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Why 75 Ohms not 50 Ohms?

H

Harry

Jan 1, 1970
0
Does anyone know why video connectors have impedance 75 Ohms not
50 or any other number Ohms?

Why is 75 Ohms better than 50 Ohms for video signals?


-- Harry
 
G

Genome

Jan 1, 1970
0
Harry said:
Does anyone know why video connectors have impedance 75 Ohms not
50 or any other number Ohms?

Why is 75 Ohms better than 50 Ohms for video signals?


-- Harry

You didn't read the FAQ......... did you?

Sheesh

DNA
 
R

Reg Edwards

Jan 1, 1970
0
Why is 75 Ohms better than 50 Ohms for video signals?
It isn't.
 
J

Joerg

Jan 1, 1970
0
Hello Reg,
It isn't.

Reight, technically it isn't. But then again 75Ohms -> less copper for
center conductor -> more plastics -> lower cost -> more profit...

That could all change with the current oil price trend.

Regards, Joerg
 
Does anyone know why video connectors have impedance 75 Ohms not
50 or any other number Ohms?

Why is 75 Ohms better than 50 Ohms for video signals?


-- Harry

I once saw this explained ( maybe in the ARRL handbook): the higher
impedance gives better voltage transfer,lower(35 ohms?) give better
current match . 50 is a compromise. Actually, some missle and radar
systems use 95 ohms.
 
J

J. Davidson

Jan 1, 1970
0
I once saw this explained ( maybe in the ARRL handbook): the higher
impedance gives better voltage transfer,lower(35 ohms?) give better
current match . 50 is a compromise. Actually, some missle and radar
systems use 95 ohms.

Isn't it related to the FM/TV dipole antenna which has 300 ohm impedance????
I'm not quite sure but the relation may have someting to do with 300 / 4
giving 75.

Jerry
 
J

J. Davidson

Jan 1, 1970
0
I once saw this explained ( maybe in the ARRL handbook): the higher
impedance gives better voltage transfer,lower(35 ohms?) give better
current match . 50 is a compromise. Actually, some missle and radar
systems use 95 ohms.

Isn't is related to the dipole TV/FM antennas with 300 ohm impedance?
300 ohms / 4 giving effectively 75 ohms.

Jerry
 
J

Joerg

Jan 1, 1970
0
Hello Jerry,
Isn't is related to the dipole TV/FM antennas with 300 ohm impedance?
300 ohms / 4 giving effectively 75 ohms.

This ratio had to be 4:1 because of baluns. However, it depends on which
part of the world you are looking at. In much of Europe the flat cable
and thus antennas were set at 240 Ohms and consequently the coax used to
be 60 Ohms. I believe they also went to 75 Ohms now, probably assuming
the old 240 Ohms antennas have all corroded away. Most of the late
models had preamps anyway.

Regards, Joerg
 
G

Guest

Jan 1, 1970
0
Harry said:
Does anyone know why video connectors have impedance 75 Ohms not
50 or any other number Ohms?

Why is 75 Ohms better than 50 Ohms for video signals?

73.2 ohms is the radiation resistance of a dipole. 2 dipoles in a balanced
configuration are thus 293 ohms. These numbers are rounded off to 75 and
300 ohms respectively.

Norm Strong
 
P

Phil Allison

Jan 1, 1970
0
<[email protected]...
"Harry" ...

73.2 ohms is the radiation resistance of a dipole. 2 dipoles in a
balanced configuration are thus 293 ohms. These numbers are rounded off
to 75 and 300 ohms respectively.


** But what has this got to do with ** video ** ????





.......... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Paul Hovnanian P.E."
Whn the video guys needed some coax, they just grabbed what was handy.


** Sounds quite plausible.

The 75 ohm standard must date back to WW2.



........... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Tam/WB2TT"
"Phil Allison"

You get more voltage for the same power.

** So what ?

My point is that video is being confused here with RF.




........... Phil
 
R

Richard Henry

Jan 1, 1970
0
Phil Allison said:
"Tam/WB2TT"

** So what ?

My point is that video is being confused here with RF.

What frequency would you pick as the low end of RF?
 
P

Phil Allison

Jan 1, 1970
0
"Richard Henry"
"Phil Allison"
What frequency would you pick as the low end of RF?


** What colour are the sox you have on ?




............ Phil
 
J

John Woodgate

Jan 1, 1970
0
(in said:
Does anyone know why video connectors have impedance 75 Ohms not 50 or
any other number Ohms?

Why is 75 Ohms better than 50 Ohms for video signals?

Better colour rendering 75 ohms is violet-green-brown, while 50 ohms is
only green-black-brown. Violet and green are complementary, so you can
get any colour along that axis by mixing. Brown is low-brightness yellow
(yellow would have given 750 kohms, which is obviously too high), thus
introducing the missing red element.

Or perhaps it's just a historical accident, based on early designs of
coaxial cables in different countries. In Germany, 60 ohms was used.
 
R

Reg Edwards

Jan 1, 1970
0
Coaxial cable impedance has nothing to do with radio antennas or
video. Its a matter of engineering economics.

50-ohm cable maximises power handling capability for a given amount of
expensive copper.

75-ohm cable minimises attenuation for a given amount of expensive
copper.

If the price of copper had been different in the 1920's when coax was
developed we might have had two different standard impedances.
 
P

Phil Allison

Jan 1, 1970
0
"John Woodgate"
Better colour rendering 75 ohms is violet-green-brown, while 50 ohms is
only green-black-brown.



** Oh dear ..........




........... Phil
 
R

Reg Edwards

Jan 1, 1970
0
Because 50-ohm coax maximises power handling capacity it is most
appropriate for feeding radio transmitting antennas and hence other
radio applications.

Because 75-ohm coax minimises attenuation versus distance it is most
appropriate for long and short distance communications purposes such
as the telephone system.

Far more millions of miles of 75-ohm coax have been manufactured than
50-ohm coax since the 1930's.
 
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