[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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