aerials - again :-(

[Andy Luckman (AJL Electronics)]

All other things being equal, a bigger version of the same aerial will have a narrower beamwidth. This can give rise to all sorts of grief unless the line of sight is "clean". The most likely problem will be multipath (ghosting on analogue). Although the new modulation will help a bit, digital still isn't keen on it either. :-)

Agreed, but here I am talking digital. We work to a maximum input (any channel) of 75dB which is fairly easy to exceed. Analogue can take much more (80-90dB) until the picture distorts or more likely another channels image will float past in the background.

[Andy Luckman (AJL Electronics)]

No, the only thing that needs to watched is the waterproof cap. Unless you clip it down properly, it can come off in the wind and write off the aerial and cable. The build standard has dropped a little since Triax was bought out, but they are still the best in my opinion. Have a look at the way the directors are fitted, much better than Antiference's (TC range) aluminium rivet idea that comes loose just by looking at it!

Can't praise the Televes highly enough. We use the Pro 45 as a problem solver. It is everything an aerial should be. The DAT versions have an onboard 13dB pre-amp which again is done properly. The only slight criticism is the time it takes to assemble the boxed versions. The bagged ones usually come in with elements broken sadly.

[Ashley Booth]

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

[tony sayer]

In article , Andy Luckman (AJL Electronics) writes

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?..

[N]

I agree about the cap, once secured properly I always used to then wrap a fair bit of tape around it just to make sure some giant bird or something didn't disturb it. The only TC aerial I have used is a wideband TC10 that I use myself for my touring caravan, it's a few years old now, been put up, taken down, packed away, driven hundreds of miles many times and is still intact. I only selected it because of the compactness of the reflector compared to others and it performs better than a contract aerial, it did a surprisingly good job for OnDigital/ITV Digital etc. although it was helped by a Televes set back variable amp with an ultra low noise figure and a substantial gain - something like up to 36db, I got reception in a number of places where officialy you couldn't. I tend to take a sky minidish now so the TC10 is in semi- retirement so I may not find out how long it takes to fall apart!

I didn't realise the DAT had a pre-amp it's the Pro series I must have been thinking of, might try one soon.

Regards

[Jonathan C]

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

[Ashley Booth]

He must know what he's talking about - he's a member of CAI ! :)

Ashley

[Andy Luckman (AJL Electronics)]

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.

[Andy Luckman (AJL Electronics)]

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?

[Dave Plowman]

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.

[Dave Plowman]

No, I have my own sig. ;-) If you mean tags - then yes, I've got the entire world supply.

[Jim Crowther]

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...

[John Stumbles]

In article , [e-mail] says... ...

Ahhh! That's where one of my sigs went - have you got the rest of them too, Dave? ;-)

John S [sigless]

[R W]

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!

[Andy Luckman (AJL Electronics)]

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.

[Andy Luckman (AJL Electronics)]

In article , Jim Crowther crane or a tree, however...

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.

[Andy Luckman (AJL Electronics)]

Provided the aerial is aligned to the correct signal and not looking at a reflection.

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?

[PoP]

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.

[Andy Luckman (AJL Electronics)]

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!