All posts by Dave Lindsay
Below are all of Dave Lindsay's postings, with the most recent are at the bottom of the page.
Malcolm Wood: I'm not sure how effective that might be or whether it may have an adverse effect on the sensitivity and directivity of your aerial. I'm not an RF engineer, so I can only but wonder:
- The interferer is at 90 degrees to the desired signal. Will putting foil on one side somehow unbalance the aerial and affect its directivity/sensitivity properties?
- The closer you put the foil to the aerial, the more affect it will have on it.
- The further away from the aerial you put the foil, the more the interfering signal will "get around" it and hence the larger the piece of foil will need to be to achieve the same effect.
In any case, choose whether you mount the aerial in the loft or on the back of the house, you may find that in certain weather conditions that reception isn't as good. I'm thinking that it's not so much that reception from Hastings might reduce in quality (although perhaps it might), but maybe the interfering signal might degrade reception more at times.
As I say, I'm not an expert in RF, so these are just my own musings. Perhaps one of the pros could chip in with some suggestions for Malcolm.
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MF: I'm not sure what is to be gained by complaining.
There are other places where similar effects have manifested themselves after switchover. Local relays usually degrade or make impossible the reception of the COM channels for those who do not use the relays (as the relays don't carry the COMs).
I imagine that it's been done in such a way as to minimise the number of people who end up loosing channels that they could get before.
There is also the fact that channels 31 to 37 and 61 to 68 have, or will soon be cleared. The former were or are to be auctioned for more Commercial multiplexes and the latter sold off to 4G mobile operators.
So the reduction in the number of available channels has played its part in creating situations where transmitters are co-channel in closer proximities where in days of old they would never have been.
It's the way things are done these days. The COMs do not have the extensive coverage that the PSBs do. All that's happened is a few people have lost access to them or have had it made difficult.
Others on here have reported this problem with Hastings' COMs being wiped out by Eastbourne and have said that they will have to look at changing to Heathfield. Unfortunately Malcolm isn't in a position to do likewise.
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Dave H: The fact that you can get QVC but that others are poor suggests that the signal being fed into your TV is too high and overwhelming the tuner.
For a list of services and multiplexes, see here (those with a bullet in the "E" England column):
DMOL Post-DSO Multiplex Channel Allocations
COM4 remains on low power until 27th June when there will be another retune. All the rest are up to their final power.
The booster should be removed from the roof-top aerial. If an aerial with line of sight for 22 miles to a transmitter at 20kW won't feed a single TV without any amplification, then there's something wrong. I suggest that it is unnecessary to have such a setup now.
If the booster connected to the loft aerial also acts as a splitter (it has two outputs), then it probably needs turning down. You may find that it can be removed and replaced with an unpowered splitter. Indeed, you "may" even be able to run all three from the same aerial with an unpowered splitter.
See ukfree.tv - independent free digital TV advice
See the website of Aerials and TV in Sheffield for lots of information as well as an online shop:
Television Aerial Boosters / Amplifiers, Splitters, Diplexers & Triplexers
If you choose to run all off one aerial, then you could leave the other as a reserve. If the cable from the roof-top aerial were to run into the loft, then you could easily swap them (the feed to the main TV would obviously need routing from the loft).
It's best to use threaded F-connectors where connections are generally perminant, such as in lofts, instead of using coax plugs.
With F-connectors you can then swap aerials using a small spanner (although you could use coax connectors if that's what's on your distribution amp).
Obviously you may find that having completed these changes, you are unable to receive COM4.
Digital reception signal strength is about getting somewhere between upper and lower thresholds. So long as you are above the lower threshold for a receiver to resolve a picture, then the picture will be as good as it will ever be, providing that quality is excellent. If you go above the upper threshold then you are potentially overloading your tuner with too much signal.
This is why I say that the booster on the roof-top aerial can be removed as doing so wouldn't impact on the picture, unless it put it below the lower threshold, and as I say, at 22 miles if it does then there's something wrong.
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Dave H: One further thing. It's always worth checking that a receiver has tuned to the desired transmitter and that you aren't looking at the output of another transmitter.
For Heathfield they are:
PSB1=C52
PSB2=C49
PSB3=C47 (HD mux)
COM4=C48 (to move to C42 on 27th June)
COM5=C44
COM6=C41
The information at the top of this page which says that COM4 is now on C42+ at 20kW is incorrect.
Check the signal strength screen whilst on one service on each of these multiplexes. The first one on each are BBC One, ITV1, BBC One HD, ITV3, Pick TV and Yesterday. (Refer to the DMOL page of multiplexes I gave you.).
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John: The short answer is shortage of channels (frequencies). And the fact that the Commercial services' coverage isn't as extensive as the Public Service ones.
To understand the long answer, it is necessary to explain that there are two types of services:
- Public Service Broadcaster (PSB) | These being BBC, Channel 3 (the various ITV1 regions in England), Channel 4 and Channel 5. The PSB multiplexes (signals) also carry ITV1+1, ITV2, Channel 4+1, E4, More 4 and the four HD services.
- Commercial (COM) | These carry other services like ITV3, Pick TV, Yesterday, Film 4 and Dave.
