[time-nuts] altinex switches

[Bob Camp]

Hi

Using common WWII coax materials, 75 ohms is roughly the minimum loss per
foot design. 50 ohms is the maximum power handling design. The low loss /
high power duality is what still has us matching transmitters into 50 ohms
and running cable TV at 75 ohms. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Chuck Harris
Sent: Friday, February 22, 2013 9:40 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] altinex switches

Hi Robert,

The only way it would make sense to make the center pin diameter
smaller on a 75 ohm BNC is if the center section of the connector
has dielectric in it.  So, I would venture then that if you find a
75 ohm BNC that has dielectric in the mating area, around the pin,
it will be a problem.

The original coax illustrates this action.  Some of the earliest
commercial coax was air insulated, and had optimally sized center
and shield diameters for low loss power transmission.  Which results
in 75 ohm impedance.  It was a dodgy affair, as it had very thin
lucite disks every foot, or so, that kept the center conductor
centered in the shield.  This worked ok for rigid coax, and even
semi-rigid coax where large radius arcs, over long runs, are
possible... like on an antenna tower, or a intercontinental under
sea telephone trunk cable...

At some point, it became desirable to make short flexible runs of
coax, WWII as I recall, and the disk scheme wasn't optimal, so the
coax was entirely filled with polyethylene plastic, and because of
the dielectric constant of the polyethylene, the 75 ohm air
dielectric impedance dropped to 50 ohm.  The loss went way up, but
flexibility trumped loss in short pigtails for interconnecting cables,
and 50 ohm became popular.... RG-8 coax was born.

-Chuck Harris

Robert Atkinson wrote:
> Hi Chuck, some early 75R BNC designs did use a smaller diameter center
contact.
> The 75R male can make intermittent contact when used with a worn or top of
limit
> 50R female. The 75R female can be damaged for use with the small contact
male if
> used with a 50R or large contact 75R male. The do exist, but are pretty
rare. A
> lot got changed out as faulty. I've not seen one on any modern (last 20
years)
> equipment.
>
> Robert G8RPI.
>
_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.