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

[Jim Lux]

On 3/25/14 9:39 AM, Jim Brown wrote:
> On 3/25/2014 9:31 AM, Tom Osborne wrote:
>> I remember good old 'twisted pair' that we used 'back in the day.' I
>> had a Heathkit AT-1 and just bend one wire and stuck it inside the
>> coax jack and taped the other wire to the outside of the coax jack.
>>
>> Who knew about PL-259's and SWR back then :-)  Think the twisted pair
>> was about 75 ohms.
>
> Hi Tom,
>
> Twisted pair is great stuff, and far better at RF than most folks
> realize. I'd guess Zo to be between 50 and 75 ohms, which makes it a
> decent match to a dipole.

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.