In message , Brian
Morrison writes
On Fri, 14 Mar 2014 15:51:51 -0700
Bob E. wrote:

75-ohm RG-6 coax: quad shield differs from "standard" RG-6 in that
the dielectric is reduced in diameter to accomodate the extra
shielding.

How does this affect the performance? I'm looking at 1 GHz (HDTV
use).

Thanks.


If the dielectric is reduced in diameter then to maintain the same Zo
for the same Er the inner conductor will have to be reduced in diameter
too. This will tend to increase the loss in the cable because of the
increased resistive losses and the dielectric loss will be higher
because the field is more concentrated in the dielectric.

What is your prime requirement? Do you need the low loss or the good
screening more?

See existing thread.
--
Ian

--- news://freenews.netfront.net/ - complaints: ---

On 17/03/2014 13:14, Jerry Stuckle wrote:
On 3/17/2014 3:45 AM, Jeff wrote:

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!

Not in the United States. It was the minimum that the cable industry
provides to the TV set.

We are talking a signal 4.25Mhz wide signal, not SSB or CW.


dBm is not a bandwidth dependant measurement such as CNR which is.
Putting +7dBm into a tv receiver is madness, it would cause severe
overload and inter mods. +7dBm is 50mW and that equates to about 61mV in
a 75 ohm system which is an enormous signal.

Jeff

Wrong. TV's are made to handle at least 20 dbm. And cable tv companies
must deliver at least 10 dbm to the premises.

TV signals (at least in the U.S.) are not measured by CNR - they are
measured by dbm. CNR is not important because the bandwidth does not
change.

Your insistence on using CNR shows you know nothing about how the
industry measures signal strength.


I have not insisted that CNR was used, it that was another poster who
mentioned CNR, what I was pointing out was the error that you made in
taking the 43db CNR value that was posted and then going on about
dbMmlevels.

I also dispute that televisions are made to handle +20dBm; that is 100mW
far in excess of what a tv tuner can handle without overload!!!!

+20dbmV may be but not +20dbm.

Jeff

On 3/17/2014 11:32 AM, Ian Jackson wrote:
In message , Jerry Stuckle
writes
On 3/17/2014 3:45 AM, Jeff wrote:

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!

Not in the United States. It was the minimum that the cable industry
provides to the TV set.

We are talking a signal 4.25Mhz wide signal, not SSB or CW.


dBm is not a bandwidth dependant measurement such as CNR which is.
Putting +7dBm into a tv receiver is madness, it would cause severe
overload and inter mods. +7dBm is 50mW and that equates to about 61mV in
a 75 ohm system which is an enormous signal.

Jeff

Wrong. TV's are made to handle at least 20 dbm. And cable tv
companies must deliver at least 10 dbm to the premises.

You do realise that 20dBm (appx 68dBmV) is a massive 100mW? With a
modest 50 channel analogue cable TV system, that would be a total input
power of 5W - which would have a TV set or set-top box sagging at the
knees - if not even beginning top smoke!

TV signals (at least in the U.S.) are not measured by CNR

Well of course they aren't. CNR is a ratio - not a level.

- they are measured by dbm.

No. The US and UK cable TV industry definitely uses dBmV.


Which is generally shortened to dbm here. What you are talking about is
dBmW - which, unfortunately, is also often shortened to dBm. But most
people on this side of the pond who are in the business understand that.

0dBmV is 1mV - a reasonable signal to feed to a TV set (especially
directly from an antenna).

0dBm is appx 48dBmV (250mV) - and that's one hell of a TV signal!

With a 75 ohm source impedance (antenna and coax) - and no significant
levels of outside noise-like interference, a 0dBmV (1mV) analogue NTSC
signal, direct from an antenna, will have a CNR of around 57dB. A TV set
with a decent tuner noise figure (5dB?) or a set-top box (8dB) will
produce essentially noise-free pictures.

