75 ohm terminals.

"Graham." wrote in message
...

Brian Gaff wrote:

it needs a balun/ transformer there.

In practice though, you can do all sorts of things and get away with it.

I couldn't bring myself to recommend the O/P just sucks it and sees after
Bill's "that's what's wrong with this country" comment ;-)

I made the mistake of commenting to a total stranger thet the aerial
on his roof could not possibly work properly, because two feeders
were connected directly to it serving separate rooms.

His reponse was the colour was perfect on all the Freeview channels in
both
rooms, and I obviously was talking through my arse, as indeed I must have
been.

There's no justice.

It probably wouldn't be so good for analogue.
In strong signal areas there is considerable chance for bodged aerial
installations "working perfectly" with digital.
Also I suspect many people don't realize that occasional pixellation, or
even missing channels is related to poor reception and just blame the cheap
"unreliable" freeview box they bought.
One stupid young lady somewhere on the internet (I forget where) told me
"rubbish [it's not because I'm using on indoor aerial because] the colours
are as good as on sky on BBC4".

--

Brian Gregory. (In the UK)

To email me remove the letter vee.

"Jim Lesurf" wrote in message
...
In article
,
David
Paste wrote:
On 10 Aug, 15:31, Jim Lesurf wrote:

To complicate matters, the answer will depend on the cable and the
'load' (i.e. input to the tuner) since the optimum depends both on the
inherent radiation coupling of the antenna and the 'match' into
whatever it is conected to.

Well, you have reached and exceeded my understanding of these things.
Actually, I still have no idea what the impedance rating of a cable
refers to.

It refers to the 'inherent' behaviour of a length of the cable. Imagine a
*very* long length of cable. Now apply a voltage (potential difference)
between the wires at one end. This extracts some charge from one wire and
shoves it onto the other. (This is necessary for the wires to now have
different potentials.) This movement of charge, and the change in voltage
between the wires now 'ripples along the wires' at the speed of light.

The result is that a given applied voltage induces a current, and both
ripple out along the wires of the cable.

For any particular cable a given voltage will set up a specific amount of
current. The ratio gives you the 'characteristic impedance' of the cable.

If you now connect a load *with that resistance* at the other end of the
cable it will 'match' the cable. So when the voltage and current reach the
load, the voltage causes the load to demand the same amount of current as
now is flowing along the cable at the load. Result: All the conveyed
energy
is 'eaten' by the load.

However if the load has the wrong impedance (i.e. not the same value as
the
cable's) that won't work correctly. Some of the energy arriving at the
load
'bounces' and is returned to the source. (In this case the antenna.) The
system then doesn't work properly.

That's an interesting way of explaining it which I've not heard before.

Thanks.


It's not the resistance to DC current in the wire, that is for sure, as
impedance is an AC thing as far as I am aware. But what does it mean? Is
it per unit length of the cable? I don't think that's right, either. Ah,
for a little more education!

The cable will have an amount of capacitance per length, and inductance
per
length. These do relate to the cable's characteristic impedance. But the
impedance is an inherent general property.

FWIW The UK tradition for domestic VHF and UHF is to use '75 Ohm' coaxial
cables. The reasons are somewhat blurred by the mists of time. :-)
However
the 'natural' output impedance of a simple dipole tends to be in the
region
of 70 - 95 Ohms. So the combination is a convenient one.

Professional RF/microwave/test systems in other uses may well use other
types of cable. So for example '50 Ohm' is common in lab gear. All very
awkard if you work in a field like TV or Audio (i.e. FM tuners, etc) since
you often have to have convertors, etc. Some DAB tuners also have used 50
Ohm, just to stir the pot a bit, I guess. 8-] Can all be a PITA. Some
old
kit also uses weird and now obsolete types of connector. Presumably
because
'It seemed a good idea at the time'. ;-

LF/MF/HF communications receivers and transceivers (including those for
amateur radio) all tend to use 50 ohm.

--

Brian Gregory. (In the UK)

To email me remove the letter vee.

In article , Brian
Gregory
[UK] wrote:
"Jim Lesurf" wrote in message
...
[snip]

That's an interesting way of explaining it which I've not heard before.

I guess I developed that way of thinking about it because I came into
trying to understand cable behaviour from the POV of a physicist rather
than have been taught by EEs. :-) That was then strenthened when I worked
for a time with a colleague who used arrangements of lengths of
transmission lines as forms of 'pulse transformer' to get ultra-high
voltage pulses.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Jim Lesurf wrote:

snip


FWIW The UK tradition for domestic VHF and UHF is to use '75 Ohm' coaxial
cables. The reasons are somewhat blurred by the mists of time. :-) However
the 'natural' output impedance of a simple dipole tends to be in the region
of 70 - 95 Ohms. So the combination is a convenient one.

Professional RF/microwave/test systems in other uses may well use other
types of cable. So for example '50 Ohm' is common in lab gear. All very
awkard if you work in a field like TV or Audio (i.e. FM tuners, etc) since
you often have to have convertors, etc.

I'm sure I read an explanation of this once, although I can't remember
the details, that 75 Ohm cable is more efficient for reception but 50
Ohm is better for transmission, thus as domestic use is strictly receive
only, 75 Ohm is the better choice ...

Some old
kit also uses weird and now obsolete types of connector. Presumably because
'It seemed a good idea at the time'. ;-

Bush used to use a Belling moulded two pin plug for the FM aerial -
though I suspect this was 300 Ohm ...

.... but their Band I only TVs had a screw terminal and saddle clamp
arrangement for the coax ...

