75 ohm terminals.

In message , tony sayer
writes
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...

From my experiences (starting early 60s), the USA have never used 50 ohm
for TV (cable TV or TV sets). Cable was always 75 ohms. Aerials were
usually 300 ohm, with a 300 ohm twin drop cable. TV sets used to have
300 balanced inputs, and needed a 300:75 ohm baluns if the fed from a
cable system.

The reasons for the choice of 50 and 75 ohm cable (and the reasons for
both) is out there on somewhere the internet. I recall that one reason
is that 50 ohm has better high power handling (hence its use for
transmission), while 75 ohm has lower loss (and, of course, if usually a
better match for halfwave dipoles).

As for the differences between 50 and 75 ohm BNC connectors, these are
mechanically mateable (both ways). The impedance difference is caused by
the 75 ohm having far less dielectric around the pins (both the male and
the female). I believe that there were some instances of early BNCs
being un-mateable, but that this was resolved when I were a lad, and had
hardly heard of BNCs. In any connection operating at less than 200 or
300MHz, you'd probably never know which impedance BNC was used.
--
Ian

In article , Ian Jackson
wrote:


The reasons for the choice of 50 and 75 ohm cable (and the reasons for
both) is out there on somewhere the internet. I recall that one reason
is that 50 ohm has better high power handling

Again, I'd be interested in some actual details of how that would be the
case.

50 Ohm means more current per watt than 75. Agreed that the I/D for a given
O/D will be bigger, but does that more than compensate by a significant
amount?

50 Ohm will have a smaller gap than 75 for the same O/D. So I'm not sure if
dielectric loss or breakdown limits would be better.

I've seen all kinds of 'reasons' given over the years. But I can't recall
any that really stood up to support "50 is better for TX but 75 is better
for RX" beyond the quite basic "75 gets closer to a simple typical
half-wave dipole's feed impedance".

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 ,
Ian Jackson wrote:

As for the differences between 50 and 75 ohm BNC connectors, these are
mechanically mateable (both ways). The impedance difference is caused by
the 75 ohm having far less dielectric around the pins (both the male and
the female). I believe that there were some instances of early BNCs
being un-mateable, but that this was resolved when I were a lad, and had
hardly heard of BNCs. In any connection operating at less than 200 or
300MHz, you'd probably never know which impedance BNC was used.

Certainly in the '60's the two had different sized pins. 50R being the
thicker, if I remember correctly. As a result you could put a 50R plug into
a 75R socket, but once you'd done that a 75R would never make a proper
connection - and that was at 5MHz!

--
From KT24

Using a RISC OS computer running v5.16

In message , charles
writes
In article ,
Ian Jackson wrote:

As for the differences between 50 and 75 ohm BNC connectors, these are
mechanically mateable (both ways). The impedance difference is caused by
the 75 ohm having far less dielectric around the pins (both the male and
the female). I believe that there were some instances of early BNCs
being un-mateable, but that this was resolved when I were a lad, and had
hardly heard of BNCs. In any connection operating at less than 200 or
300MHz, you'd probably never know which impedance BNC was used.

Certainly in the '60's the two had different sized pins. 50R being the
thicker, if I remember correctly. As a result you could put a 50R plug into
a 75R socket, but once you'd done that a 75R would never make a proper
connection - and that was at 5MHz!

You're not thinking about N-connectors, are you? These are certainly
incompatible. The thin 75 ohm male pin in a 50 ohm female doesn't make
contact*, and a wide 50 ohm male in a 75 ohm female will splay and wreck
the female.

But as for BNCs, I've never known them to be incompatible**. Some older
50 ohm males have a pin which has a point which is blunter than a 75
ohm, but the pin diameter is essentially the same size. It certainly
causes no damage. This was discussed at length around six months ago, in
rec.radio.amateur.antenna, and lots of documentary evidence was produced
to show that for a long time, the male and female are exactly the same
dimensions.

