Why choice 50 ohm system?

[TQSU]

We always choice 50 ohm to match what we need,
but why?
and 75 ohm seems for video system ?

 

[flatulent]

coax cables

These impedances produce convenient conductor ratios in coaxial cable. Also they are in the areas of minimum attenuation and maximum power capability. Another reason is that the parasitic effects of shunt capacitance and series inductances in circuits are minimized.

In the days of open parallel wire lines 300 and 600 ohms were common.

 

[eirp]

I think It also arises from "coaxial-epoch" around world wars --> Flatulent is right with some optimal values for 50/75 ohms.
In microwave techniques (planar transm. lines..) 50 or 75 ohm isn't doctrine.

 

[TQSU]

But we can choice operation frequency even 0~100GHz,
I think 50 ohm should be not a good point to match to.
Am I right?

 

[BigBoss]

Most of measurement equipments work with 50 Ohm. In fact 75 Ohm impedance gives minimum attenuation for coaxial transmission lines
(see Microwave Filters , Impedance Matching Networks and Coupling Structures by G. Matthaei ) , however due to historical reason, 50 Ohm became a standard for RF&MW.

It's not very difficult to match 50 Ohm system impedance to any impedance if you're familiar with the techniques.

Rgrds

 

[boy]

This article may help.

 

[StoppTidigare]

The 75 ohm coax comes from the fact that the radiation resistance of a free standing half wave dipole antenna can be calculated analytically and is 73.1 Ohm.
See page 617 David K. Cheng "Field and wave Electromagnetics" 2ed.

I read something similar that you said, flatulent at the newsletter archive
at http://www.sigcon.com
do you have any other references for your statements ?

Is the book by Matthei "the book" if one want to see a derivation of flatulents statements ?

/StoppTidigare

 

[inhand]

but in MW monolithic ic, it seems the 50 ohm can have the best performace, no reflection, so the 50 ohm coax resource not fit for MMIC or HMIC tech.
i think

 

[bomba]

Could anybody suggest me an e-book refer to this subject?
Because I have doubt with adapting impedances in 50 Ohms transmission line, MV, using Op-Amps.

 

[MentorSI]

Good article.

 

[jupitorcuu]

Due to consider two side of one problem .Get best power handling and least attenuation. 33oHm is best power handling but big attenuation.75 oHm has a small attenuation. So 50oHm is middle of this two .

 

[nickwang]

Tom Lee's book would be good stuff for reading, especially it covers some history of electronic circuit design.