Re: [TowerTalk] 75 or 70 Ohm twinlead or ladderline cable - does it exist?

[Tom Osborne <w7why@frontier.com>]

Hi Jim

I wonder if we are talking about the same 'twisted pair?'

The wire I was talking about was the old telephone wire they had back in 
the 50's and 60's that came from the pole to the house.  I think most of 
it was probably number 12 or so copperweld wire.  It had a funny 
insulation - almost like tarred rubber on some of it.  73
Tom W7WHY


> Perhaps for AWG 16 magnet wire or something.. but for  AWG24, it's about
> 100 ohms (e.g. Cat 5 is specified at 100 +/- 15 ohms)
>
> twisted pair is much like any other parallel wire transmission line..
> Z = 120/sqrt(epsilonr)*acosh(s/d)
>
> so to get Z low, you need high epsilon and/or very close spacing.
> acosh(1) = 0, so there is hope..
> But acosh(1.1) = 0.433, so air insulation with spacing of 10% of the
> diameter is 53 ohms.
>
>
> Virtually all of the loss in transmission line
> > below VHF is due to copper loss, so big copper means low loss, and a
> > good twist minimizes both radiation and pickup on the feedline from
> > differential mode current. A good choke is still required to kill common
> > mode current.
>
>
> As the Z of the transmission line increases, the dielectric losses
> increase (because they're tied to voltage) and the ohmic losses decrease
> (because they're tied to current).  Going from 50 to 200 ohm Z increases
> the dielectric loss by a factor of 4 and decreases the ohmic loss by a
> factor of 4.
>
>
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