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Question about high gain aerials and dB's



 
 
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  #1  
Old December 14th 06, 09:43 PM posted to uk.tech.digital-tv
Marky P
external usenet poster
 
Posts: 1,971
Default Question about high gain aerials and dB's

Hello,

I've often wondered what is the actual difference between dB's is.
If, for example you have an aerial with a gain of 14dB & you get a
slightly grainy picture or occasional break up on freeview, how much
extra dB gain would cure it? And is an aerial such as the Triax Unix
100 overkill for even fringe area jobs? I'm still thinking about my
possible install of an aerial for Freeview in Ilfracombe early in the
new year.

Marky P
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  #2  
Old December 15th 06, 12:00 AM posted to uk.tech.digital-tv
Andy Wade
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Posts: 263
Default Question about high gain aerials and dB's

Marky P wrote:

I've often wondered what is the actual difference between dB's is.
If, for example you have an aerial with a gain of 14dB & you get a
slightly grainy picture or occasional break up on freeview, how much
extra dB gain would cure it?


As a _very_rough_guide_, for analogue/PAL, a 4 dB change in video S/N is
worth about one ITU-R (CCIR) picture grade. For pictures that are "a
bit noisy" a 1 dB improvement will be noticeable, 2 dB is well worth
having, 3 dB or more can be a substantial improvement.

Here are a couple of relevant BBC R&D reports that you might find
interesting:

http://www.bbc.co.uk/rd/pubs/reports/1987-23.pdf
http://www.bbc.co.uk/rd/pubs/reports/1988-12.pdf

As to impulse interference break-up on DTT, another 3 or 4 dB will again
usually give a worthwhile improvement.

--
Andy
  #3  
Old December 15th 06, 12:41 AM posted to uk.tech.digital-tv
Bill Wright
external usenet poster
 
Posts: 8,408
Default Question about high gain aerials and dB's


"Andy Wade" wrote in message
...
Marky P wrote:

I've often wondered what is the actual difference between dB's is.
If, for example you have an aerial with a gain of 14dB & you get a
slightly grainy picture or occasional break up on freeview, how much
extra dB gain would cure it?


As a _very_rough_guide_, for analogue/PAL, a 4 dB change in video S/N is
worth about one ITU-R (CCIR) picture grade. For pictures that are "a bit
noisy" a 1 dB improvement will be noticeable, 2 dB is well worth having, 3
dB or more can be a substantial improvement.

Here are a couple of relevant BBC R&D reports that you might find
interesting:

http://www.bbc.co.uk/rd/pubs/reports/1987-23.pdf
http://www.bbc.co.uk/rd/pubs/reports/1988-12.pdf

As to impulse interference break-up on DTT, another 3 or 4 dB will again
usually give a worthwhile improvement.

--
Andy


In the hard commercial world, where a 'worthwhile improvement' of 3dB leads
to DTT reception that breaks up quite a lot less often than it did before,
the result is likely to be, "Can you go back to Bash Street Flats? It still
isn't right."

I like to see c/n at least 10dB above threshold all the way through a
system, with signal levels at the outlets at least 10dB above receiver
threshold. The latter reduces come-backs caused by faulty flyleads,
resistive splitters in the wallplate, deaf boxes, etc.

It's no good ****ing about with the odd dB in this job. When you leave it it
has to be rock solid. If you can't do that you either take the appropriate
steps (bloody big ones) or you write a disclaimer letter.

I went to one today where they want DTT on the systems. It's a whole series
of small (8 dwelling) blocks. The DTT off the aerial is 50dB behind the
analogue. It's actually at a useable level, but because it's off the side of
the TX (about 30dB below the main lobe(s), a lot of the signal is reflected
from city towers. Yes, OK, I could install a stacked pair of grouped aerials
to null out the reflections as far as poss and discriminate against the
analogues, I could put all the muxes and analogues through channel filters
to get the levels right. So they're gonna spend 700 per (small block) to
get unreliable reception, when we'll be ripping it all out in about 52
months? I said "Let's give you a nice system with satellite on it, and then
you can all get Freesat." You've got to be pragmatic. Forget the niceties,
just do whatever gives the customer best value. They're thinking about it.

Bill


  #4  
Old December 15th 06, 08:34 AM posted to uk.tech.digital-tv
Jim Lesurf
external usenet poster
 
Posts: 787
Default Question about high gain aerials and dB's

In article ,
Andy Wade wrote:
Marky P wrote:


I've often wondered what is the actual difference between dB's is.
If, for example you have an aerial with a gain of 14dB & you get a
slightly grainy picture or occasional break up on freeview, how much
extra dB gain would cure it?

[snip]

Here are a couple of relevant BBC R&D reports that you might find
interesting:


http://www.bbc.co.uk/rd/pubs/reports/1987-23.pdf
http://www.bbc.co.uk/rd/pubs/reports/1988-12.pdf


As to impulse interference break-up on DTT, another 3 or 4 dB will again
usually give a worthwhile improvement.


