New TV aerial for 'strong' or 'weak' signal?

[Dave Plowman (News)]

It's not just losses but picking up rubbish on a poorly screened downlead. Made worse by many cheap aerials not actually being matched to 75 ohms.

[Derek ^]

A good one, with low/medium gain but a very clean polar diagram and wide bandwidth.

If that's what wolfbane is suggesting there must be reason for it, not that WB can't be wrong, but if it's just a matter of sourcing a log periodic against a random alternative, if I were you that's what I'd use.

You could do worse than make a posting on uk.tech.broadcast reporting your Wolfbane results. There very helpful professional aerial contractors on there. Bill Wright is one of them.

[Matt]

[Lobster]

OK, I'm learning fast today (from a standing start as regards TV aerials etc!).

Here's my Wolfbane table:

or

I assumed the top transmitter was the one to go for being nearest (1 mile away); that's the one which said "log periodic" aerial. But I gather that's not the transmitter to go for then?! In fact, my PC TV card has come back to life (sort of) and shows it was tuned into Winter Hill, 30 miles away. All the aerials in the street point that way (and it's consistent with the quoted bearing of 329 deg. And for that, "amplified extra hi-gain" aerial is specified. "Weak" signal then?

I'm a bit pissed off the aerial's come down, as I had a guy up there about a year ago rewiring it when I set up an internal distribution system (yes, all CT100 cable!) and funnily enough he didn't mention it was obviously on its last legs (the square section has corroded right through and snapped). Anyway, the bugger's not getting a return trip out of it as I'm planning to diy.

So do I gather the Screwfix aerials (16203 looks very like the old one, and I think the neighbours) are a bit crap then?

Thanks David

[tony sayer]

Yes, well a mile away from a small repeater no problem at all unless you have a stonking great big hill in the way.

That TX is on Group A for analogue transmissions but the log would have been recommended for future digital services unless their already in existence. A wideband type will do fine I can't see any engineering reason why a log should be used.

And with those sort of powers involved no worries about overload.

The neighbours aerials were probably put up a long time ago before the existence of the relay station, and perhaps some of them are now tuned to that perhaps, but not pointing at it!. In any case they'd be from group C and well of resonance for the local..

[The Natural Philosopher]

No. Poor coax will attenuate - some frequencies more than others, but if the antenna kicks out enough, it will not be an issue.

Poor coax unless its totally BROKEN and in high signal areas, will not introduce ghosts or pickup MUCH in the way of noise and interference. Good coax is better, but bad coax and a decent antenna beats good coax and a piece of coat hanger any day :D

[Mark]

Both Macclesfield and Langley are only local repeaters which may have only recently been erected and at 36 and 6 watts ERP respectively are very low power and intended to fill dead spots locally, fine if you are in one of these, but note that they are Vertically polarised. Winter Hill would be your nearest main transmitter, which of the three you get the best reception from is a matter of trial and error without a signal strength meter, but note that the first two are group A and WH is CD and horizontally polarised. A broadband Ariel may work fine with the two local transmitters but may struggle receiving digital from Winter Hill.

-

[Andy Luckman (AJL Electronics)]

Your knowledge of this subject is lacking.

Rubbish. A kink or badly made off connector WILL intoduce reflections and losses. Poor quality cable will also have poor impedance consistency, again adding to the problem. Couple that with the increase in attenuation with higher frequency and you have a recipe for disaster.

I do understand YOUR motivation to do every thing "cheap", but don't advise others to waste their money in the same way please.

Not necessarily.

[Andy Luckman (AJL Electronics)]

Have you checked what is being broadcast? Most repeaters carry only 4 analogue channels.

I fear that any sort of washing powder will not receive digital TV services. :-)

[Christian McArdle]

Log periodics are used for transmitters that are located very close to the receiving station. The advantages of them for this situation are:

1. Very even frequency response means that a lower frequency high power transmission (think analogue) won't overpower the weak low power transmission which might happen to be at a higher frequency (think digital). Log periodics are inherently wideband and suitable for digital transmissions that may be far from the original analogue band of the site. They tend to be more expensive and have lower gain than a conventional Yagi, though.
2. Very tight side and rear lobes help prevent the reflections and ghosting that are characteristic of being in a high power area close to the transmitter.

However, most of the aerials might use a high gain pointed at the far away transmitter either because

1. the relay wasn't known about.
2. the relay is of exceptionally low power or is not line of sight
3. the relay doesn't support digital.
4. there is a large reflective object, such as a gasometer causing an exceptionally hard ghost image from that relay which is easier solved by pointing at Winter Hill than by fiddling about with stacked arrays.

Christian.

[tony sayer]

In article , Christian McArdle writes

Umm, that doesn't sound like very precise science Christian. Can you give your reasoning in a bit more detail?...

