Re: Type 519 -- Why 125 ohms?


Tom Lee
 

Pretty much everyone on this list has the analysis chops to do it. It's just that everyone has better things to do with their time. So many Netflix series to binge-watch! :)

And your intuition is spot on about those other factors mattering. The numbers you cite come from assuming a coaxial line with air dielectric.  The answers will be different for other geometries, and the introduction of dielectrics makes the answers also depend on frequency. But that's too complicated, so the industry settled on values from the air-dielectric coax calculation. You have to standardize on something.

If anyone gives a rat's patootie about the derivation, please contact me off list and I can send you a couple of pages.

-- Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 5/13/2021 13:33, Jim Ford wrote:
I thought it was about 77 ohms.  75 ohm TV coax being very close.  Maximum power handling supposedly occurs for about 30 ohms, and 50 ohms is a compromise between lowest loss and highest power handling.  I've always thought that there would be variations In both  with dielectric and conductor materials and geometry (e.g. coax, microstrip, coplanar waveguide, etc).  Don't have the analysis chops to calculate it myself....             Jim Ford Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: Göran Krusell <mc1648pp@gmail.com> Date: 5/13/21 12:55 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Type 519 -- Why 125 ohms? My memory tells me that the Germans in the 1930s (?) made calculations to figure out if there would be a certain characteristic impedance that would give you the lowest possible cable losses and found 60 ohm as the result. Or am I out in the blue?Göran


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