Why 75 Ohms not 50 Ohms?

a

rounded

You made me look. ;)

Luckily today both are the same color, a sort of linty brownish blue.

Yes- lots of people know and it is well-documented.

Given the basic constraint of coaxial cable geometry with center conductor, its diameter, and distance to the shield and its type, with intervening dielectric of some loss factor, it is possible to adjust all these physical parameters in order to mathematically optimize cable performance for the various applications. Some applications require high withstanding voltage, some high power transfer, some signal integrity across wide bandwidths. The cable geometries that result from these optimizations turn out to have greatly different impedance levels. The high power transfer capability solution lands at 30 ohms, the high voltage at roughly 50, and the low loss, broadband, signal integrity at

1. Since most video applications are comparatively broadband small signal, requiring good signal fidelity for end performance, the 75 ohm cable solution is best for them, and that is why it was adopted. According to experts within the cable industry, Robert Fano's volume 9 of the MIT Radiation Laboratory Series is the best scientific exposition and documentary available on the topic- and the last word on it actually.

"Harry" ...

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

......... Phil

Ah, no. It is like saying that a 2:1 turns ratio transformer has 6 dB of gain. It does not. For dB to make any sense whatsoever, you should be working in the same impedance.

Jim

** Sounds quite plausible.

The 75 ohm standard must date back to WW2.

.......... Phil

"Tam/WB2TT"

** So what ?

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

.......... Phil

--- Well, let's just see:

Assume a voltage source feeding 1 watt into a 50 ohm coax line through a 50 ohm resistor and the other end of the line terminated by a 50 ohm load. Under those conditions each of the resistors will be dissipating 0.5W and, since:

E² P = --- R

we can rearrange to solve for the voltage across the load like this:

E = sqrt(PR) = sqrt (0.5W * 50R) = 5V

Now, if we do the same thing with a 75 ohm setup we'll have:

E = sqrt(PR) = sqrt (0.5W * 75R) ~ 6.124V

Is 6.124V not 1.76 dB > 5V ?

-- John Fields Professional Circuit Designer

Does anyone really care ?

Apart from the dBm power level jihadists of course !

Graham

Makes sense for feeder cables.

Makes sense for general signal distribution.

Graham

"Richard Henry" "Phil Allison"

** What colour are the sox you have on ?

........... Phil

========================================

If that's the best you can do as a professional circuit designer then you are obtaining money under false pretences.

Gee, Phil, the US had 75 ohm coaxial cables to connect tv stations in different cities all the way back to the early days there was no other way to have a live TV feed through a wide region of the country. When JFK was assassinated, ATT engineers quickly lashed all of the different networks and segments together for the first live, nationwide feed that was shown on every TV station that was part of a network. It was live coverage for days in low resolution B&W because the signal had to go through a lot more amplifiers than the system was designed for so both the bandwidth and noise figures suffered. In fact, if it had happened on one coast, it would have barely made it to the other, due to the extra connections along the way. It did show that it was possible to feed wide areas, though. The coaxial cables were still in place after C-band network feeds were available, in case a transponder or a bird died. Now, they use fiber optic backups, and to connect some TV stations to local cable TV systems.

--
?

Michael A. Terrell
Central Florida

--
Of course it does. It has 6dB of _voltage_ gain.

"John Woodgate"

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

.......... Phil

"Joe McElvenney"

** Err - why the heck not ???

** There should be none unless the co-ax cable is very long ( eg 1000 feet

• ) and di-electric losses come into play.

.......... Phil

I said not a damned thing about dBu. I said dB. For those of us in the RF world, it means something significantly different.

To say that a transformer has a dB gain means you know nothing about the subject.

Jim

--
Care to elaborate?

Ah, but you WERE working in the "same impedances" if you consider POWER gain, and I agree. THe posters were playing games with voltage gain and were doing the mental masturbation to convince us that transformers have gain. Twits.

Jim

I read in sci.electronics.design that Paul Hovnanian P.E. wrote (in ) about 'Why 75 Ohms not 50 Ohms?', on Tue, 13 Sep 2005:

Officer in charge at New Orleans on the Monday.....

Nothing better to do now.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk