All posts by Dave Lindsay
Below are all of Dave Lindsay's postings, with the most recent are at the bottom of the page.
jb38: It's interesting the point you make about using an aerial with wider acceptance in combination with an amplifier where there is difficult non-line-of-sight. I've always wondered why you wouldn't just go with a higher gain aerial so as not to have to resort to amplification. This makes sense; thanks for the insight.
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Nick: I do not believe that jb38 has implied that a lower gain aerial plus booster is a "substitute" for a high gain aerial. He has said that a lower gain aerial plus booster can be better in difficult non-light-of-sight situations.
That does not mean that high gain antennas don't have their uses. What ATV is probably saying is that a low gain antenna plus booster is NOT the equivalent of a high gain antenna without booster.
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Nick: Quite a lot.
Think of car headlights a night when they are coming towards you over the brow of a hill. Before they get over the brow, you do not have direct line of sight. You can see the growing mass of light above the horizon (which is the highest point).
Now imagine that the light is a signal you are trying to receive.
I believe that what jb38 is saying, is that it is best to use a low gain/wide acceptance aerial to collect this "mass of light" rather than a high gain/narrow acceptance one.
Read ATV's page on amplifiers:
Television Aerial Boosters / Amplifiers, Splitters, Diplexers & Triplexers
Amplification is fine so long as you are amplifying a small good quality signal. If you amplify a small poor quality signal then you will end up with a large poor quality signal.
Because a high gain aerial has a narrow range of acceptance, it is imperative for the signal to be "good" quality along that narrow path where it is most sensitive. If it's not, then all you'll have is a poor quality signal.
The suggestion that a low gain antenna is better means that the signal that it is "collecting" is over a wider surface area of the aerial. Therefore, if "some" of that is "poor" but most is "good", the "good" will win through and therefore the signal carried down the aerial lead will be "good".
Or to think of it another way, consider the high gain antenna with small acceptance angle. Let's suppose that "most" of that angle is filled with a signal that is "poor". The signal carried down the lead will therefore be "poor". Now imagine replacing the high gain aerial with a low gain one that has a wider acceptance angle. Because the "poor" bit is spread over much smaller proportion of the acceptance angle, it therefore follows that the signal from the aerial is better (albeit perhaps of smaller magnitude).
jb38, am I on the right lines?
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Andrew: You may find this discussion interesting:
Freeview on Sudbury TV transmitter | ukfree.tv - independent free digital TV advice
jb38 says that where there is difficult non-line-of-sight a lower gain aerial with booster is better than a high gain one. The reason being that lower gain aerials have wider acceptance angles.
As you don't have line of sight, I thought that this might be useful to you.
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will snell: I quite certain that it doesn't apply for Belmont. There is a retune due for Sandy Heath viewers on 9th May, but you're firmly outside its area!
Have a look in the 800s for another transmitter's services. Unless it was transmitted by Belmont, which would be doubtful and the fact that only one TV gives this message suggests that it may not be.
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Ann: This has happened in other areas. People who could receive the full Freeview service before switchover (and therefore on low power) found that a relay transmitter adjacent to them which they do not use wiped out any chance of receiving the Commercial channels (which the relay doesn't transmit anyway) when it came on air at switchover.
As an example, Steyning transmitter relays Midhurst's Public Service channels and rebroadcasts them on the same channels as Midhurst uses for its Commercial services. So those living in places where they can receive directly from Midhurst but close to Steyning transmitter will probably find that they can no longer get the Commercial services from Midhurst.
The reason that it's like this is because of scarcity of frequencies and the requirement to fit it more services, albeit that they (the Commercial services) are quasi-national.
In the days of analogue, all transmitters had four channels each. Now the main ones that serve large areas have six and the "filler-in" relays have three.
Midhurst is one of those where the transmission power of the three Commercial channels are lower than that of the Public Service channels. The "re-use" of these channels in closer proximity to the re-use of the Public Service ones is probably the reason for this.
As KMJ,Derby says, a wideband aerial isn't the best for Rowridge in poor reception areas. The sensitivity of these aerials is lower at Group A (bottom third) channels which Rowridge uses exclusively. See these plots:
Gain (curves), Again
See also:
Rowridge Transmitter
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Steve: Your most likely Anglia transmitter would seem to be Rouncefall and Sudbury (which operate on the same channels). These are higher up that Crystal Palace, so run the automatic tuning scan and unplug the aerial when it gets to 30%. If it's Sandy Heath that you're picking up, then its Public Service channels are 27, 24 and 21, so the unplugging aerial trick won't avoid these if they are able to be picked up with your aerial.
What I would say is that, presumably, Crystal Palace is the "main" transmitter and Sudbury or maybe Sandy Heath is/are in the 800s. If this is so, then what you've experienced suggests that the TV displays messages applicable to all transmitters to which it is tuned and not just the "main" one.
It "may" be that the message relating to the Sandy Heath retune is also broadcast by Sudbury and relays of the two. I wonder if this is the case as if it weren't, then there would need to be a way of "inserting" this message for Sandy Heath broadcasts and not Sudbury ones.
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John F: Channel numbers start at 21 and go up to 68 or 69. 69 minus 21 is 48, so perhaps that's where the number comes from.
At your location, Sandy Heath is probably available. Belmont's BBC is C22 and Sandy's is C27. Yet Belmont's ITV is C25 which is after Sandy's which is C24.
My guess is that it doesn't select the stronger signal but the first one!
It might be worth researching the channels for the transmitter you will be using (as well as those that you may pick up which you are seeking to avoid) at a particular site. Digital UK Tradeview predictor is good for this. Enter the post code and tick the box to say that you're in the trade.
Digital UK - Postcode checker
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Wednesday 25 April 2012 9:22PM
Ann: If you can pick up one of the muxes from Rowridge off the back of your aerial, then perhaps it isn't so bad. As I said, Rowridge's COMs are weaker horizontally than vertically
As for not getting anything on C50, your aerial is most sensitive in the direction that it is facing and the polarisation it is set for. Clearly it is picking up too much signal from Horndean and this is trashing the Midhurst signal. The interfering signal does not have to be at such a level as to be useable if the Midhurst one wasn't there, which is what I think you may have been expecting by attempting to tune in C56 from Horndean.
If your aerial were to face Horndean, and it was vertical, it would get more signal from it and the level of rejection of Midhurst's C50 would be much greater than it is now which means that you may well be able to receive C50 from Horndean.
One of the problems you've got is that to get a good signal from Midhurst, I guess that the aerial needs to be high. But that's exactly where the stronger interfering signal from Horndean is likely to be!
I can see the tree-covered hill on Streetview from Hawthorn Road where there are quite a few large aerials on Rowridge. Clearly you are closer to the foot of that slope than they are, which is why your chances of reception are lower.