frequencies of DTT muxes after switchover - do aerial riggers know?

"Robin" wrote in message
.uk...

There is some relevance, even to simple domestic installations - there
must be people who receive adequate analogue reception through a
reasonably high gain grouped aerial.

As one of them may I seek advice?

I need to replace the current (25 year old) aerial/cable in order to get
a decent digital signal from CP. (Analogue is fine; and Telewest cable
means no pressing need.)

CP is just 14.5 km away. So my initial reading suggested a simple Group
A (e.g. XG8) would do fine. But then I see nearly all the neighbours
with new kit have wideband aerials - some of enormous size. There is
some risk of blockage/ghosting from a hospital near the line of sight.
But I've been on the roof and it isn't that near. So I guess it's
insurance against change.

So what do I do: go wideband and get a worse signal now? Go wideband
and risk a poor signal now? Go wideband but with a bigger aerial (at
greater cost - and some risk to the Victorian terrace's chimney)?

All the people using CP who have not so far had to change their aerials for DTT
might well have to change if the CP transmissions are no longer in Group A. I
hope that is one reason why the CP transmissions will remain in Group A.

When the power is increased it may be that a Group A aerial which gets adequate
signal now, would still collect enough signal from a non Group A transmission in
the future.

Frankly I think that is wrong that the government is forcing a change on us,
without bothering to tell us what exactly the change will be.

--

Michael Chare

On 3 Oct 2005 08:58:08 -0700, "
wrote:

Paul D.Smith wrote:
I don't agree with your advice Paul - there are plenty people for who
fitting a wideband aerial would be the worst thing to do - the ~8
million served by crystal palace for a start! There are many other
places where a grouped aerial is still the best bet (if not downright
essential!) - in these regions it would be very valuable to know what's
coming after switch off.

Why single out CP? Asks a man about to replace an aging TV aerial and
planning on using a wideband one :-)

Paul (and the other Paul),

because all the transmissions from Crystal Palace are in group A...

http://www.dtg.org.uk/retailer/tx_lse.html

As you can see from that website, there are many many transmitters
where the official advice is still to use something _other_ than a
wideband aerial for receiving analogue and digital. That advice is
unlikely to change post switch off (though exact frequencies may, hence
my original question). The aerial grouping may remain correct even if
further services launch after switch off.

There is some relevance, even to simple domestic installations - there
must be people who receive adequate analogue reception through a
reasonably high gain grouped aerial. They could swap to a wideband
aerial now and receive poor analogue and digital reception, or they
could wait until switch over, save a few quid, and receive adequate
digital reception through their existing grouped aerial. There must be
at least some people in this situation, but they (and we - the
"informed" techies) don't know because the information hasn't been made
available.

Maybe the CAI want to hide this fact from the public: that many folks
need do nothing to their aerials, and come switch over, digital will
just magically work for them. Far better to tell everyone that they
must get a DIGITAL aerial now otherwise they will lose TV at switch
over. Sounds like a great scam if you ask me!

My CAI installed did not seem to know anything about aerial groups.

Scott

wrote:
I was reading this document...

http://www.digitaltelevision.gov.uk/...odpractice.pdf

and came across this paragraph on page 19, just after an explanation of
"grouped" aerials, and the need to change them in some cases for
current DTT broadcasts...

"After switchover, while all public broadcasting services will remain
in the same group as before, this might not be possible for some other
digital channels. An appropriately qualified local installer should be
able to advise on the best type and group of aerial for each location."

So Bill (and others), have the lovely people at
www.digitaltelevision.gov.uk told you which frequencies will be used in
each location after switchover?

Or are you as in the dark as the rest of us, meaning that "An
appropriately qualified local installer" will have no better idea of
what aerial might be best after 2012 than anyone else reading
uk.t.d-tv?

Cheers,
David.


If you think you have problems, here in Northern Ireland, Divis has
digital muxes on 23, 26, 29, 33, 34, 48 with analogue 21, 24, 27, 31.

RTE from Claremount Carn analogue on 52, 56, 66, 68 and proposed digital
on 53, 57, 60, 63, 30, 34.

How do we combine all that on a domestic install???

Regards
Glenn...

"Paul Schofield" paul_AT_sonifex_DOT_co_DOT_uk wrote in message
...
Okay I can see that you don't need to have a wideband aerial to receive
analogue or digital signals from CP, but is there a reason why you
shouldn't
use a wideband aerial? In any case this is not due for another 7 years for
this region so hardly a desperate hurry.

