For a given OD the centre conductor will be SMALLER diameter for 75 ohm
cable wrpt 50 ohm cable.
Google for the whole minimum loss/highest power xfer capability etc issue as
regards coax cable diameter and impedance. All std textbook stuff. Or used
to be!
DaveB, NZ
----- Original Message -----
From: "Hal Murray" <[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Sent: Monday, June 13, 2011 1:38 PM
Subject: Re: [time-nuts] Lowloss cable?
[email protected] 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.
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