Correct..

Well if the main TX is sufficient then yes why bother to move/change the aerial..

Yes..

Is there such a large object?. And if so, wonder hoe Wolfbane would have known about it?...

[Christian McArdle]

The gain response of a log periodic is very smooth, without and rear or side lobes that can make setting up in a reflective environment so difficult. A quick browse of the internet should find you some charts of log periodics and yagi arrays for comparison.

Christian.

[Andy Wade]

Actually the gains of typical TV log-periodics are not so low as commonly supposed. They compare well with the so-called 'wideband Yagi' types, particularly at the LF end of the band. The trouble is that the manufacturers only tend to quote the gain of Yagis at the HF end of the band, where they are actually working as Yagi arrays. At the LF end they are really corner reflectors - all the gain comes from the arrangement of the dipole and the angled reflector and the directors are doing very little. The gain at channel 21 is typically 3 dB (or more) less than at channel 68. In the past the manufacturers haven't told you that, and some may have quoted exaggerated gain figures anyway.

Take a look at the DTG/CAI document "Guidelines for the use of Benchmarked Aerials"

of the wideband Yagi products which have been approved under this scheme fall in Standard 2 or 3, the former requiring forward gain of 7 to 10 dBd and the latter 5 to 8 dBd. In comparison the spec. for log-periodics (Standard 4) requires a flat 7 dBd. So in Group A, a small Yagi (Standard 3) actually has 2 dB less gain than the log and the quite large Standard 2 product only equals the log's performance.

For an up-to-date list of benchmarked aerials see

list on the DTG site seems to be out of date.

[Christian McArdle]

I hadn't realised that the wideband yagis were quite so bad. I thought that the wideband criteria would favour log periodic, but I hadn't realised that they would actually surpass yagis in gain for large parts of the spectrum.

Christian.

[Lobster]

OK, update from the OP... I went up to the chimney for a shufti yesterday and really wasn't happy up there, so decided I'd best get a pro in.

Today I was quoted 47+vat for a straightforward replacment of the existing bog-standard 8-element aerial; or about 90+vat if I wanted a digital-compatible one (and that's definitely just for the aerial, as all the cable is already new CT100). Wow!

The guy said it would be one like 'next door's - which from my uneducated viewpoint looks something like this one: . I reckon about 1 in 20 aerials in our street are like that; the rest are like mine. Is this beast really so much more expensive than a common or garden aerial, and is it necessary? Assuming labour is the same for both aerials, this fancy thing alone is about 50 quid *more* than the standard item. And Screwfix sell a digital aerial (Maxview) for 7 quid. So is this rigger guy taking the proverbial? Seriously thinking about diy'ing again, if I'm going to have to pay >100 quid :-( I'd definitely rather be future-proof and have a definitely digital-compatible aerial.

Incidentally, I already use a feed off the (now broken) aerial for the digital TV card in my PC, and that seemed to work perfectly OK. Does that mean a standard aerial should be OK? As in, is digital TV reception "all or nothing"??

Thanks David

[Christian McArdle]

Nah. It's not all or nothing. The bit rate gets worse and you start getting drop out, particularly in bad weather. If you're going to pay for someone to be up there, you might as well fit something decent rather than a crappy contract aerial.

Christian.

[Bob Eager]

But I guess you get what you pay for. I got a nice 45 element wideband antenna for about 30 quid (plus VAT). The 75 element one was about 10 quid more, AFAIR. That's a Televes DAT 45 (or 75) which is on the DSG 'recommended' list. Oh, that's from CPC.

[tony sayer]

In article , Christian McArdle writes

How do you work that one out?. Are we at the TX or RX end?. Actually logs are used extensively at small relay stations but mainly for logistic reasons..

That can be either way round and why should a log aerial be sooo much better?. Sorry but that reasoning is rather weak...

Not so simple as that. Wideband yes, but if your digital transmissions are in a group setting like Crystal palace in London, no particular sense in a wideband aerial there..

Not really that simple as the AGC will adjust the TV RX gain accordingly. Anyway digital, does it matter?.

[Andy Wade]

Bad in Group A - my point exactly. And I was talking about 'quality' benchmarked product. If you look at the cheapo contract wideband

10-eles you can find gain at 470 MHz of less then 0 dBd! (Also beware vendors who quote gains in dBi and hope you haven't noticed.)

NALOPKT. Log-periodics make a lot of sense, especially if you want to fit Group W for future-proofing.

[Lobster]

OK, thanks. So is the highly-expensive (IMHO!) digital aerial that this bloke wants to fit sound like it might be what's required?

If I was to diy I'd be very grateful for pointers as to where to buy a decent aerial of the correct spec! What about this one?

Still way less than the apparent cost from my local riggers...

Thanks David