However, with an analogue TV signal from a large cable TV system, the
signal CNR will be much worse than 57dB (regardless of its level). If I
recall correctly, the NCTA ( National Cable Television Association)
minimum spec is a CNR of 43dB (UK is 6B). At this ratio, it is judged
that picture noise is just beginning to become visible.

CNR is not important because the bandwidth does not change.

You're havin' a laff - surely?!

Nope.


Your insistence on using CNR shows you know nothing about how the
industry measures signal strength.

I'm not insisting on anything. However, an analogue with a poor CNR will
produce noisy pictures - regardless of the signal level. Similarly, a
digital signal with a too poor an SNR/MER will fail to decode -
regardless of the signal level. I think the UK cable TV spec for digital
signals is 25dB (although a good set-top box will decode down to the
mid-teens).

External noise is somewhat consistent. Front ends are pretty much
comparable in their S/N ratio. The only problems with noise are
generally if you have something generating noise locally. But that is
not a problem with the signal nor the receiver.

That is why the real world uses signal strength to determine proper
signal levels. CNR in TV is not used nor is it required when the other
parameters are known.

--
==================
Remove the "x" from my email address
Jerry Stuckle

==================

On 3/17/2014 11:43 AM, Ian Jackson wrote:
In message , Jerry Stuckle
writes
On 3/17/2014 3:38 AM, Jeff wrote:

No one said the NTSC had to be noiseless. But the 43dB is a bit
high,
even for older sets. Input from the cable tv company to our
equipment
was 10-20dB; we tried to push 10dB to all of the outputs but never
had a
problem even down to 7dB (the lowest we would let it drop to).

That makes no sense; a 7dB CNR would be pretty much unwatchable on
analogue, it would be a very very noisy picture, if it even locked at
all!

Jeff


I'm not talking CNR - I'm talking signal strength. 7dbm is plenty of
signal. Most later TV's would work even at 0dbm.

Well the "43dB"that you were stating "was a bit high" was expressed as
CNR, so it is reasonable to think that your other figures were also CNR
as you did bot state otherwise.

Also 7dBm (5mW) is a very high signal and would cause most sets to
intermod like crazy. Perhaps you meant 7dBmV.

Jeff


Yes, I should have been more clear. It is 7dBmV - but the TV industry
generally shortens it to dbm (and that's how the test equipment is
labeled). Just like other industries which use dBmW generally shortens
it to dbm.

No. You are absolutely wrong. No one in the professional cable TV would
even think of referring to 'dBmV as 'dBm'. There's around 48dB
difference between the two.


Then why, pray tell, does the several $K Sencore signal analyzer sitting
on the back shelf (because it's now pretty much obsolete) say "dbm"? It
has been that way since I first started with MATV systems back in the
early 70's. It's so common many cable techs wouldn't know there even is
a dBmW.

However, you are right about 'dBmW' - which is invariably (and
regrettably) shortened to 'dBm'.

Sorry for the confusion - it's been about 10 years since I've been in
the field - I've been away from it for too long.

Well, I think it is beginning to show! [Sorry for being personal, as
it's something I always try to avoid.]





I've been in management for several years now. I still get out in the
field some - but I'm too old to be pulling cables. Leave it to the
youngsters.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 3/17/2014 11:58 AM, Jerry Stuckle wrote:
Then why, pray tell, does the several $K Sencore signal analyzer sitting
on the back shelf (because it's now pretty much obsolete) say "dbm"? It
has been that way since I first started with MATV systems back in the
early 70's. It's so common many cable techs wouldn't know there even is
^
company

a dBmW.


--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

Jerry Stuckle wrote:
On 3/17/2014 10:45 AM, Rob wrote:
Jerry Stuckle wrote:
On 3/16/2014 11:42 AM, Ian Jackson wrote:
In message , Jerry Stuckle
writes





HDTV requires a stronger signal than the old NTSC.

It really depends on how good your old analogue NTSC was. For a
noiseless picture, you would need around 43dB CNR, but pictures were
still more-than-watch-able at 25dB, and the picture was often still
lockable at ridiculously low CNRs (when you certainly wouldn't bother
watching it). Digital signals can work at SNRs down to around 15dB for
64QAM and 20dB for 256QAM (although if it's a little below this, and you
will suddenly get nothing).



