What your mother didn't tell you about transmission lines

Posted by Fred Townsend on Sep 11, 2011; 8:39am
URL: http://elecraft.85.s1.nabble.com/What-your-mother-didn-t-tell-you-about-transmission-lines-tp6780383.html

Was KPA500 RF output wiring:

Gary there are several distinctions:
First there is a difference between a jumper and a transmission line. For
short distances you will have perturbations at the start and finish that
will average out because you do not really establish the transmission line
effect. Losses are usually insignificant.  So to make a twisted pair put two
insulated wires in your drill chuck and twist them together. Works great.
Things get very different when the distance moves out.

For twisted pair (TP) to be an effective transmission line the twist must be
constant. This is called Unified Twist Pair (UTP) Examples of UTP are CAT5
and CAT6 cable. If you twist it yourself the twist will not be constant so
you will get dispersion effects and other problems. Dispersion is usually
not too big a problem at HF but it does cause phase noise.

Rubber (parallel) lamp cord can be used as transmission line and it is
cheap. Z0 runs 95 to 120 Ohms depending on insulation thickness.
Z0, the surge impedance of coax runs 50 to 91 Ohms. (91 Ohm is pretty hard
to work with because the center conductor is so thin.)
Z0 runs 95 to 900 Ohms for twin lead but really widens out beyond 300 Ohms
(AKA ladder line).
The limits for both are determined by physics. You can push them but you
can't change them so in short there is no such thing as 50 Ohm twin lead or
300 Ohm coax.

Finally if you are making your own TP I suggest you use either Kynar or
Teflon insulated wire. Both will be silver plated and have lower losses than
copper.

73
Fred, AE6QL
   
-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Gary Hinson
Sent: Saturday, September 10, 2011 11:06 PM
To: [hidden email]
Subject: Re: [Elecraft] KPA500 RF output wiring

> Sure. Twisted pair is a VERY effective transmission line, and that is
> what is needed ...

Hey Jim, what kind of twisted pair could I use to feed my [mostly 50 ohm] HF
antennas?  Coax is expensive in ZL so I'm willing to try alternatives.  I'm
even thinking of homebrew balanced line with baluns at the bottom (a
coax-fed remote antenna switch in my case).

73
Gary  ZL2iFB


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