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Looks like I managed to find the cause of 4g getting into the Ch 62and Ch 64 modulators....



 
 
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  #11  
Old July 20th 19, 08:34 PM posted to uk.tech.digital-tv
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Default Looks like I managed to find the cause of 4g getting into the Ch62 and Ch 64 modulators....

All of these 32 downleads all go to triax shielded triplexer wallplates.

Not all of these wallplates are connected to TVs or radios. So in theory the wall plate unused sockets should have terminators screwed/plugged in.

I've *never* seen a terminator screwed/plugged into a wall plate socket.

So why should a damaged screen on a coax cable be any different to a wall socket that has been left unused and unterminated?
  #12  
Old July 20th 19, 08:41 PM posted to uk.tech.digital-tv
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Default Looks like I managed to find the cause of 4g getting into the Ch62 and Ch 64 modulators....

As those unterminated sockets could also allow unwanted signals to get in and travel back to the multiswitch......
  #13  
Old July 21st 19, 04:01 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Default Looks like I managed to find the cause of 4g getting into the Ch62 and Ch 64 modulators....

On 20/07/2019 20:34, wrote:
All of these 32 downleads all go to triax shielded triplexer wallplates.

Not all of these wallplates are connected to TVs or radios. So in theory the wall plate unused sockets should have terminators screwed/plugged in.

I've *never* seen a terminator screwed/plugged into a wall plate socket.

So why should a damaged screen on a coax cable be any different to a wall socket that has been left unused and unterminated?

The reason for terminating outlets is to absorb signal by converting it
to heat. That is a necessity in some configurations because the signal
would otherwise return to whence it came; would arrive there with a
random phase relationship to its outgoing self with unpredictable
results. However there are very few devices used to output signal that
would be affected by such a return signal. The outputs are generally
well isolated from each other, and also little or no signal can get to
the device input.

Instances where non-termination of an outlet would matter? Nowadays,
few. In the old days we sometimes had tap-off lines that used
non-directional resistive taps, and there was always the theoretical
possibility that a return signal could get back through a very low value
tap and cause phase cancellation at the previous tap in the line. More
likely were cases where a number of outlets were fed from a resistive
splitter. There really would be trouble if some outlets weren't
terminated, and given that many TV sets didn't actually terminate across
the band some very strange effects were possible. "If the kitchen telly
is on ITV the living room telly has no sound on BBC2."

Non-termination of actual tap-off lines can be guaranteed to cause havoc.

Unterminated outlets of good quality and design radiate or receive very
little signal. I can't tell you why. Discontinuities in the screen along
a feeder, however, can radiate and receive quite readily. If the screen
isn't connected at an outlet nothing much will happen if the outlet
isn't used, but as soon as something is connected to it things will go
awry.

Bill
  #14  
Old July 21st 19, 08:44 AM posted to uk.tech.digital-tv
Roderick Stewart[_3_]
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Default Looks like I managed to find the cause of 4g getting into the Ch 62 and Ch 64 modulators....

On Sun, 21 Jul 2019 04:01:26 +0100, Bill Wright
wrote:

Unterminated outlets of good quality and design radiate or receive very
little signal. I can't tell you why.


Some semiprofessional video equipment had outputs that could detect
the presence of a 75 Ohm termination and automatically switch the
signal off if it appeared that nothing was connected. I wonder if your
good quality RF equipment was doing the same?

Rod.
  #17  
Old July 21st 19, 10:08 AM posted to uk.tech.digital-tv
Ian Jackson[_7_]
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Default Looks like I managed to find the cause of 4g getting into the Ch 62 and Ch 64 modulators....

In message , Bill Wright
writes


screen along a feeder, however, can radiate and receive quite readily.
If the screen isn't connected at an outlet nothing much will happen if
the outlet isn't used, but as soon as something is connected to it
things will go awry.

Indeed. If there's a break in the screen, after the break the inner is
no longer screened at all, and will readily pick up any RF flying around
(and, of course, radiate). If there's no on-going cable after the break
(as at an unused wall outlet) there's very little RF pick-up (apart from
on very tip of the inner that the RF can 'see').

If the break is only partial (high-resistance), the inner that follows
is partially unscreened. If the screening is simply 'poor' and leaky, it
is effectively a continuous high-ish resistance RF-wise - and so again
the on-going inner is poorly screened.
--
Ian
  #18  
Old July 22nd 19, 12:18 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Default Looks like I managed to find the cause of 4g getting into the Ch62 and Ch 64 modulators....

On 21/07/2019 08:44, Roderick Stewart wrote:
On Sun, 21 Jul 2019 04:01:26 +0100, Bill Wright
wrote:

Unterminated outlets of good quality and design radiate or receive very
little signal. I can't tell you why.


Some semiprofessional video equipment had outputs that could detect
the presence of a 75 Ohm termination and automatically switch the
signal off if it appeared that nothing was connected. I wonder if your
good quality RF equipment was doing the same?

Rod.


No.

Of course unterminated baseband video was always a recipe for trouble.

The saving grace with UHF and above is that cable losses will absorb
some of the reflected signal.

Bill
  #19  
Old July 22nd 19, 09:47 PM posted to uk.tech.digital-tv
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Default Looks like I managed to find the cause of 4g getting into the Ch62 and Ch 64 modulators....

Basically I had a superhub 2 and was offered an upgrade to a superhub 3 free of charge.

I signed up for the upgrade. VM sent a parcel to my front door.

Upon opening the box, there was also a cable set top box, a splitter and three white coax cables with crimped on connectors.

The instructions said to use one of the cables between the VM wall socket to the splitter.

The splitter had two markings of TV and modem. The instructions said to use the remaining two cables to connect the TV set top box and the modem to their respective ports on the splitter.

S.
  #20  
Old July 26th 19, 07:00 PM posted to uk.tech.digital-tv
Terry Casey[_2_]
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Default Looks like I managed to find the cause of 4g getting into the Ch 62 and Ch 64 modulators....

In article ,
says...

In the old days we sometimes had tap-off lines that used
non-directional resistive taps, and there was always the theoretical
possibility that a return signal could get back through a very low value
tap and cause phase cancellation at the previous tap in the line. More
likely were cases where a number of outlets were fed from a resistive
splitter. There really would be trouble if some outlets weren't
terminated, and given that many TV sets didn't actually terminate across
the band some very strange effects were possible. "If the kitchen telly
is on ITV the living room telly has no sound on BBC2."


The Stock Exchange system I worked on in the 70s used these
resistive splitters and as we used the full band from 45MHz to
230MHz, any reflections were virtually guaranteed to affect
something!

One I recall was a double ring on one channel which made it
impossible to read. I checked all the obvious things but found
nothing wrong but then went out into the corridor and looked
up and there was a nice fresh cable running along the top of
the wall. On investigation, the install had only been done the
previous day.

I found a ladder and went up to the original tee which had, of
course, been changed from the original terminated one and
popped a terminator in the output socket. Perfect pictures!

Down the corridor to the newly installed tee. On
investigation, the 75 ohm terminating resistor had been fitted
on the tap instead of the through feed, leaving the main feed
unterminated!

--

Terry

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