That has not been our experience. We had a number of customers here in
the DC area who had great pictures on NTSC sets, but got either heavy
pixilation or no picture at all when the switchover occurred. We sent
them to a company which does tv antenna installations (we do a lot of
low voltage, including tv - but not antennas). In every case,
installing a better outdoor antenna solved the problem.

Most likely the company reduced the transmitted power by a factor
of 10 at the time of the switchover, to put the added link margin
in their own pockets.
(transmitting a megawatt of ERP as was regular in the analog days
puts a serious dent in your electricity bill, even when you have
a lot of antenna gain)


Not at all. If anything, they raised their power.

Here they went from 1 megawatt to about 50 kilowatt (ERP).
And then there are several programmes on one transponder, instead
of one analog programme. This gives significant savings in power.

Ian Jackson wrote:
Yes, I should have been more clear. It is 7dBmV - but the TV industry
generally shortens it to dbm (and that's how the test equipment is
labeled). Just like other industries which use dBmW generally shortens
it to dbm.

No. You are absolutely wrong. No one in the professional cable TV would
even think of referring to 'dBmV as 'dBm'. There's around 48dB
difference between the two.

However, you are right about 'dBmW' - which is invariably (and
regrettably) shortened to 'dBm'.

I think here it is more customary to express voltage levels in dBuV.

In message , Jerry Stuckle
writes
On 3/17/2014 11:32 AM, Ian Jackson wrote:
In message , Jerry Stuckle
writes
On 3/17/2014 3:45 AM, Jeff wrote:

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!

Not in the United States. It was the minimum that the cable industry
provides to the TV set.

We are talking a signal 4.25Mhz wide signal, not SSB or CW.


dBm is not a bandwidth dependant measurement such as CNR which is.
Putting +7dBm into a tv receiver is madness, it would cause severe
overload and inter mods. +7dBm is 50mW and that equates to about 61mV in
a 75 ohm system which is an enormous signal.

Jeff

Wrong. TV's are made to handle at least 20 dbm. And cable tv
companies must deliver at least 10 dbm to the premises.

You do realise that 20dBm (appx 68dBmV) is a massive 100mW? With a
modest 50 channel analogue cable TV system, that would be a total input
power of 5W - which would have a TV set or set-top box sagging at the
knees - if not even beginning top smoke!

TV signals (at least in the U.S.) are not measured by CNR

Well of course they aren't. CNR is a ratio - not a level.

- they are measured by dbm.

No. The US and UK cable TV industry definitely uses dBmV.


Which is generally shortened to dbm here.

I must emphasise that you are simply WRONG. None of the professional
cable TV engineers I've ever been associated with (both in the UK and
the USA have ever used the term 'dBm' when they mean 'dBmV'. Can you
think of a reason why? [Clue - There's 48dB difference between the two
units.]

What you are talking about is dBmW - which, unfortunately, is also
often shortened to dBm. But most people on this side of the pond who
are in the business understand that.

I can live with that. The incorrect use of 'dBm' to mean 'dBmW' is a de
facto industry standard - and I'm not going to try and change the world
by pretending that I don't understand the incorrect 'dBm'.

0dBmV is 1mV - a reasonable signal to feed to a TV set (especially
directly from an antenna).

0dBm is appx 48dBmV (250mV) - and that's one hell of a TV signal!

With a 75 ohm source impedance (antenna and coax) - and no significant
levels of outside noise-like interference, a 0dBmV (1mV) analogue NTSC
signal, direct from an antenna, will have a CNR of around 57dB. A TV set
with a decent tuner noise figure (5dB?) or a set-top box (8dB) will
produce essentially noise-free pictures.

