A Sky, cable and digital tv forum. Digital TV Banter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » Digital TV Banter forum » Digital TV Newsgroups » uk.tech.digital-tv (Digital TV - General)
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

Question about high gain aerials and dB's



 
 
Thread Tools Display Modes
  #11  
Old December 16th 06, 08:44 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 , Steve
wrote:


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.


I doubt that is the case for two reasons.

1) The noise figures for the two amps are nominally similar, and both are
low.

2) The problems show the symptoms of being due to impulse interference, not
random noise.

FWIW The DTTV RX reports a CNR value, and this also remains high, but I
have no idea how the RX estimates this so it may be spurious.

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
Ads
  #12  
Old December 16th 06, 08:46 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 , tony sayer

wrote:
In article , Jim Lesurf [email protected]



And did it do anything for the FM?...


I use completely different antenna and coax systems for FM and TV.

That said, I did wonder about the units that have inputs for TV, FM
and DAB antennas and then MUX them all down the same leads. Do any
of those work well enough to be worth trying?

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
  #13  
Old December 16th 06, 05:34 PM posted to uk.tech.digital-tv
Steve
external usenet poster
 
Posts: 23
Default Question about high gain aerials and dB's

On Sat, 16 Dec 2006 09:44:07 +0000, Jim Lesurf wrote:

In article , Steve
wrote:


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.


I doubt that is the case for two reasons.

1) The noise figures for the two amps are nominally similar, and both are
low.


In your original system the noise figure was worse by the loss of the
downlead coax - I jumped to conclusions about that being high, maybe
wrongly, but I've been surprised by how bad simple downlead coax can be
when it's a few years old.

Quality throughout is certainly the key.

Steve
  #14  
Old December 16th 06, 11:19 PM posted to uk.tech.digital-tv
Marky P
external usenet poster
 
Posts: 1,971
Default Question about high gain aerials and dB's

On Sat, 16 Dec 2006 09:46:07 +0000 (GMT), Jim Lesurf
wrote:

In article , tony sayer

wrote:
In article , Jim Lesurf [email protected]



And did it do anything for the FM?...


I use completely different antenna and coax systems for FM and TV.

That said, I did wonder about the units that have inputs for TV, FM
and DAB antennas and then MUX them all down the same leads. Do any
of those work well enough to be worth trying?

Slainte,

Jim

Oddly enough, when I used to use a DAB/FM/TV diplexer, I found the
signal on FM & DAB to be slightly better, when really the reverse
should be true. I did use an amp to overcome the spliiting losses
though.

Marky P
  #15  
Old December 16th 06, 11:50 PM posted to uk.tech.digital-tv
Paul Ratcliffe
external usenet poster
 
Posts: 2,486
Default Question about high gain aerials and dB's

On Sat, 16 Dec 2006 09:44:07 +0000 (GMT), Jim Lesurf
wrote:

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.


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


I doubt that is the case for two reasons.

1) The noise figures for the two amps are nominally similar, and both are
low.


Even though the noise figure of the amps. may be similar, you WILL have
improved the overall system carrier/noise ratio merely by putting the
amp. at the top rather than the bottom, all other things being equal.
As you have also improved the co-ax, you will get a further benefit
in system C/N figure.
  #16  
Old December 17th 06, 09:34 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 , Paul Ratcliffe
wrote:
On Sat, 16 Dec 2006 09:44:07 +0000 (GMT), Jim Lesurf
wrote:


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.


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


I doubt that is the case for two reasons.

1) The noise figures for the two amps are nominally similar, and both
are low.


Even though the noise figure of the amps. may be similar, you WILL have
improved the overall system carrier/noise ratio merely by putting the
amp. at the top rather than the bottom, all other things being equal.


Yes, I'd agree. However the cable run is less than 10m and the old cable
was in decent condition. But as I pointed out, the symptoms and the
measurements I made indicated that the real problems were with II rather
than continuous background noise.

As you have also improved the co-ax, you will get a further benefit in
system C/N figure.


I am not certain that the new cable has lower loss than the old. What is
clear is that the screening is better. The old cable had a sparse braid,
but the new has more braiding, and also foil thick enough to cut my fingers
if I try to fold it back without due care. :-)

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
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT. The time now is 12:01 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.SEO by vBSEO 2.4.0
Copyright 2004-2018 Digital TV Banter.
The comments are property of their posters.