[time-nuts] Lowloss cable?
Dave Brown
tractorb at ihug.co.nz
Mon Jun 13 02:57:00 UTC 2011
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" <hmurray at megapathdsl.net>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Monday, June 13, 2011 1:38 PM
Subject: Re: [time-nuts] Lowloss cable?
>
> 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.
>
>
>
>
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