All posts by Dave Lindsay
Below are all of Dave Lindsay's postings, with the most recent are at the bottom of the page.
Nigel: There are two possibilities that I can think of as to why you might have to retune your TV:
1. The thing keeps automatically tuning itself at will and every time it picks up another transmitter.
2. The signal level is too high and you need to remove amplification/attenuate.
3. The Winter Hill channels are high up and so are found later in the scan. Some sets don't have enough memory to store all transmitters that are received and by the time it gets to the one you want the memory is full.
Check that it is tuned to Winter Hill for all channels. The UHF channel number or frequency is usually given on the signal strength screen. You need to look at one service from each multiplex: BBC One, ITV, BBC One HD, ITV3, Pick TV and Film4. See above for channel numbers and frequencies.
If, when reception goes poor, the channels are still tuned to Winter Hill, then suspect that the signal level is too high and overwhelming the tuner which causes poor quality, much like turning up the sound full blast on a hifi.
Try removing the aerial plug and holding it close by in an attempt to "loose" a bit of signal.
4G won't affect TV signals being broadcast, at all; they will remain as they are (albeit that channels 61 upwards are being cleared for 4G use). It is the 800MHz 4G base stations that are likely to affect some TV receivers.
There are a whole variety of factors and one is the strength of the TV signal. Chorley is close to Winter Hill and if it has a good view to the transmitter then chances of being affected are lower than where the signal is weaker.
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Roy Barton: The short answer is yes, rotate your aerial for vertical polarisation with Rowridge; there isn't, generally, a requirement to replace the aerial. The information at the top of this page is correct and says that each multiplex is broadcast horizontally and vertically.
The gain (sensitivity) of yagi aerials drops on lower channels. A wideband yagi is a greater compromise than a Group one as by the time you get down to Group A channels the gain is low. Manufacturers will quote the gain on the highest channel which is likely to be not what you're using it for. Rowridge is all Group A, so for that reason if you use a yagi then use a Group A one. For example gain curves that illustrate the point, see:
Gain (curves), Again
Above it does say "To receive Freeview from the Rowridge transmitter you will require an aerial of group A positioned horizontally." I will alert the site administrator to see if it can be changed.
The longer explanation: There are the PSB (Public Service) channels and there are the COM (Commercial) channels. The COMs don't have as extensive a coverage as the PSBs. This is due mainly to the fact that small relays don't carry them, but there will also be a few who won't get them reliably even though they use a transmitter that broadcasts them.
Basically, the PSB-network is designed to serve all areas that the former four-channel analogue network did and without requirement for existing aerials to be adjusted or replaced. The COM transmitters fit in where they can and in some cases require a replacement aerial, or in the case of Rowridge, vertical polarisation may be beneficial.
Rowridge broadcasts horizontally so as to be backward-compatible with existing aerials.
Because the COM channels fit in where they can, the channels that are used for them are reused by other transmitters in closer proximity than those of the PSB channels. Rowridge's COMs are co-channel with those of Stockland Hill's in the west and Crystal Palace's in the east. This is the reason that Rowridge's COMs are effectively restricted power horizontally; it's so they don't spread as far. Turning your aerial for vertical polarisation will increase the "rejection" of the unwanted Stockland signals (which are only horizontal).
Vertical polarisation may also reduce co-channel interference during lift conditions. Like the UK, France uses horizontal polarisation for high-power transmitters. (Rowridge being the one exception.)
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Roy Barton: A yagi wideband is not a good idea for a Group A transmitter.
Log periodic aerials have very flat gain curves and therefore the same issue does not apply; they are fine. See:
Rowridge Transmitter
Using the aerial amplifier is fine. It needs to be as close to the aerial as possible because it will be amplifying the noise picked up in the aerial lead, which is what you don't want (poorer snr). See:
Television Aerial Boosters / Amplifiers, Splitters, Diplexers & Triplexers
If you have a Group A aerial, then you should re-orientate it for vertical polarisation. A wideband aerial isn't necessary (over that of a Group A one) because all of Rowridge's channels are within Group A.
