[TowerTalk] 75 or 70 Ohm twinlead or ladderline cable - does it exist?
w7why at frontier.com
Tue Mar 25 19:40:37 EDT 2014
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
> 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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