[aprssig] Why is coax 50 ohm ?

[Steve Noskowicz]

  I don't remember if this is urban legend or not, but I remember something like this.
   
  The geometry of 75 ohms is the lowest loss as Wyatt states.  When they filled that geometry with (poly) dielectric, 50 ohms resulted.
   
  As to 93 ohms, if you can make anything you want, why not 93...   Don't know why 93.  Perhaps, as Wyatt also suggests,  there's some phase delay or other esoteric reason.
   
  73, Steve, K9DCI
   
   Google may know...
   
  
Message: 5
Date: Tue, 28 Mar 2006 21:09:51 +1000
From: "Andrew Rich" 
Subject: [aprssig] Why is coax 50 ohm ?
To: "TAPR List" 

  Is there a reason why coax is 50 ohm ?

And 75 ohm ? 

And 92 ohms ?

Is it historical or is there a scienific reason why ?
-----------------------------------------
Andrew Rich
Amateur radio callsign VK4TEC  email: vk4tec at tech-software.net  web: http://www.tech-software.net    Brisbane AUSTRALIA 

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Message: 9
Date: Tue, 28 Mar 2006 14:29:57 +0000
From: wyattfoard at comcast.net (Wyatt Foard)
Subject: Re: [aprssig] Why is coax 50 ohm ?
To: TAPR APRS Mailing List 

Andrew Rich wrote:
> Is there a reason why coax is 50 ohm ?
> 
> And 75 ohm ?
> 
> And 92 ohms ?
> 
> Is it historical or is there a scienific reason why ?
Both 50 ohms and 75 ohms have been selected for specific reasons. Going back to Maxwell's equations for coax as a transmission line, maximum power transfer is at Z=30ohms and mimimum loss in the transmission line is at Z=77ohms. 50ohm coax is a good trade off for general purpose applications, as well as a nice round number. Video signals are not high power and so use 75ohms to minimize loss, again picking a round number.

I have no idea about 92 ohms, but would suspect there is another spec that was minimized or maximized by its selection.

Thanks,
Wyatt Foard - KR5WF


			
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