For a full list of what's carried on what, see DMOL Post-DSO Multiplex Channel Allocations
This page shows that there are three PSB multiplexes and three COMs. Each is carried on a single UHF channel; each single analogue service used one UHF channel.
The design of the network of transmitters means that PSBs are given the same coverage as the former four-channel analogue. The plan was for aerials that worked with analogue before to work for the PSBs.
There are 81 main and larger relay transmitters that carry all six multiplexes. The 1,000 or so other small relays are PSB-only. The COMs achieve about 90% coverage of the population, whereas the PSBs cover 98.5%.
The COMs don't have a "Public Service" obligation which is why they don't have the extensive coverage that the PSBs do. So no one who can receive the PSBs is "guaranteed" to receive the COMs.
In some cases, aerials may need to be replaced to receive COMs. Or in the case of Rowridge viewers, they may need to adjust their aerials to vertical.
The horizontal signals from Rowridge are therefore primarily to ensure compatibility with horizontal aerials.
Due to the shortage of channels (frequencies) available, Rowridge's COMs use channels that are also used by other transmitters, including Crystal Palace and Stockland Hill (PSBs are not co-channel). These two transmitters are exclusively horizontal. Stockland's COMs are co-channel with those of Rowridge, but lower power to its PSBs. So you can see that they have had to restrict the coverage of Stockland's COMs so as to prevent interference. Or to look at it another way, channels are reused by transmitters in much closer proximity that they would have been previously.
So fringe viewers on Stockland, for example, will only COMs intermittantly, or may have to spend money on a more sensitive aerial, or may only be able to receive PSBs.
For the reason of these other horizontally polarised transmitters using the same channels, Rowridge's COMs have restricted power horizontally. That is, to protect against interference where Stockland and Crystal Palace overlap Rowridge.
The PSBs get the better channel allocations whereas COMs may get ones where they are restricted in coverage and in places where other transmitters prevent reception because they are co-channel.
Previously with analogue, each transmitter had four channels. With digital, the 81 most powerful transmitter require six. Plus there is the fact that the number of channels available is reduced due to the plan to sell of some for 4G mobile operators and some more being ringfenced for selling to three more COM licencees. If the channels set aside for future COMs had been used, then there would have been less clashes and less people who have to replace or adjust their aerials.
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stuart smith: Communal aerial systems are sometimes filtered to only allow channels that are used through (or only allow group(s) of channels through).
Look at the table above, under the heading "Transmission frequencies". Accepting that there is an error which says that SDN is currently on 42+ when it is on 48. On 27th it will move to C42.
You will note that prior to switchover, with the exception of 29 and 34, the lowest channel was 47.
A filter that cuts off at one channel won't have an absolute cut-off where one channel gets through unscathed and the other gets attenuated completely. It may reduce a neighbouring channel a bit. So perhaps up to C39 or C40 is filtered which is why 41 is not good.
You "may" see some affect on 42 when it comes on air, if it is the aerial system that is the cause of the problem, and if it hasn't been rectified by then.
You need to establish, as firmly as you can, that the problem lies with the aerial system and not your equipment. If you have more than one TV or box, then try each in turn. When trying the TV, connect the aerial lead from the wall directly to the TV and not via a box, so as to discount the box as being the source of the problem.
On each receiver, assess the signal strength of each multiplex and compare, perhaps writing each down. Different receivers have different scales and tolerances, so don't compare a single multiplex between receivers; only compare all multiplexes on each receiver. See if 41 is down relative to the other muxes with all receivers. Obviously 48 is expected to be lower then the others.
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toniswift: No, only Absolute is available on Freeview.
See DMOL Post-DSO Multiplex Channel Allocations
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Peter: If manufacturers started using F-connectors, apart from the fact that adapters would often be needed, people would go connecting satellite dishes to terrestrial receivers.
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Stuart: See the explanation of my experience with a loft aerial:
ukfree.tv - independent free digital TV advice
I believe that I have line of sight, or near line of sight, from the roof whereas you clearly don't.
A cause of poor quality at night could be caused by dampness forming on trees which the signal path passes through. Or of course, it could be damp forming on the roof which is affecting the signal quality.
I used Megalithia to plot the path from your location to Hannington:
Terrain between ( m a.g.l.) and (antenna m a.g.l.) - Optimising UK DTT Freeview and Radio aerial location
As you can see, in the direction of the transmitter the ground goes up about a mile away and you are therefore receiving from over the brow of a hill.
Click the link below that trace to see a map of the area, complete with a line joining you and the transmitter.
I zoomed out until the two points were on the screen. I then placed a ruler up to the screen and found the line to be able 21cm long. The distance to the transmitter is about 21 miles. This allowed me to work out where the brow of the hill is.
It is around Ferhill Road, near Hawley Park and around which there are trees...
The flat bit coloured red that you see on the plot is Hawley Hill which also has woodland on it.
I think that you need to have your aerial mounted outside, preferably above the roof level.
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Saturday 16 June 2012 1:05AM
Floyd: I can only be very general without knowing a location and which transmitter you are using.
If you're receiving from Sandy Heath, then many others have reported issues via this site in different places.
We know that there are due to be changes on or after 27th June that should improve the signal, so see how it goes.