*Unless you slip about 7mm of ~26 SWG copper inside the female first!
**I did experience some Russian BNCs which were subtly a different size
- but that was in the Moscow Ostankino TV transmission tower!
--
Ian

In article , Ian Jackson
wrote:
In message , charles
writes
In article , Ian Jackson
wrote:

As for the differences between 50 and 75 ohm BNC connectors, these are
mechanically mateable (both ways). The impedance difference is caused
by the 75 ohm having far less dielectric around the pins (both the
male and the female). I believe that there were some instances of
early BNCs being un-mateable, but that this was resolved when I were a
lad, and had hardly heard of BNCs. In any connection operating at less
than 200 or 300MHz, you'd probably never know which impedance BNC was
used.

Certainly in the '60's the two had different sized pins. 50R being the
thicker, if I remember correctly. As a result you could put a 50R plug
into a 75R socket, but once you'd done that a 75R would never make a
proper connection - and that was at 5MHz!

You're not thinking about N-connectors, are you? These are certainly
incompatible. The thin 75 ohm male pin in a 50 ohm female doesn't make
contact*, and a wide 50 ohm male in a 75 ohm female will splay and wreck
the female.

No, these were BNC (B***** Nasty Connectors) - but I am talkinga bout the
1960s.

--
From KT24

Using a RISC OS computer running v5.16

In message , Jim Lesurf
writes
In article , Ian Jackson
wrote:


The reasons for the choice of 50 and 75 ohm cable (and the reasons for
both) is out there on somewhere the internet. I recall that one reason
is that 50 ohm has better high power handling

Again, I'd be interested in some actual details of how that would be the
case.

50 Ohm means more current per watt than 75. Agreed that the I/D for a given
O/D will be bigger, but does that more than compensate by a significant
amount?

50 Ohm will have a smaller gap than 75 for the same O/D. So I'm not sure if
dielectric loss or breakdown limits would be better.

I've seen all kinds of 'reasons' given over the years. But I can't recall
any that really stood up to support "50 is better for TX but 75 is better
for RX" beyond the quite basic "75 gets closer to a simple typical
half-wave dipole's feed impedance".

There is a short discussion he
Newsgroups: rec.radio.amateur.antenna
Subject: Why 50 ohms?
From: phaedrus
Date: 14 February 2010 09:33:58
One useful reference is
http://www.microwaves101.com/encyclopedia/why50ohms.cfm

--
Ian

In article , Ian Jackson
wrote:
In message , Jim Lesurf
writes

I've seen all kinds of 'reasons' given over the years. But I can't
recall any that really stood up to support "50 is better for TX but 75
is better for RX" beyond the quite basic "75 gets closer to a simple
typical half-wave dipole's feed impedance".

There is a short discussion he Newsgroups:
rec.radio.amateur.antenna Subject: Why 50 ohms? From: phaedrus
Date: 14 February 2010 09:33:58 One useful
reference is http://www.microwaves101.com/encyclopedia/why50ohms.cfm

Thanks. That is quite a good reference. I've just also found the
expressions for losses in Ramo, et. al. So if I get a chance I'll see if
that agrees with the above.

However from the above it seems to come down to 'anywhere in the 30 - 80
Ohm region depending on the assumptions and construction'. So I'm not clear
if the above was the *reason*, or a post hoc 'justification'. And I'll need
to read the above more carefully to see if it can explain why "50 for RX
and 75 for TX" makes sense. :-)

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

On Aug 11, 8:01*pm, "Brian Gregory [UK]" wrote:
"Jim Lesurf" wrote in message

'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.

....and me.

Bill

In article
,
wrote:
On Aug 11, 8:01 pm, "Brian Gregory [UK]" wrote:
"Jim Lesurf" wrote in message

'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.

...and me.

FWIW I've often felt that the 'standard' explanations in textbooks are
rather opaque. I tend to take this as a sign that either

A) The author has simply accepted and re-written the same 'explanation'
from finding it in an older textbook, and faithfully duplicated it without
actually understanding or checking.

and/or

B) I am too dim to understand the 'standard' explanation.

