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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.

The Terminator - I'll not be back



 
 
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  #21  
Old June 20th 17, 10:29 PM posted to uk.tech.digital-tv
Woody[_5_]
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Posts: 1,781
Default The Terminator - I'll not be back


"Terry Casey" wrote in message
...
In article ,
says...

Effectively your unterminated feed will become a receiving
aerial, injecting local interference, short wave radio, baby
alarms, etc. into the network.

How does an unterminated coax act as an antenna?


Probably needs a better brain than mine to give a detailed
explanation but I've noticed this: "a transmission line with
a high SWR tends to act as an antenna, radiating
electromagnetic energy away from the line, rather than
channeling all of it to the load" in:

https://www.allaboutcircuits.com/textbook/alternating-
current/chpt-14/standing-waves-and-resonance/

Except for the signal direction, there is no intrinsic
difference between transmitting and receiving antennae.


[snip]

Some may argue about it, but there is a difference between a
transmitting and a receiving antenna, viz that the length of a simple
omni-directional transmitting antenna needs to be matched to the
frequency used - assuming all other impedences are the same* - to
achieve maximum efficiency. However within reason a receive antenna
only needs to be a piece of wire in the air. (*I.e. 50R cable, 50R
source/load impedence.) Think of a receiver with a telescopic aerial?

On my 2m (145MHz-ish) amateur radio antenna I could** listen to
anything in the mobile radio hi-band (let us say 160-174MHz) with no
problem, but if I transmitted in that frequency range there would be a
serious mismatch which, apart from making the antenna an inefficient
radiator, could do serious damage to my transmitting equipment by
reflected power in the antenna system. (**Since the move of the
Ambulance service from hi-band to Airwave, the use of more and more
digital radio, and greater use by (such as) taxi systems to data with
only little speech there is very little these days to listen to!
Marine (156MHz and thereabouts) and aircraft are now relatively busy
bands compared with PMR.)

A directional antenna however is different and does need to be matched
to frequency for both tranmit and receive.


--
Woody

harrogate3 at ntlworld dot com


  #22  
Old June 20th 17, 10:37 PM posted to uk.tech.digital-tv
Ian Jackson[_7_]
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Posts: 76
Default The Terminator - I'll not be back

In message ,
Terry Casey writes
In article ,
says...

Effectively your unterminated feed will become a receiving
aerial, injecting local interference, short wave radio, baby
alarms, etc. into the network.

How does an unterminated coax act as an antenna?


Probably needs a better brain than mine to give a detailed
explanation but I've noticed this: "a transmission line with
a high SWR tends to act as an antenna, radiating
electromagnetic energy away from the line, rather than
channeling all of it to the load" in:

https://www.allaboutcircuits.com/textbook/alternating-
current/chpt-14/standing-waves-and-resonance/

Except for the signal direction, there is no intrinsic
difference between transmitting and receiving antennae.

Monitoring the return path feed in a CATV headend with a
suitable receiver can be very enlightening to anybody who
doubts this ...

The basic answer is NO (or, sort-of, not really).

An unterminated coax line does not act as an antenna - except that when
the 'un-termination' is an open circuit, an RF signal floating around
the area can 'see' the unscreened end of the inner of the O/C connector.
This will allow a tiny bit of pickup (or, if RF is being fed into the
coax, a tiny bit of radiation).
--
Ian
  #23  
Old June 21st 17, 04:07 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 1,992
Default The Terminator - I'll not be back

On 20/06/2017 20:32, Ian Field wrote:

Nothing so entertaining - an unterminated transmission line merely
causes reflections and ringing.


A few kW into an unterminated line can be fun.

Bill
  #24  
Old June 21st 17, 11:36 AM posted to uk.tech.digital-tv
Terry Casey[_2_]
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Posts: 740
Default The Terminator - I'll not be back

In article , harrogate3
@ntlworld.com says...

"Terry Casey" wrote in message
...
In article ,
says...

Except for the signal direction, there is no intrinsic
difference between transmitting and receiving antennae.


