On 2006-06-06, Christian McArdle wrote:
1. TV aerial gain is not constant over its rated bandwidth

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.

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.

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

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.

... You'd have
to be really in a properly marginal area to make that 3dB really count. ...

Unfortunately the extent of the "amplified extra high gain aerial"
region at http://www.wolfbane.com/articles/ukdcmap.htm 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".

... 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.

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.

--
John Phillips