However, with an analogue TV signal from a large cable TV system, the
signal CNR will be much worse than 57dB (regardless of its level). If I
recall correctly, the NCTA ( National Cable Television Association)
minimum spec is a CNR of 43dB (UK is 6B). At this ratio, it is judged
that picture noise is just beginning to become visible.

CNR is not important because the bandwidth does not change.

You're havin' a laff - surely?!

Nope.

OK. Are you by any chance related to John McEnroe?
http://www.youtube.com/watch?v=ekQ_Ja02gTY


Your insistence on using CNR shows you know nothing about how the
industry measures signal strength.

I'm not insisting on anything. However, an analogue with a poor CNR will
produce noisy pictures - regardless of the signal level. Similarly, a
digital signal with a too poor an SNR/MER will fail to decode -
regardless of the signal level. I think the UK cable TV spec for digital
signals is 25dB (although a good set-top box will decode down to the
mid-teens).

External noise is somewhat consistent. Front ends are pretty much
comparable in their S/N ratio. The only problems with noise are
generally if you have something generating noise locally. But that is
not a problem with the signal nor the receiver.

That is why the real world uses signal strength to determine proper
signal levels. CNR in TV is not used nor is it required when the other
parameters are known.

So pray tell me why, in my many years in the cable TV industry, I spent
so many pointless hours measuring (among all the other parameters) CNR?


--
Ian

On 17/03/2014 16:01, Jerry Stuckle wrote:
On 3/17/2014 11:58 AM, Jerry Stuckle wrote:
Then why, pray tell, does the several $K Sencore signal analyzer sitting
on the back shelf (because it's now pretty much obsolete) say "dbm"? It
has been that way since I first started with MATV systems back in the
early 70's. It's so common many cable techs wouldn't know there even is

I can't comment onyour Sencore signal analyzer as I have never used one,
BUT every other signal generator and spectrum analyser I have come
across and used, from HP/Agilent, R&S, MI etc etc when labelled dBm mean
dB relative to a milliwatt. Also every other RF engineer I have come
across universally understands dBm to mean dB relative to a milliwatt
NOT dBmV.

Just check the specs of any rf test gear line you will see that they
refer to dbm meaning dB relative to a milliwatt. Even Sencore's website
with the specs of their latest equipment, Where they mean dBuV or dBmV
they say so.

Jeff

In message , Rob
writes
Jerry Stuckle wrote:
On 3/17/2014 10:45 AM, Rob wrote:
Jerry Stuckle wrote:
On 3/16/2014 11:42 AM, Ian Jackson wrote:
In message , Jerry Stuckle
writes





HDTV requires a stronger signal than the old NTSC.

It really depends on how good your old analogue NTSC was. For a
noiseless picture, you would need around 43dB CNR, but pictures were
still more-than-watch-able at 25dB, and the picture was often still
lockable at ridiculously low CNRs (when you certainly wouldn't bother
watching it). Digital signals can work at SNRs down to around 15dB for
64QAM and 20dB for 256QAM (although if it's a little below this, and you
will suddenly get nothing).



That has not been our experience. We had a number of customers here in
the DC area who had great pictures on NTSC sets, but got either heavy
pixilation or no picture at all when the switchover occurred. We sent
them to a company which does tv antenna installations (we do a lot of
low voltage, including tv - but not antennas). In every case,
installing a better outdoor antenna solved the problem.

Most likely the company reduced the transmitted power by a factor
of 10 at the time of the switchover, to put the added link margin
in their own pockets.
(transmitting a megawatt of ERP as was regular in the analog days
puts a serious dent in your electricity bill, even when you have
a lot of antenna gain)


Not at all. If anything, they raised their power.

Here they went from 1 megawatt to about 50 kilowatt (ERP).
And then there are several programmes on one transponder, instead
of one analog programme. This gives significant savings in power.

That's quite s drop in power. In the UK, it seems that the digitals are
being run at 1/5th of what the analogues were. Certainly the main
transmitter for London, Crystal Palace, was 1MW erp, but is now 200kW on
the main six digital muxes. [There are also a couple more running around
10dB less.]
--
Ian