Some may argue about it, but there is a difference between a
transmitting and a receiving antenna, viz that the length of a simple
omni-directional transmitting antenna needs to be matched to the
frequency used - assuming all other impedences are the same* - to
achieve maximum efficiency. However within reason a receive antenna
only needs to be a piece of wire in the air. (*I.e. 50R cable, 50R
source/load impedence.) Think of a receiver with a telescopic aerial?


I think you are missing something here as you are
concentrating on single frequency high(ish) power
transmitters.

In a CATV network, the 'transmit' frequencies are a band
encompassing 85-750MHz and the receive band is 5-65MHz.

(Note that this 65/85MHz revese/forward split was that in
common use when I last worked on CATV 10 years ago. With the
increase in upstream traffic since then, it is quite
possible that Band II has been abandoned in favour of an
85/105MHz split. Certainly, when I scrounged a filter for
the RF installation at the BVVS Museum at Dulwich it came
from a large box containing lots of identical used 65/85MHz
filters!)

Considering that all of the CATV distribution is underground
except for the last few metres at the customers home - more
if is a multi storey block of flats, of course! - it is
surprising the plethora of short wave transmissions that
could be heard in our headend before we started work
commissioning the network for the initial introduction of
Broadband Internet ...


--

Terry
  #25  
Old June 21st 17, 06:13 PM posted to uk.tech.digital-tv
Terry Casey[_2_]
external usenet poster
 
Posts: 740
Default The Terminator - I'll not be back

In article -
september.org, lid says...


Probably needs a better brain than mine to give a detailed
explanation ...


A fellow member of the
http://golbornevintageradio.co.uk/
forums has pointed me to this:

http://www.antenna-theory.com/defini...eciprocity.php

which says:

"Reciprocity

Reciprocity is one of the most useful (and fortunate)
property of antennas. Reciprocity states that the receive
and transmit properties of an antenna are identical. Hence,
antennas do not have distinct transmit and receive radiation
patterns - if you know the radiation pattern in the transmit
mode then you also know the pattern in the receive mode.
This makes things much simpler, as you can imagine."

So, if a poor SWR - such as might result from an
unterminated feeder - can cause it to radiate, the
reciprocity property means that it must be able to receive
as well - screening or no screening.

--

Terry
  #26  
Old June 21st 17, 06:50 PM posted to uk.tech.digital-tv
Ian Field
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Posts: 160
Default The Terminator - I'll not be back



"Bill Wright" wrote in message
news
On 20/06/2017 20:32, Ian Field wrote:

Nothing so entertaining - an unterminated transmission line merely
causes reflections and ringing.


A few kW into an unterminated line can be fun.


Only if you're stupid enough to do it.

  #27  
Old June 21st 17, 07:44 PM posted to uk.tech.digital-tv
Ian Field
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Posts: 160
Default The Terminator - I'll not be back



"Tweed" wrote in message
news

Effectively your unterminated feed will become a receiving
aerial, injecting local interference, short wave radio, baby
alarms, etc. into the network.

How does an unterminated coax act as an antenna?


Its radiation characteristics are worse than twin feeder to start with.

Reflections have nodes (algebraic sums of amplitudes) that can produce
pretty sharp current peaks. "antenna" may not be the best word - you
probably won't recover much information from what's radiated.

  #28  
Old June 21st 17, 09:02 PM posted to uk.tech.digital-tv
Ian Jackson[_7_]
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Posts: 76
Default The Terminator - I'll not be back

In message ,
Terry Casey writes
In article -
september.org, lid says...


Probably needs a better brain than mine to give a detailed
explanation ...


A fellow member of the
http://golbornevintageradio.co.uk/
forums has pointed me to this:

http://www.antenna-theory.com/defini...eciprocity.php

which says:

"Reciprocity

Reciprocity is one of the most useful (and fortunate)
property of antennas. Reciprocity states that the receive
and transmit properties of an antenna are identical. Hence,
antennas do not have distinct transmit and receive radiation
patterns - if you know the radiation pattern in the transmit
mode then you also know the pattern in the receive mode.
This makes things much simpler, as you can imagine."