A wideband aerial is almost by definition a poor aerial. There are enough
compromises needed to get the bandwidth wide enough to cover a whole group,
covering the massive range (not far off a whole octave) needed for a
wideband aerial results in a low gain aerial.

--

Brian Gregory. (In the UK)

To email me remove the letter vee.

Usenet wrote:


If you think you have problems, here in Northern Ireland, Divis has
digital muxes on 23, 26, 29, 33, 34, 48 with analogue 21, 24, 27, 31.

RTE from Claremount Carn analogue on 52, 56, 66, 68 and proposed digital
on 53, 57, 60, 63, 30, 34.

How do we combine all that on a domestic install???

Well, channel 34 is certainly going to present a challenge !


--
Mark
Please replace invalid and invalid with gmx and net to reply.

Paul (and the other Paul),

because all the transmissions from Crystal Palace are in group A...

http://www.dtg.org.uk/retailer/tx_lse.html

As you can see from that website, there are many many transmitters
where the official advice is still to use something _other_ than a
wideband aerial for receiving analogue and digital. That advice is
unlikely to change post switch off (though exact frequencies may, hence
my original question). The aerial grouping may remain correct even if
further services launch after switch off.

Thanks for the clarifications however there was a posting to this group only
a couple of weeks ago which quoted "a source" as saying that post analogue
switch off, it was being proposed that all digital signals be moved to the
centre of the current 21-69 channels leaving the top and bottom sections to
be sold off. If this were to happen, my (almost brand new) group A aerial
would be useless. And with my cynic hat on, there is no point in the push
to digital unless the government senses another big payout a'la mobile 3G!

Now looking at the Antiference TC range (partially reproduced below), the
wideband seems to have extra gain compared to the A so would this
compensate? Alternatively, where would I find real specs. on the gain
across the channel spectrum?

Technical Specification and Performance Data
TC18A TC18B TC18C/D TC18E TC18W
Forward Gain dBd 12.3 12.8 12.8 12.3 13.5
Forward Gain dBi 14.5 15.0 15.0 14.5 15.7
Front/Back Ratio dB 22 22 22 22 22
Acceptance Angle deg 18 17 18 19 19
Length mm 1809 1523 1394 1394 1404
Width mm 343 265 235 265 343
Windage N (at 130km/h) 91 73 65 68 75

Bottom line - I currently have an old aerial which gets good analogue (apart
from CH5 which is slightly worse) and digital but since the aerial and mast
is no it's last legs, and I want to put up an FM dipole too, I'm replacing e
verything. The only question is, with what sort of aerial?

ARGH........

Paul DS.

wrote in message
oups.com...

In other words, we're starting this switch over malarky in 3 years, but
we still haven't figured out what frequencies will be used, so it's not
possible to install a communal system now which won't need some further
work in 3-6 years time - unless no channel filters are used in the
system.

I think you're confusing communal aerials with MATV? Where I live the main
analogue and digital signals are in Group B and the communal system just
passes these via a 4-channel block filter and massive levels of
amplification. It doesn't demodulate/remodulate the channels in any way.
(There is an additional block filter for Channel 5 which is in Group A).
After switchover it will only be neccessary to remove the channel blocks
(and possibly some of the amplification) as the remaining digital signals
will all be in Group B.

(kim)

Paul D.Smith wrote:
Thanks for the clarifications however there was a posting to this group only
a couple of weeks ago which quoted "a source" as saying that post analogue
switch off, it was being proposed that all digital signals be moved to the
centre of the current 21-69 channels leaving the top and bottom sections to
be sold off.

I think I saw that quote - it's incorrect. The proposed sell off plans
have been outlined already...

"Once Digital Switchover is complete it is proposed that spare
frequencies will be allocated: Channels 21-30 and 41-62 will remain for
digital television broadcast. Channels 31-40 (excluding 36 and 38) and
63-68 will be sold off (possibly for other broadcast use). Channels 36
and 38 will be used for their current uses."

(lifted straight from http://wwww.dtt-tx-info.org/dso.html - there are
other "official" sources, but that website in particular seems to be
more accurate than "official" sources on many things!)

Remember digital signal work fine in adjacent channels (unlike
analogue), so it's quite easy for all digital transmissions to remain
"in group" for existing aerials, even with those frequencies sold off.

It's also quite possible that, if the "sold off" spectrum is used
(totally or partially) for DTT, these additional DTT muxes will also be
arranged to lie "in group" from each transmitter.

Possible, but not certain. I have _some_ confidence that this has been
thought through properly, but no one can be sure.

