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.
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