So, if a poor SWR - such as might result from an
unterminated feeder - can cause it to radiate, the
reciprocity property means that it must be able to receive
as well - screening or no screening.

But a poor SWR is not responsible for a feeder radiating or picking up.
--
Ian
  #29  
Old June 22nd 17, 12:40 AM posted to uk.tech.digital-tv
Graham.[_12_]
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Posts: 416
Default The Terminator - I'll not be back

On Tue, 20 Jun 2017 16:43:18 +0000 (UTC), Tweed
wrote:

Graham. wrote:
On Tue, 20 Jun 2017 12:22:03 +0100, Dan wrote:

On Tue, 20 Jun 2017 08:24:07 +0100, UnsteadyKen wrote:

Why on earth would you even consider not fitting it?

Sorry, I seem to have missed the part of my OP where I said was considering
not fitting it.

I was merely asking why it was required, oh yes you gave a sensible &
concise answer, bookended by ranting nonsense:

Circuits work best when correctly terminated.
The terminator will probably allow Virgin's kit to determine if the
circuit is currently in use and to test the line in a known condition.


You moan about the poor service, how about keeping your end of the
bargain up by using the equipment correctly?

Eh?

Does anyone remember the good old days of usenet when you could have
sensible, friendly discussions without people going off on pointless rants?


No, that never happened on Usenet.
Anyway you were only postulating what would happen, you didn't say you
weren't going to fit it.

Alan_m on the other hand, did seem to suggest he wouldn't have fitted
the terminator. Tut tut. Hope he doesn't remove the cap on his gas
main should he dispense with that service ;-)


I think what Virgin meant to say, but failed to due to over simplification,
is that you need to fit the terminator if you still have another service
from them. It is possible to have TV in one room and a cable modem in
another. In that case there is usually a splitter in the brown box on the
outside of the house. If, say, you got rid of the cable modem but kept the
TV, you would need to terminate the cable modem outlet otherwise the
splitter may become influenced by the reflections and thus degrade the
signal to the TV box. If you got rid of all of their services I very much
doubt that they would care at all. The active amplifier/splitter in the
street box needs to be able to cope with any type of load on any of its
ports. Otherwise one malicious customer could destroy the signals to all
the other customers. Malicious customers can (or perhaps could, this may be
cured now with properly authenticated cable modems) suck up all the
broadband bandwidth with a suitably modified uncapped cable modem. A
colleague had a problem with sudden loss of performance at random times,
and came across a Virgin van in the area and the technician said they were
trying to trace the uncapped modem.



I rather expect you are right, the tap-offs to individual properties
probably have high attenuation rendering the need for termination no
more than a theoretical nicety (BICBW)

OTOH Bill will remember when high-rise flats had the trunk cable to
all the flats below looped through each wallplate, with the ones on
the ground floor having a 75 ohm resistor wired across the coax at the
saddle clamp.
The wallplates had different attenuation values depending on which
floor.

Loads of scope for induced faults.



--

Graham.
%Profound_observation%
  #30  
Old June 22nd 17, 02:06 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 1,992
Default The Terminator - I'll not be back

On 22/06/2017 00:40, Graham. wrote:


I rather expect you are right, the tap-offs to individual properties
probably have high attenuation rendering the need for termination no
more than a theoretical nicety (BICBW)

OTOH Bill will remember when high-rise flats had the trunk cable to
all the flats below looped through each wallplate, with the ones on
the ground floor having a 75 ohm resistor wired across the coax at the
saddle clamp.
The wallplates had different attenuation values depending on which
floor.

Loads of scope for induced faults.


These were called 'padded outlet' systems. They were a nightmare.
Tenants would replace their wallplate (usually with a chrome or brass
one) and not connect the outgoing trunk. Then, with everyone on the
floors below complaining of 'no telly' they would refuse to let the
engineer in. This only made their final bill higher of course.

Bill

 




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