[NJARC] Interesting question from the AWA Reflector

[Jim Whartenby]

It is my understanding, from reading some long forgotten history, is that 75 ohm coax became a standard because of copper pipe size!

Early coax was ridged and used two standard pipe sizes, one for the shield the other for the center conductor.  This resulted in an impedance of 70 some odd ohms.  When the space between the two pipes was filled with a dielectric to support the center conductor, the 50 ohm (originally 52 ohms)impedance standard resulted.

I think all of the original work was done at Bell Labs, don't know about the Japanese connection.
That's my story and I'm sticking to it!
Jim




--- On Sat, 10/11/08, John Ruccolo <jr6v6gt at yahoo.com> wrote:

> From: John Ruccolo <jr6v6gt at yahoo.com>
> Subject: [NJARC] Interesting question from the AWA Reflector
> To: njarc at mailman.qth.net
> Date: Saturday, October 11, 2008, 8:44 AM
> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Hi Folks,
> 
> Any ideas?
> 
> Regards,
> 
> John -- Kilo Charlie Two Uniform Alpha Kilo
> 
> 
> From: va3awa <VA3AWA at sympatico.ca>
> Subject: [AWA] Anyone have the answer for my UK friend?
> To: antiquewirelessassociation at yahoogroups.com
> Date: Saturday, October 11, 2008, 8:58 AM
> 
> Can somebody explain to me why if a diople impedance is 75
> ohms in free 
> space. 
> And eary manufactures like KW and Vangaurd, Pye etc and
> also Marconi 
> with their lab test equipment all used 75 ohms as the
> standard. 
> When and Why did later manufactures (Japanise) settle for
> 50 Ohms as 
> stadard. It is impossible to match 75 ohms to 50 ohms
> without using a 
> matching device. 
> I have been running some simulated test on the computer for
> various 
> wire antennas and without exception all were heigher than
> 50 Ohms. 
> Mostly between 65 and 80 ohms. 
> Which of course means than 75 ohm coax should be used and
> not 50 ohm, 
> leaving the ATU to match the 75 ohm to the TX output of 50
> ohms. 
> 
> (I believe that Walt's 30 metre antenna uses 75 ohms
> ?). 
> 
> Was it because they can not design Transistor transmitter
> to hadle 75 
> ohms, maybe because of collector secondary breakdown ? 
> 
> There must be a reason, any ideas ? 
> 
> 
> 
> 
>       
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