[snip]

Now looking at the Antiference TC range (partially reproduced below), the
wideband seems to have extra gain compared to the A so would this
compensate? Alternatively, where would I find real specs. on the gain
across the channel spectrum?

Technical Specification and Performance Data
TC18A TC18B TC18C/D TC18E TC18W
Forward Gain dBd 12.3 12.8 12.8 12.3 13.5
Forward Gain dBi 14.5 15.0 15.0 14.5 15.7
Front/Back Ratio dB 22 22 22 22 22
Acceptance Angle deg 18 17 18 19 19
Length mm 1809 1523 1394 1394 1404
Width mm 343 265 235 265 343
Windage N (at 130km/h) 91 73 65 68 75

Well, 1dB is neither here nor there - and (with my cynic hat on) they
are probably quoting the peak gain - i.e. in the middle of the band the
aerial is designed to cover. How does performance hold up at the edges
of the frequency band?

See the graph in this article by Bill...

http://www.wrightsaerials.tv/digitalterrtvrecep.htm

I suspect it's a made up graph, but it illustrates the point - the gain
of the wideband aerial falls off at lower frequencies, and (not shown)
so does its directivity.

Bottom line - I currently have an old aerial which gets good analogue (apart
from CH5 which is slightly worse) and digital but since the aerial and mast
is no it's last legs, and I want to put up an FM dipole too, I'm replacing e
verything. The only question is, with what sort of aerial?

You'll _probably_ find either will be better than what you have, but
I'd fit a good modern group A aerial and some CT100 cable.

Whatever happens elsewhere, even the government (= technical
incompetants) aren't going to change the grouping of signals from
Crystal Palace and require most of ~8 million people to buy new
aerials!

There _may_ be new services which appear (out of group A) after
analogue switch off (2011) - if these are new TV services, you won't
receive them, and neither will anyone else in London without a wideband
aerial.

ARGH........

Indeed.

Cheers,
David.

P.S. If neighbours are using wideband aerials, go and look at their
reception. If digital and analogue are both good enough for you, you
have your answer. If not, then similarly you have your answer!

kim wrote:
I think you're confusing communal aerials with MATV? Where I live the main
analogue and digital signals are in Group B and the communal system just
passes these via a 4-channel block filter and massive levels of
amplification. It doesn't demodulate/remodulate the channels in any way.
(There is an additional block filter for Channel 5 which is in Group A).
After switchover it will only be neccessary to remove the channel blocks
(and possibly some of the amplification) as the remaining digital signals
will all be in Group B.

That's exactly the situation I was thinking of kim.

It may be that "just" removing the filters will give good reception.

However, it may be that the filters are necessary to remove locally
generated or inserted RF junk (TETRA, RSL TV, on or co-channel
interference in a 2-aerial diplexed system etc).

You'll still need to prevent high powered unwanted transmissions from
entering the system (otherwise the main amplifier will be overloaded),
you'll still need to equalise signals of different power (either from
different transmitters, or due to local nulls etc), and you'll still
need to set up any diplexing or selective combining filters correctly.

In short, the system will need "some work" (= some expense!), but it's
not clear yet exactly what this work will be, even though switch over
is 3 years away in some regions. This makes "future proofing" a little
harder, and knowing exactly what work will be required impossible (at
this stage).

Cheers,
David.

wrote in message
oups.com...

If you've lived in rented accommodation, then you might know that there
are communal aerial systems out there that have been in use for 30
years, and haven't seen any maintenance during that time!

36 years where I live (although I wasn't here at the time). The distribution
system was designed primarilly for 404-line VHF use. The cabling is of the
high loss variety and just for good measure it has been tied in a knot to
prevent it becoming detached from the wall outlet! 'Maintenance' has
consisted of fitting channel blocks to reduce co-channel (and DTT) reception
and ever increasing levels of amplificatiion to compensate for losses at
higher frequencies to the point where Channel 5 is slightly overloaded.

The 'housing association' (formerly the city council) pays an annual sum to
an aerial contractor which was fixed back in 1969. For that reason the
contractor has no intention of spending his money on renewing the system and
the housing association has no intention of spending their's. The so-called
resident's association is less than helpfull, they looked at me like I was
dolally when I mentioned there was no FM radio feed of any kind. Their chief
concern was with extracting £19,000 for the rebuilding of a toddler's
playground which was vandalised as they looked-on and is likely to be
vandalised again in the near future. The same amount would have been enough
to rewire all 140+ low-rise dwellings on the estate.

(kim)