--

Terry

In article [email protected], Terry Casey
wrote:
Jim Lesurf wrote:

snip


FWIW The UK tradition for domestic VHF and UHF is to use '75 Ohm'
coaxial cables. The reasons are somewhat blurred by the mists of time.
:-) However the 'natural' output impedance of a simple dipole tends
to be in the region of 70 - 95 Ohms. So the combination is a
convenient one.

Professional RF/microwave/test systems in other uses may well use
other types of cable. So for example '50 Ohm' is common in lab gear.
All very awkard if you work in a field like TV or Audio (i.e. FM
tuners, etc) since you often have to have convertors, etc.

I'm sure I read an explanation of this once, although I can't remember
the details, that 75 Ohm cable is more efficient for reception but 50
Ohm is better for transmission, thus as domestic use is strictly receive
only, 75 Ohm is the better choice ...

You'd have to explain the reasons given for that for me to be able to tell.
Looks like an odd idea to me. Not one I've heard previously.

The closest I've heard was that that 75 Ohms was convenient for simple
dipole antennas. But of course they should also then have a balun or use
twin feed. So I have a feeling these things are examples of where one
practice simply emerges on a 'chance' basis as people gradually tend to
adopt a common behaviour. Be interested to see historic or technical
reasons, though.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

In article , Jim Lesurf
wrote:
In article [email protected], Terry Casey
wrote:
Jim Lesurf wrote:

snip


FWIW The UK tradition for domestic VHF and UHF is to use '75 Ohm'
coaxial cables. The reasons are somewhat blurred by the mists of time.
:-) However the 'natural' output impedance of a simple dipole tends
to be in the region of 70 - 95 Ohms. So the combination is a
convenient one.

Professional RF/microwave/test systems in other uses may well use
other types of cable. So for example '50 Ohm' is common in lab gear.
All very awkard if you work in a field like TV or Audio (i.e. FM
tuners, etc) since you often have to have convertors, etc.

I'm sure I read an explanation of this once, although I can't remember
the details, that 75 Ohm cable is more efficient for reception but 50
Ohm is better for transmission, thus as domestic use is strictly
receive only, 75 Ohm is the better choice ...

You'd have to explain the reasons given for that for me to be able to
tell. Looks like an odd idea to me. Not one I've heard previously.

The closest I've heard was that that 75 Ohms was convenient for simple
dipole antennas. But of course they should also then have a balun or use
twin feed. So I have a feeling these things are examples of where one
practice simply emerges on a 'chance' basis as people gradually tend to
adopt a common behaviour. Be interested to see historic or technical
reasons, though.

Sometimes it is just local custom. In the UK, and most of Europe, video
distribution uses 75R - in the USA it's 50R.

--
From KT24

Using a RISC OS computer running v5.16

charles wrote:


Sometimes it is just local custom. In the UK, and most of Europe, video
distribution uses 75R - in the USA it's 50R.

All NTSC broadcast TV video equipment (including stuff for the US
market) is 75R. I've never encountered anything other than 75R for
baseband video or SDI anywhere in the world ?

In article [email protected], Terry Casey
scribeth thus
Jim Lesurf wrote:

snip


FWIW The UK tradition for domestic VHF and UHF is to use '75 Ohm' coaxial
cables. The reasons are somewhat blurred by the mists of time. :-) However
the 'natural' output impedance of a simple dipole tends to be in the region
of 70 - 95 Ohms. So the combination is a convenient one.

Professional RF/microwave/test systems in other uses may well use other
types of cable. So for example '50 Ohm' is common in lab gear. All very
awkard if you work in a field like TV or Audio (i.e. FM tuners, etc) since
you often have to have convertors, etc.

I'm sure I read an explanation of this once, although I can't remember
the details, that 75 Ohm cable is more efficient for reception but 50
Ohm is better for transmission, thus as domestic use is strictly receive
only, 75 Ohm is the better choice ...

Olde wives tale that one...

Tho 75 is the common impedance for RX aerial systems well as near as
they come .. whereas 50 is the one for TX systems.


Some old
kit also uses weird and now obsolete types of connector. Presumably because
'It seemed a good idea at the time'. ;-

Bush used to use a Belling moulded two pin plug for the FM aerial -
though I suspect this was 300 Ohm ...

... but their Band I only TVs had a screw terminal and saddle clamp
arrangement for the coax ...

Most tuners aren't that well matched anyway...
--
Tony Sayer

In article ,
Mark Carver wrote:
charles wrote:


Sometimes it is just local custom. In the UK, and most of Europe, video
distribution uses 75R - in the USA it's 50R.

All NTSC broadcast TV video equipment (including stuff for the US
market) is 75R. I've never encountered anything other than 75R for
baseband video or SDI anywhere in the world ?

they must have changed since the 60s.

--
From KT24

Using a RISC OS computer running v5.16

charles wrote:
In article ,
Mark Carver wrote:
charles wrote:

Sometimes it is just local custom. In the UK, and most of Europe, video
distribution uses 75R - in the USA it's 50R.

All NTSC broadcast TV video equipment (including stuff for the US
market) is 75R. I've never encountered anything other than 75R for
baseband video or SDI anywhere in the world ?

they must have changed since the 60s.


Looks like they have ! Just done a very quick Google, and found this:-

http://www.radiobanter.com/showthread.php?t=70893

Reading the thread, it does indeed suggest that older US kit was 50R.

Well, well, that must be a first, the Americans following the rest of
the world over standards, rather than digging their heels in !!!