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[aprssig] Why is coax 50 ohm ?

Sam Guccione gucci1 at mchsi.com
Wed Mar 29 01:28:05 UTC 2006


If you look at the equation for formula for the impedance of a coaxial
transmission line, you will find that an impedance of 77 ohms results in the
lowest value of attenuation. Also, if you look at the breakdown voltage
between the inner and outer conductors of a coaxial transmission line you
will find that it is the highest when the coax line is 60 ohms impedance.
Finally, if you look at the power carrying capacity you will find that the
highest capacity occurs when the line impedance is 30 ohms. So a compromise
was reached. The US Navy, as the story goes, during WWII selected 50 ohms as
the compromise impedance between these three values based upon commercially
available copper water pipe sizes which were used to construct coax during
those times.

Dr. Sam Guccione, K3BY

-----Original Message-----
From: aprssig-bounces at lists.tapr.org
[mailto:aprssig-bounces at lists.tapr.org]On Behalf Of Wyatt Foard
Sent: Tuesday, March 28, 2006 8:30 AM
To: TAPR APRS Mailing List
Subject: Re: [aprssig] Why is coax 50 ohm ?


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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