[TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

[Mike & Becca Krzystyniak]

Hi Paul and the Reflector,

    Back in the 70's, Prof. Albright from my EE353 RF course @ U.of Illinois
used to tell the genesis of 50 ohm coax story too. 
    A variant of yours it excluded the optimum voltage parts.  Doing the
math, the geometric mean of 
    max power handling per foot (my class notes had 35 ohms) and minimum
loss per foot (my notes had 75 ohms) was allegedly how this all came about.

    I assume this was based on copper and some pre WW-II poly insulator.


73  Mike K9MK


    

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Paul
Christensen
Sent: Monday, December 16, 2013 2:28 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Fwd: Fwd: Fwd: RG-149: 50 ohm/70 ohm - does it
matter?

>" If I recall my radio teacher, 50 ohms were decided sometimes during 
>WW2 or even earlier. There were some switch from (I may be wrong here) 
>from 50 to 52 ohms at one point.

Lots of folklore ranging from commonly available pipe diameters before WWII
to the geometric mean of three important values: (1) Approx. 30 ohms
represents a characteristic Z for optimum power; (2) 60 ohms for optimum
voltage; and (2) 75 ohms represents a Z for minimal loss, all other factors
being equal.

The geometric mean of 75-60-30 is 51.3 ohms and is a close approximation to
either 50 or 52 ohms.

Paul, W9AC 

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