I always thought it was because the impedance of basic antennas are
around 50 and 75 ohms. Here is another story I came across.

"In the United States, the predominant impedance for coaxial
transmission lines and connectors is 50 ohms. The theoretical impedance
for minimum attenuation is 77.5 ohm and for maximum power transfer it
is 30 ohms. The average of these two impedances is 53.75 ohm or rounded
off to 50.

Therefore, 50 ohm is a compromise between minimum attenuation and
maximum power transfer in a coaxial transmission line, and that is why
it was selected. There are connectors available with other impedances,
the next most popular impedance being 75 ohm (approximate minimum
attenuation performance) that is in fairly wide use internationally and
in long line communication systems."

75 ohm cabling is also often used for video applications.

--
Telamon
Ventura, California

Telamon wrote:
I always thought it was because the impedance of basic antennas are
around 50 and 75 ohms. Here is another story I came across.

"In the United States, the predominant impedance for coaxial
transmission lines and connectors is 50 ohms. The theoretical impedance
for minimum attenuation is 77.5 ohm and for maximum power transfer it
is 30 ohms. The average of these two impedances is 53.75 ohm or rounded
off to 50.

Therefore, 50 ohm is a compromise between minimum attenuation and
maximum power transfer in a coaxial transmission line, and that is why
it was selected. There are connectors available with other impedances,
the next most popular impedance being 75 ohm (approximate minimum
attenuation performance) that is in fairly wide use internationally and
in long line communication systems."

75 ohm cabling is also often used for video applications.

--
Telamon
Ventura, California

All single ended video ( NTSC, PAL, SECAM or ATSC) systems that I have
seen
or read about are 75 ohms.

New AES/BEU coax is also 75 ohm.

Older, and increasingly rare, balanced video is 150 Ohms. Twinax.


As to the why, inspite of 4 years of college and 30 years in the trade
I have
never heard a convincing story about the "why".

In the real world of HF receivers I have never been able to detect any
difference
between 50 and 75 ohm cable. I also have not ever seen a modern radio,
since say 1970, that has anything like a 50 ohm input impedance. Most
are at least twice that or 100 Ohms and many are closer to 300 and this
changes with frequency.


This link makes the most sense in that the math is at least correct.
http://www.edn.com/article/CA60931.html?1=1

Terry

In article .com,
wrote:

Telamon wrote:
I always thought it was because the impedance of basic antennas are
around 50 and 75 ohms. Here is another story I came across.

"In the United States, the predominant impedance for coaxial
transmission lines and connectors is 50 ohms. The theoretical impedance
for minimum attenuation is 77.5 ohm and for maximum power transfer it
is 30 ohms. The average of these two impedances is 53.75 ohm or rounded
off to 50.

Therefore, 50 ohm is a compromise between minimum attenuation and
maximum power transfer in a coaxial transmission line, and that is why
it was selected. There are connectors available with other impedances,
the next most popular impedance being 75 ohm (approximate minimum
attenuation performance) that is in fairly wide use internationally and
in long line communication systems."

75 ohm cabling is also often used for video applications.


All single ended video ( NTSC, PAL, SECAM or ATSC) systems that I have
seen
or read about are 75 ohms.

New AES/BEU coax is also 75 ohm.

Older, and increasingly rare, balanced video is 150 Ohms. Twinax.


As to the why, inspite of 4 years of college and 30 years in the trade
I have
never heard a convincing story about the "why".

In the real world of HF receivers I have never been able to detect any
difference
between 50 and 75 ohm cable. I also have not ever seen a modern radio,
since say 1970, that has anything like a 50 ohm input impedance. Most
are at least twice that or 100 Ohms and many are closer to 300 and this
changes with frequency.


This link makes the most sense in that the math is at least correct.
http://www.edn.com/article/CA60931.html?1=1

This paragraph in the article you quoted:

"A half-wave dipole in free space has a feed-point impedance of
approximately 73 Ohm. A quarter-wave antenna with a ground plane has a
feed-point impedance of approximately 37 Ohm. A compromise between the
two, ratiometrically, is 51.97 Ohm, which produces an SWR of 1.404-to-1
either way.--James C Bach"

is my rational since when connecting antennas to coax the impedance need
to match.

--
Telamon
Ventura, California

Why this value was chosen is given in a paper presented by Bird Electronic
Corp

See URL:
http://www.epanorama.net/documents/w...impedance.html

Scroll about 2/3 the way down the page.

CL

Telamon wrote:
I always thought it was because the impedance of basic antennas are
around 50 and 75 ohms. Here is another story I came across.

Another story I have heard is that at the begining of WWII, the development
of RADAR required a new type of feed line. Coaxial conductors were made of
copper pipe. The pipes used were standard pipe purchased at a plumbing store
and the ratio of inner conductor to shield was due to avaialble sizes.

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 Fax ONLY: 972-2-648-1443 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/

Radar,,, the beginning of microwave ovens,,, the Radar Range.World War
Two,,, there was a guy in Europe,,, he had super sekert information
about one of those thingys,,, he also had a guard whom was ordered to
shoot and kill the guy if his information was ''compromised''.I read the
book,many years ago.
cuhulin