In theory, more elements increases the gain on-axis and reduces it off-axis, so it should be better if you can accurately perform the more critical alignment it requires.
However, I'm sceptical that many aerials are really designed that accurately, at least as far as the physics goes. I suspect much of the design is to 'look' higher quality so they can charge more;-)
50 volts across his 'tongle', or even tongue, (sorry, couldn't pass that one up :-) ) plus another 75ac if it rang would be pretty exillerating..........
.andy
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IIRC the number of elements increases teh gain slightly, but the real gain is in drectionality and freedom from ghosting.
If the transmitter is behind hills/houses you do need LOT of height, but its easy to check yoiurloft with a set top aerial and a small TV up there. If you can get reaosnable quality from that, and move around to find teh best spot, a decent aerial in the same spot will do about 4-10 times better than the set top aerial. 4 stomes stronger and maybe 10 times better on ghosting.
Final alignment is done by ot looking at signal strength, but picture quality. You want to make sure your main ghosts are coming into the antenna best rejection lobes, this is usually a few degrees off maxiumum signal.
In the end you fiddle around with te antenna and a TV in the loft, til you are pretty sure its as good as you are going to get, then bolt it down and enjoy whatever you can get.
Frankly it will be better than a quick external installation by a cowboy rigger in 99 cases out of 100.
Doing it this way also allows you to assess the best place within the loft. Metal tanks and metal in the house will make big differences, so its not always obvious where the best place actually is till you try em all..
but
Andy Hall wrote in
Thanks for explaining I was still getting the horrors trying to work it out, the "all fours" was a clincher.
My bum is *still* going two bob, half a crown......
mike r
I live in an area where the signal is rubbish and we are not allowed to have external aerials. It was a new house and the builder put in the wiring for 3 rooms which terminated in the loft. I went to a reputable installer and they fitted a fairly small aerial but a whopping big signal booster attached to it and the picture is great. Cost me £130 which I didn't think was too bad having seen the rubbish pictures some of my neighbours have. He didn't have to do anywiring but I guess he used quality products.
Angela
I am glad that you ended up with a good result.
Generally, adding amplification to a poor signal doesn't help because the amplifier itself introduces its own noise to the signal; and usually this is worse at higher levels of amplification.
A good quality commercial grade amplifier will have a better noise figure than a cheap one so should be used when a scenario like this happens.
Normally though, it is better to install a larger antenna and not need to add amplification......
.andy
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Indeed a cheap bubble pack "Booster" for what? £7.50? Is unlikely to have a better noise figure than a £300 TV.
Lets agree on a "better" antenna (from a proper manufacturer) rather than "bigger".
DG
Yes, exactly.
.andy
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LOL I only just noticed my spelling mistake :-} - "tongue" :-}
Totally agree. Boosting a 40dB S/N ratio signal gives at best - a 40dB S/N ratio!
You muts have reaosnbable signal strenght to start with. However ofetn its not height you want, but accurate location and adirectionality - both of which are as easily achieved with a loft aerial as one up a pole.
Only if you REALLY have to get the height to get any signal at all are externals indicated.
At the moment my aerial gets a reasonable analogue picture, but with some ghosting. Would I be right in thinking that if I got a digital set-top box this setup would be more forgiving of ghosting? I think the signal itself is fairly strong, but there's just a bit of reflection off nearby trees and things.
Ghosting is the result of signals being reflected from some other object between the transmitter and your receiving antenna. Because the signal takes a longer time to reach you (think of it as corners of a triangle, you get a ghosted image offset to the right on an analogue picture.
You can roughly work out the extra distance by measuring the offset as a percentage of the total screen width. On a standard 625line/50Hz TV (which is what you have, one line takes just under 64 microseconds in total, but thre is about 53 microseconds worth visible because of synchronisation time. So your extra delay is roughly the proportion of the screen for the ghosted image multiplied by 53 uS.
