aerials - again :-(

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says...

I don't see how a 'bigger' (I assume greater number of elements) aerial can give worse multi-path.
Also I didn't think Digital Terestial TV was troubled by multi-path.
Ashley
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Yes but can you "explain" this please?. As it stands it does not make sense...

Current wisdom reckons that digi is relatively immune from multipath.

Can't be that many locations where digi will overload!....

75 dB what?..

--
Tony Sayer


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On Sat, 24 Jan 2004 10:03:07 +0000, "Andy Luckman (AJL Electronics)"

So what you're saying is, because a bigger aerial may have a narrower bandwidth (and therefore be much more sensitive to signals in the direction of the beam), it's more lightly to receive signals (multipath) outside the direction of the beam?

Ignoring for a second that your quoted figures are completely meaningless (dB relative to...), you're saying that if the signal from a channel is too strong, it's also more lightly to suffer from interference from much weaker signals - and therefore that a weaker main signal is actually less lightly to be subject to interference?
If these facts are correct you may have just revoloutionised the radiocommunications industry and changed our entire scientific understanding of electromagnetic radiation. We'll be able to improve reception substantially just by reducing the transmitter power.
Taken to it's logical conclusion, do you believe, for example that we may actually be able to provide a high quality, multi channel TV service using transmitters that have been switched off, or better yet possibly not even built?
Rgds Jonathan
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snipped-for-privacy@nospam.please says...

He must know what he's talking about - he's a member of CAI ! :)
Ashley
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Sadly that isn't always the case. We are all learning all the time, but I like to think 16 years+ in the business counts for something?
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AJL Electronics (G6FGO) Ltd : Satellite and TV aerial systems
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Signals arriving slightly off beam will cause multipath. The wider the beamwidth, the less effect multipath has, which is why "grid array" aerials are specified for areas that have reflections.

dBuV is the standard. The perceived wisdom is that 75dBuV is the limit for a digital installation. This is really only of relevance to the analogue services as the digitals are usually 15-20dB down anyway.

I said nothing of the sort. Excess signal causes intermodulation products. This has the same apparent symptom (in digital) of a weak signal, causing breakup and picture loss. On analogue the effect will be picture distortion or images floating past in the background.
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AJL Electronics (G6FGO) Ltd : Satellite and TV aerial systems
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You're confusing me here. By this definition an omni directional aerial would be best for multi-path conditions, which is not the case in practice.
--
*Procrastinate now

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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snipped-for-privacy@argonet.co.uk says... ...

Ahhh! That's where one of my sigs went - have you got the rest of them too, Dave? ;-)
John S [sigless]
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No, I have my own sig. ;-) If you mean tags - then yes, I've got the entire world supply.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Grid arrays have an extremely wide beamwidth as well as four active elements specifically to help with multipath. A correctly phased omni like the "globe" should be very good in multipath areas although the gain will (as has been shown) be useless.
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The multi-path is already there. The aerial cannot *cause* it. It can avoid the worst effects by being narrow-beam, and picking up just *one* path, not necessarily the 'main' one, which might be the most compromised.
One has to make sure the extra path found isn't a reflection from a crane or a tree, however...
--
Jim Crowther "It's MY computer" (tm SMG)
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Indeed. You have explained it better than I managed. Of course it is possible to align onto a reflection rather than the main lobe, which is less easy with a wider beamwidth. Hence my comment about narrow beamwidth being best where there is clear line of sight.
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Andy Luckman (AJL Electronics) wrote:

Er, no.
One of the major benefits of using a bigger yagi with a narrower beamwidth is REDUCED ghosting.
I thought you knew your stuff Andy!
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Provided the aerial is aligned to the correct signal and not looking at a reflection.
> I thought you knew your stuff Andy!
I do usually! I seem to be having trouble putting across what I *really* mean. :-) I suppose I could blame this virus I am recovering from and the cough / stabbing pains I have been left with, but that would just be making excuses wouldn't it?
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On Mon, 26 Jan 2004 10:55:29 +0000, "Andy Luckman (AJL Electronics)"

You are not alone.
I got a nasty cold/cough about 10 days ago. Felt a bit dreary but not the usual "I'm dead but still breathing" type of flu.
Ever since I've been whacked of energy.
PoP
Sending email to my published email address isn't guaranteed to reach me.
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Sounds like a mider version then. :-) Mine started with extreme temperature variations, loss of strength and pounding head. Then you get the cough followed by the stabbing pain in the side when you *do* cough. I am still putting up with the latter!

Mine's back now. It seems that many people have had whatever virus this was. I don't tend to get ill, so it was quite a surprise to my system!
--
AJL Electronics (G6FGO) Ltd : Satellite and TV aerial systems
http://www.classicmicrocars.co.uk : http://www.ajlelectronics.co.uk
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Have you tried connecting the twig directly to a TV with a new length of Coax, to eliminate the possibility of any bad connections anywhere?
If there is a short between the centre and the screen anywhere, then depending on your install, it could bring the whole system down.
Sparks...
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I haven`t, but I don`t have any coax going spare :-(
Might just say "ah b*ll*cks to it" and get another cheapo yellow pages plonker out... I just don`t like climbing on roofs (and at almost 18 stone I don`t think they`d like me either :-} )
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Colin Wilson wrote:

Try putting it in the loft? With the bracket in the centre, yes it can make a difference sometimes. Oh and use decent 75 ohm coax, sat stuff isn't always 75 ohms...
Niel.
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On Mon, 19 Jan 2004 11:52:09 +0000, snipped-for-privacy@soton.ac.uk wrote:

The only thing is Niel, that if the reception varies by channel outside, it will be subject to that to a greater degree in the loft.
.andy
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