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Roy Barton: Here is a plot of the terrain between you and the transmitter:
Terrain between ( m a.g.l.) and (antenna m a.g.l.) - Optimising UK DTT Freeview and Radio aerial location
You will note that one mile away, the ground obscures line-of-sight. This corresponds to Holt Heath.
Therefore, your task is to receive what is "coming over" the brow. (Imagine if it were a light.) As far as the COMs go, you may have to contend with interference from Stockland Hill's COMs, which are co-channel.
You are only 28 miles away; that said, we don't know how the COMs might have restriction WRT the PSBs, in your direction. The Digital UK Coverage Checker suggests that they may be a bit inferior to the PSBs. The predicted figures are that one third of Rowridge viewers won't get the COMs, so they must be inferior in some way.
Obviously, with Stockland being in the opposite direction, the reflector is important WRT blocking interference.
As I say, I would start by changing the existing aerial to vertical polarisation and take it from there. Only then will you know whether it is sufficient to pull in the COMs or not.
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Michael McCann: In Newtownards, I don't think that you will be receiving Freeview channels from Black Mountain. Divis is the main high power transmitter which serves your area. Black Mountain does broadcast Freeview, but only at low power so as to cover a small area that can't receive from Divis.
However, Black Mountain does broadcast the NIMM. For a lot of the large area that it is intended to serve, Divis and Black Mountain are on near enough the same bearing that a single aerial will work. This was also the case for Channel 5 analogue (which was Black Mountain rather than Divis).
I'm not familiar with the i-Can boxes, but can you try doing a manual tune on UHF channel 39? Select DVB-T2 mode, rather than DVB-T mode, if there is such an option. Once you've entered the channel number wait before pressing the button to scan the channel; see if it gives any indication as to the strength and quality on that channel.
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Dr. John Pritchard: The symptom of changing from colour to black and white is not one of the digital system; this is only a possibility with analogue. Therefore, the issue must be somewhere from your box to the TV and not the aerial. How have you got the 20 year old TV connected to the box?
Have you confirmed that the TV is tuned to Dover transmitter for affected services, and not another transmitter? This is generally the first thing to check for in these circumstances.
There should be no 4G transmitters operating at 800MHz as the licences have not been auctioned off yet. Trading Standards aren't likely to be interested in complaints about TV reception. The propagation of RF signals are governed by the laws of physics, much as politicians may wish to rewrite them in order to cram more services in.
I have made two observations which could potentially affect reception of RF signals:
1. See this terrain plot between you and the transmitter:
Terrain between ( m a.g.l.) and (antenna m a.g.l.) - Optimising UK DTT Freeview and Radio aerial location
For around a mile, the signal path (the line between the transmitter and you) runs close to the ground. The higher ground approximately half a mile from you corresponds to the Westwood and Northwood areas. Looking at satellite images there are large industrial buildings in Westwood. Are these new and might their appearance coincide with the poor TV reception?
2. The airport runway is perpendicular to the path of your TV signal. Therefore, presumably, airplanes fly across it. They can act to cause issues with reception as they act to reflect the signal back down to earth. If the reflection happens to shine on your location then it could cause difficulties with reception.
Do any of these sound plausible explanations?
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Dr. John Pritchard: I will go out and come in again. I now see that you are in Northwood and therefore that the Westwood industrial buildings are not between you and the transmitter.
However, I do note that on satellite images, there are a number of 'stub' of roads off New Haine Road which makes me think that the industrial estate will spread into the fields behind you, which are obviously likely to be in the way as far as reception from Dover is concerned. Or more to the point, perhaps it already has done and the photos are out-of-date.
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Dr. John Pritchard: Of course, what I said about the signal path being low to the ground for about a mile still applies. It is just that I hadn't corresponded the terrain to the right location.
So have there been other developments in the area that may be the cause?
Below the terrain plot there is a link to see a line between the two points. Whilst it isn't exact, it should give you a rough idea.
For example, Streetview photos taken in April 2009 show the early development of buildings on Manston Road, with new roads being shown on maps.
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Saturday 16 February 2013 10:36AM
harry atkins: Yes.
Make sure that the mounting bracket is perpendicular to the elements if it uses a cradle. If it is parallel to them then it will act to block the signal.
See these pictures of correctly and incorrectly fitted cradles:
Aerial Positioning Tests