The TV signals travel at approx. the speed of light (299,792,458 metres/sec); so from this you can work out the extra distance. Generally the transmitter is a lot further from you than the reflecting object, so to a good approximation, the distance you calculate is the distance the object is from you. Sometimes, if the objects are very close, e.g. surrounding walls and other objects, the ghosted image will be so close that it just appears to be a fuzziness of the verticals of the picture.
With digital TV, you would not see ghosting directly, since the digital information is placed on an analogue signal and the decoder uses that. The decoder is taking the signal and making decisions millions of times per second (the data bit rate) about whether the signal is 1 or 0 and uses that. However it is doing that by sampling the level of the signal at that rate. This works well in principle, but there are problems with it. Interference to the signal or poor signal will mislead the decoder about whether given bits should be 0 or 1. Currently digital TV transmissions use a very low power in comparison with the analogue TV, which is one of the reasons why people are having to use better antennas for digital TV.
If you factor in an additional delayed signal (which is what ghosting on analogue is), the decoder will see it as interference to the wanted signal. Depending on how strong the delayed signal is relative to the wanted one, the decoder may make mistakes as it will with interference.
Up to a point, the decoder can take care of this because the transmitted digital signal has extra data added that is used to help the decoder detect and correct such errors. Beyond a certain rate of errors, these mechanisms fail and the result is the the picture and sound will freeze or the picture breaks into small blocks (called macroblocks). Obviously you don't want this.
Therefore, when an antenna is set up for digital reception, it is not a case of pointing and hoping or looking on the screen for best picture or minimum ghosting. A proper installer will have a piece of test gear which measures the rate of errors coming in the signal. This takes account of all the interference and multiple reception paths, and generally the objective is to minimise the error rate.
You could try with your existing antenna, but don't be surprised if you have to get a new, more directional one which will give a stronger signal and because it is more directional, reject unwanted reflected signals.
.andy
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Overall length is a better guide to gain than the element count, especially when the vendors cheat and don't count the elements properly.
I would have THOUGHT so, but....I don't know for sure.
You may need a slightly different aerial too.
(i) All digital transmission methids have one theing in common. They work better at high signals, and tend to stop working altogether on low ones. They do not, for the most part, degrade gradually and gracefully.
(ii) Andy sez, and I have no reason to douvbt it, that digital signals are at lower signal strength..
There are two points here. The first is that if the signals are weaker so are the ghosts. So unless you have almost double images now - in which case a better antenna pointing in a different direction slightly, is called for, that ought to work OK.
But the second point is I don't know what the modulation method is. I do know that it invloves digital compression to get teh data rate down, but how that data is encoded is not somethig I am aware of - however it would seem strange indeed if it were done in such a way as to make te signal peculiarly sensitive to low apmlitude time delayed analogues. Ghosts. Given the amount of computing power and the bitrate, it would alos seem that adaptive filtering to detect static ghposts and eliminate them would in fact be pretty simple. So my guess is that digital signal qre bothe encoded to elminintae ghost effects, and probably filtered to remove even more - at leats in better designed sets.
The technology of all this gubbins has come on very fast with mobile phones - fast and stable chips to do FFT stuff etc exist, and de-ghosting is pretty much basic algorithims for DSP's.
Another point to consider is that Yagi aerials are high "Q" which means they can be detuned by nearby solid (especially metal) objects. So try and mount the aerial as far away from dividing walls, water tanks etc as possible.
Paul
There is compression anyway (MPEG-2 for the video) and forward error correction. Modulation is Coded Orthogonal Frequency Division Multiplexer (COFDM).
More that you ever wanted to know can be found by Google searching using 'DVB-T' as a keyword.
.andy
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The critical parameters are the carrier to noise ratio and the error rate........
.andy
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Many thanks for the advice, I guess it's a try-it-and-see situation, but at least I'm better informed now :-)
It is, unless you want to get a professional in.
Try it over a week or two and also on different channels. The behaviour can vary according to which frequency is used and with the weather.
.andy
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