Right TV aerial?

[tony sayer]

In article , Christian McArdle writes

I think you'll find in general Christian, its more than that in a lot of instances!..

[Christian McArdle]

Not necessarily. Indeed, take the Blake DMX10A (Band A) mentioned earlier. According to Blake, it has 12.89dB gain, just 0.29dB more than the DMX10WB (Wideband) at 12.60dB.

Obviously, you could go for a genuine wideband log periodic rather than a Yagi. These definitely have lower gain figures. However, the gain doesn't tell the whole story, as the log periodic has a much smoother response spectrum and smaller side lobes and so can often provide better performance in actual service.

Christian.

[tony sayer]

In article , Christian McArdle writes

Well the Yagi array isn't a very wideband device to start with!, and no matter what you do you simply cannot change that!...

In some circumstances.. but I think you'll find people such as Bill Wright who spend at LOT of time on practical installation, will recommend a grouped aerial for difficult reception cases over either a log or the abortion known as a wideband Yagi.....

[Chris Bacon]

You lot ought to x-psot to uk.tech.digital-tv or whatevetr it'sgh called, and taLK to Mr. Wright.

[John Phillips]

This is not a valid comparison for receiving group A signals. Because:

1. TV aerial gain is not constant over its rated bandwidth
2. "Official" aerial gain figures are usually the peak gain value
3. Peak gain for a WB aerial is typically at the top end of the UHF band
4. Group A frquencies are at the bottom end of the UHF band so you will not get close to the WB aerial's "official" gain figure for signals in group A.

In actual practice the DMX10WB will be 3 or more dB worse *for signals in group A* than the DMX10A. If you are receiving a grouped set of signals it is certainly true that a matching grouped aerial will be better than a wide band aerial.

Right now with many parts of the country only marginally covered until the transmitter power is increased by region over the next few years an aerial that has 3 dB better gain may make the difference between good and poor reception.

[The Natural Philosopher]

I'd settle for a digital feed that didn't randomly have digital errors on it, on different MUXes on different days..

Bags of signal level is no use if what is being fed to the transmitters is garbage..

[Christian McArdle]

Indeed, which is one of the reasons why the apparently lower gain log periodics perform well against transmitters with a wide frequency band compared with a Yagi, which is always a bit of a compromise for this.

However, even 3dB really isn't that much in the scheme of things. You'd have to be really in a properly marginal area to make that 3dB really count. I'd prefer to fit a wideband aerial with a few extra directors than go for a smaller aerial banded to a frequency range that might change or be expanded for HDTV and, thus, require replacement in 5 years time.

Indeed, this is exactly what I did by putting up the DAT75. From a marginal area where not all multiplexes should be receivable, I get them all on full signal with a remarkably good error rate. I will continue to get any new services, whether they be DVB-T after the changeover or HDTV in five or ten years time.

Christian.

[John Phillips]

I agree. A LP has a good broadband match that the tuned yagi cannot equal but you do get low gain from the LP. A LP is the aerial of choice in a strong signal wideband situation.

I might agree in an analogue system. However in a digital TV system, although the "digital cliff" between working and not isn't quite as bad as some say, 3 dB can in practice be very significant for performance.

Unfortunately the extent of the "amplified extra high gain aerial" region at

shows that many will indeed be in that marginal situation until the DTTV power levels go up at analogue switch-off. The "amplified" bit only gets you a 3 dB or so improvement in link budget over "extra high gain".

Yes. I investigated the gain vs. frequiency of the DAT45 and DAT75 in the Televes data book. They are remarkable, if big and heavy, aerials. For some they will be the solution. However in a group A area the group A Blake DMX10A is as effective as the wideband DAT45 and much smalller/lighter (and cheaper). The DAT75 is another thing if you can accommodate it.

[tony sayer]

Wouldn't give one of them rooftop room.! If I needed more gain than what a standard Yagi could give then I'd phase Two of them together!...

[Christian McArdle]

All I can say is that the DAT75 works very well in practice. Where I live, almost all aerials point towards Crystal Palace, as the correct transmitter (Hannington) is a fair distance away on the other side of a hill and suppresses the digital signal in our direction. Typical contract UHF aerials give a very poor digital performance on Hannington. The DAT75 gives 100% on the signal meter on every multiplex.

Whilst phasing two Yagis together might get you a nice aerial, it would be a lot of work compared to buying the DAT75, which is a bit like 3 Yagis sharing a dipole and reflector.

Christian.

[Andy Wade]

A well-designed log-periodic _should_ give a good broadband match, but many of the commercial designs don't. Note that the relevant CAI/DTG aerial benchmarking standard (Standard 4) only requires the aerial to have 6 dB return loss at the feedpoint (VSWR = 3), which is the same as for the Yagi-derived types.

Also note that the gains of "wideband Yagis" at the bottom of the band is not necessarily higher than for a log: e.g. a Standard 2 Group W Yagi is only required to have 7 dBd gain over ch. 21-36, which is exactly the same figure as for a Standard 4 log. (More reading in

>However, even 3dB really isn't that much in the scheme of things. ... >

Quite agree - it all depends on how near your are to the cliff edge. During the "equalisation" programme many transmitter powers were doubled (+3 dB) which brought about worthwhile reception improvements for a lot of people. Similarly for the change from 64-QAM to 16-QAM for four of the muxes, which is worth about 4 dB of link budget.

I've always thought that Wolfbane should be taken with a very large pinch of salt.

Hmm, a masthead amp can be worth quite a bit more than that. Typical system without a preamp will have about 4 dB feeder loss (including flylead) and 6 dB STB tuner noise figure (some better, some worse), so a typical system noise figure seen at the antenna o/p is ~10 dB. Put in a decent preamp (NF = 2 dB) with only modest gain (say 10 dB) and the system NF will come down to under 4 dB. That (with the usual assumption that the antenna noise temperature is 290 K) gets you a 6 dB improvement.