Either way, I have tended to find that this nags me until I can think out
my own 'explanation' based on the underpinning physics. Sometimes others
also find that clearer, sometimes not. Doing this does slow me down, but in
the end I prefer it.

In the process I've also encountered a number of cases where it becomes
clear that textbooks simply repeat erronious or misleading 'explanations'.
A sign that the author didn't bother to check for themself and just
accepted what they read on the basis, "In the learned text" = "Must be
right."

Alas EM is prone to this as people are often bamboozed by the equations and
just get used to applying them without actually understanding the physics.
To the point where I've seen people given hundreds of thousands of pounds
for research proposals based on fundamental misunderstandings of EM! In
effect due to being 'lost in the equations' rather than thinking critically
about the simple physics involved. (Also seen cases of the reverse. i.e.
good ideas turned down because people didn't understand them and thought
the books said otherwise.)

Alas, in academic work people often regard being obscure and presenting
difficult 'explanations' as a sign that "Gosh! He must be cleverer than me
to understand that". So there can be a premium on giving obscure
explanations. Also helps to prevent people spotting errors. Baloney Baffles
Brains. :-)

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

On Mon, 16 Aug 2010 10:45:04 +0100, Jim Lesurf
wrote:

In article
,
wrote:
On Aug 11, 8:01 pm, "Brian Gregory [UK]" wrote:
"Jim Lesurf" wrote in message

'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.

...and me.

Me too, I tried and failed to grasp this several times in the past and
gave up. I just accepted that microphones and other inputs needed to
be used in sockets with certain impedances because they did G

FWIW I've often felt that the 'standard' explanations in textbooks are
rather opaque. I tend to take this as a sign that either

A) The author has simply accepted and re-written the same 'explanation'
from finding it in an older textbook, and faithfully duplicated it without
actually understanding or checking.

and/or

B) I am too dim to understand the 'standard' explanation.

Either way, I have tended to find that this nags me until I can think out
my own 'explanation' based on the underpinning physics. Sometimes others
also find that clearer, sometimes not. Doing this does slow me down, but in
the end I prefer it.

In the process I've also encountered a number of cases where it becomes
clear that textbooks simply repeat erronious or misleading 'explanations'.
A sign that the author didn't bother to check for themself and just
accepted what they read on the basis, "In the learned text" = "Must be
right."

There's certainly a bunch of that in dietary and metabolic research
too. Appeals to "authority" with no attempt to discern whether The
Authorities have any basis for their beliefs other than "it's what
other authorities believe".

The worst is when there is then an attempt to fit newly discovered
pathways into a false model and the equivalent of epicycles starts up.
Those who are clueful enough to go back to first principles and
re-work the entire theory often end up ostracised.

Alas EM is prone to this as people are often bamboozed by the equations and
just get used to applying them without actually understanding the physics.
To the point where I've seen people given hundreds of thousands of pounds
for research proposals based on fundamental misunderstandings of EM! In
effect due to being 'lost in the equations' rather than thinking critically
about the simple physics involved. (Also seen cases of the reverse. i.e.
good ideas turned down because people didn't understand them and thought
the books said otherwise.)

Alas, in academic work people often regard being obscure and presenting
difficult 'explanations' as a sign that "Gosh! He must be cleverer than me
to understand that". So there can be a premium on giving obscure
explanations. Also helps to prevent people spotting errors. Baloney Baffles
Brains. :-)

Some people actually are brilliant at understanding things but crap at
explaining it. Ron Krauss is in serious need of a translator. Some of
his papers resemble Vogon poetry, and even in interviews he tends to
leave crucial bits out because he takes them for granted. Other
researchers, bloggers etc. serve as a user-friendly interface.

The other side of the coin is when complex stuff is oversimplified for
the plebs, and it is done badly

cf. Horizon from a few years back compared to the likes of Carl Sagan