Just to complicate matters... :-)

In some cases the level - as such - may not be the significant factor. Some
months ago I changed from using a low noise preamp near the receiver to
having a distribution amp up in the loft. The distribution amp has a lower
gain than the LNA. I also changed the cabling from old co-ax to
screen-and-braid 'CT100' co-ax.

Despite the lower signal level at the receiver, occurances of picture or
sound problems are now much rarer. I presume this is due to a combination
of having higher levels than before on the downlead(s) and the improved
screening. i.e. that the interference was 'local' and entering via the
downlead co-ax.

Alternatively, if there is interference from a source almost in the line of
sight, then a higher gain antenna may raise the level of the interference
as well as the signal.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html
  #5  
Old December 15th 06, 10:30 AM posted to uk.tech.digital-tv
Marky P
external usenet poster
 
Posts: 1,971
Default Question about high gain aerials and dB's

On Fri, 15 Dec 2006 01:41:33 -0000, "Bill Wright"
wrote:


"Andy Wade" wrote in message
.. .
Marky P wrote:

I've often wondered what is the actual difference between dB's is.
If, for example you have an aerial with a gain of 14dB & you get a
slightly grainy picture or occasional break up on freeview, how much
extra dB gain would cure it?


As a _very_rough_guide_, for analogue/PAL, a 4 dB change in video S/N is
worth about one ITU-R (CCIR) picture grade. For pictures that are "a bit
noisy" a 1 dB improvement will be noticeable, 2 dB is well worth having, 3
dB or more can be a substantial improvement.

Here are a couple of relevant BBC R&D reports that you might find
interesting:

http://www.bbc.co.uk/rd/pubs/reports/1987-23.pdf
http://www.bbc.co.uk/rd/pubs/reports/1988-12.pdf

As to impulse interference break-up on DTT, another 3 or 4 dB will again
usually give a worthwhile improvement.

--
Andy


In the hard commercial world, where a 'worthwhile improvement' of 3dB leads
to DTT reception that breaks up quite a lot less often than it did before,
the result is likely to be, "Can you go back to Bash Street Flats? It still
isn't right."

I like to see c/n at least 10dB above threshold all the way through a
system, with signal levels at the outlets at least 10dB above receiver
threshold. The latter reduces come-backs caused by faulty flyleads,
resistive splitters in the wallplate, deaf boxes, etc.

It's no good ****ing about with the odd dB in this job. When you leave it it
has to be rock solid. If you can't do that you either take the appropriate
steps (bloody big ones) or you write a disclaimer letter.

I went to one today where they want DTT on the systems. It's a whole series
of small (8 dwelling) blocks. The DTT off the aerial is 50dB behind the
analogue. It's actually at a useable level, but because it's off the side of
the TX (about 30dB below the main lobe(s), a lot of the signal is reflected
from city towers. Yes, OK, I could install a stacked pair of grouped aerials
to null out the reflections as far as poss and discriminate against the
analogues, I could put all the muxes and analogues through channel filters
to get the levels right. So they're gonna spend 700 per (small block) to
get unreliable reception, when we'll be ripping it all out in about 52
months? I said "Let's give you a nice system with satellite on it, and then
you can all get Freesat." You've got to be pragmatic. Forget the niceties,
just do whatever gives the customer best value. They're thinking about it.

Bill


Is that why you use TC18's on all your jobs?

Marky P

  #6  
Old December 15th 06, 10:41 AM posted to uk.tech.digital-tv
Andy Wade
external usenet poster
 
Posts: 263
Default Question about high gain aerials and dB's

Bill Wright wrote:

In the hard commercial world, where a 'worthwhile improvement' of 3dB leads
to DTT reception that breaks up quite a lot less often than it did before,


I take your point, although at does depend where the 3 dB improvement is
starting from of course. The change from 64-QAM to 16-QAM for four of
the muxes was only worth about 4 dB of C/N but seemed to make a big
difference to the perception of DTT to the public.

I like to see c/n at least 10dB above threshold all the way through a
system, with signal levels at the outlets at least 10dB above receiver
threshold. The latter reduces come-backs caused by faulty flyleads,
resistive splitters in the wallplate, deaf boxes, etc.


I agree with that 100%. In fact for 64-QAM the thermal noise threshold
will be in the low-30s of dBuV into the receiver. (~4 dBuV noise floor
from source assumed to be at 290 K, plus 6 dB or so receiver noise
figure, plus say 20 dB min. C/N gives 30 dBuV). The generally
recommended min. DTT signal for reliable reception is 45 dBuV, so the
noise margin is a healthy 10 to 15 dB for 64-QAM and even more for the
present 16-QAM. 45 dBuV seems to give freedom from impulse interference
problems in most cases, with a reasonable standard of installation - but
if you want it to go through old cabling, outlets and flyleads add at
least another 5 dB. Would you agree with that?

I went to one today where they want DTT on the systems. It's a whole series
of small (8 dwelling) blocks. The DTT off the aerial is 50dB behind the
analogue. It's actually at a useable level, but because it's off the side of
the TX (about 30dB below the main lobe(s),


50 dB - gosh, where's that? Is this analogue and digital from the same
TX, or analogue from a relay and DTT from its parent? Surely that's
going to be well-nigh impossible to receive reliably - even if you can
equalise the levels for the system, the DTT reception's likely to be
interference limited and might fall over at the first hint of any lift.

