference rather than to increase coverage. Similar power increases occurred elsewhere in similar circumstances.
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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]
~~
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
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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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Monday 8 April 2024 3:29PM
Paul Dursley: It sounds like your TV may have performed an automatic retune of its own accord which has resulted in the wrong regional HD channels. I would be very surprised if the Backwell transmitter is broadcasting the wrong region and think the more likely explanation is that the TV has tuned to Wenvoe, the main transmitter near Cardiff, for these channels.
There are a number of approaches you could take to address the issue. One way is to set preference for the region in the TV's menu. See if there is something like demonstrated in this video: https://youtu.be/XsXBnHF8cDM?t=158
In the video, with respect to regional selection the first selection is 'GBR' (Great Britain). From there it is possible to specify 'Best signal quality (All regions)' or specify a region. You will be looking to select GBR > England > West.
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Another approach is to wipe the tuning and manually tune the Backwell channels. Being that the TV apparently takes it upon itself to retune itself (which, as in this instance, can be a nuisance), it might do so and select the wrong region again.
The signal strength/information screen should tell you which transmitter you are tuned to, by virtue of the UHF channel number (or frequency). PSB3 multiplex carries all HD programme channels. The UHF channel (C) numbers are as follows:
Backwell:
PSB1 = C25
PSB2 = C28
PSB3 = C22
Wenvoe:
PSB1 = C41
PSB2 = C44
PSB3 = C47
Mendip:
PSB1 = C32
PSB2 = C34
PSB3 = C35
Judging by the fact that there is the hill in the way, I think it's unlikely you will get Mendip, but I've included it just in case. You can use this to see which transmitter it is that you are watching.
PSB1 carries BBC standard definition TV and BBC radio. PSB2 carries ITV1, Channel 4, Channel 5 and some ITV and Channel 4 sister channels (all standard definition). PSB3 carries all HD programme channels.