[time-nuts] Lowloss cable?

[Hal Murray]

richiem at hughes.net said:
> What's the best small diameter (<0.25") low loss coax? I need to run about
> 30' from my GPS antenna to a TBolt. 

There are two sources of attenuation.  One is the dielectric losses.  The 
other is resistance, primarily skin effect on the center conductor.

Most modern coax uses foam polyethylene for the dielectric.  It's pretty good.

To reduce the resistive losses, you want a bigger center conductor.  The 
useful cross section is the circumference times the skin depth.

There are two ways to get a bigger center conductor.  One is to use 75 ohm 
coax rather than 50.  For most GPS gear, that gives an impedance mismatch, 
but that is probably smaller than the reduced attenuation.  (It obviously 
depends on the length.  We should be able to compute the cross over length.)

The other approach is to use a bigger outside diameter.  The impedance 
depends on ratio of the inside of the shield and the outside of the center 
conductor.  So if you make the center conductor bigger to reduce skin effect, 
you have to make the outside bigger to keep the same impedance.

There are 2 types of 75 ohm coax readily available.  One is RG-59 at roughly 
1/4 inch dia.  The other is RG-6 at roughly 1/3 inch dia.

You can get all sorts of numbers (attenuation vs frequency, size) with a bit 
of googling.

Any consumer electronics place will have them in the cable TV section.  They 
come in various lengths with F connectors.  You will need adapters and/or to 
install connectors.




-- 
These are my opinions, not necessarily my employer's.  I hate spam.