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: [email protected] [mailto:[email protected]] 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 -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