--
Andy
  #7  
Old December 15th 06, 11:56 AM posted to uk.tech.digital-tv
Bill Wright
external usenet poster
 
Posts: 8,408
Default Question about high gain aerials and dB's


"Marky P" wrote in message
...
On Fri, 15 Dec 2006 01:41:33 -0000, "Bill Wright"
wrote:
I went to one today where they want DTT on the systems. It's a whole
series
of small (8 dwelling) blocks. The DTT off the aerial is 50dB behind the
analogue. It's actually at a useable level, but because it's off the side
of
the TX (about 30dB below the main lobe(s), a lot of the signal is
reflected
from city towers. Yes, OK, I could install a stacked pair of grouped
aerials
to null out the reflections as far as poss and discriminate against the
analogues, I could put all the muxes and analogues through channel filters
to get the levels right. So they're gonna spend 700 per (small block) to
get unreliable reception, when we'll be ripping it all out in about 52
months? I said "Let's give you a nice system with satellite on it, and
then
you can all get Freesat." You've got to be pragmatic. Forget the niceties,
just do whatever gives the customer best value. They're thinking about it.

Bill


Is that why you use TC18's on all your jobs?

Marky P

We use Blake log periodics where we need wideband, two stacked Blake log
periodics to get more gain or better directivity, then the same using TC18s
for non-wideband. Very rarely we will use a large XG whatsit.

Bill


  #8  
Old December 15th 06, 02:02 PM posted to uk.tech.digital-tv
Bill Wright
external usenet poster
 
Posts: 8,408
Default Question about high gain aerials and dB's


"Andy Wade" wrote in message
...
Bill Wright wrote:
50 dB - gosh, where's that? Is this analogue and digital from the same
TX, or analogue from a relay and DTT from its parent? Surely that's going
to be well-nigh impossible to receive reliably - even if you can equalise
the levels for the system, the DTT reception's likely to be interference
limited and might fall over at the first hint of any lift.


On some of the roofs of the flats complex at Riverdale, Sheffield. The tx is
maybe 2km away and clear LOS. Typical levels on a Blake LP are analogue main
four 33dBmV, C5 analogue 0dBmV and hopelessly ghosty, since the main signal
is bounced off the Manor Top flats, DTT very variable and eratic, with big
slopes and notches in the muxes, but generally at about -20dBmV. On one
block it is about -10, but poor BER even with the analogues removed by
filters. 100m down the road there's no problem.

Bill


  #9  
Old December 15th 06, 05:19 PM posted to uk.tech.digital-tv
Steve
external usenet poster
 
Posts: 23
Default Question about high gain aerials and dB's



In some cases the level - as such - may not be the significant factor. Some
months ago I changed from using a low noise preamp near the receiver to
having a distribution amp up in the loft. The distribution amp has a lower
gain than the LNA. I also changed the cabling from old co-ax to
screen-and-braid 'CT100' co-ax.

Despite the lower signal level at the receiver, occurances of picture or
sound problems are now much rarer. I presume this is due to a combination
of having higher levels than before on the downlead(s) and the improved
screening. i.e. that the interference was 'local' and entering via the
downlead co-ax.


More likely the former - your system noise figure has reduced appreciably
and so the signal/noise ratio is better.

Steve
  #10  
Old December 15th 06, 07:58 PM posted to uk.tech.digital-tv
tony sayer
external usenet poster
 
Posts: 5,001
Default Question about high gain aerials and dB's

In article , Jim Lesurf [email protected]
and.demon.co.uk writes
In article ,
Andy Wade wrote:
Marky P wrote:


I've often wondered what is the actual difference between dB's is.
If, for example you have an aerial with a gain of 14dB & you get a
slightly grainy picture or occasional break up on freeview, how much
extra dB gain would cure it?

[snip]

Here are a couple of relevant BBC R&D reports that you might find
interesting:


http://www.bbc.co.uk/rd/pubs/reports/1987-23.pdf
http://www.bbc.co.uk/rd/pubs/reports/1988-12.pdf


As to impulse interference break-up on DTT, another 3 or 4 dB will again
usually give a worthwhile improvement.


Just to complicate matters... :-)

In some cases the level - as such - may not be the significant factor. Some
months ago I changed from using a low noise preamp near the receiver to
having a distribution amp up in the loft. The distribution amp has a lower
gain than the LNA. I also changed the cabling from old co-ax to
screen-and-braid 'CT100' co-ax.

Despite the lower signal level at the receiver, occurances of picture or
sound problems are now much rarer. I presume this is due to a combination
of having higher levels than before on the downlead(s) and the improved
screening. i.e. that the interference was 'local' and entering via the
downlead co-ax.

Alternatively, if there is interference from a source almost in the line of
sight, then a higher gain antenna may raise the level of the interference
as well as the signal.

Slainte,

Jim

And did it do anything for the FM?...
--
Tony Sayer

 




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