RadioBanter - View Single Post - Why are cables and connectors usually 50 ohms?

Telamon · January 21st 07, 05:00 AM posted to rec.radio.shortwave

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