RadioBanter - View Single Post

Ralph Mowery · July 5th 20, 06:31 PM posted to rec.radio.amateur.antenna

In article ,
says...

What are the units of distance? Inches? CM? MM?
For 300 ohm ladder line, I get .390.

Thanks

On Tuesday, March 30, 2004 at 8:43:04 PM UTC-5, Tom Bruhns wrote:
Impedance is about 120*ln(2*D/d) ohms, where ln is the natural
logarithm, and D is the wire spacing, center to center, and d is the
wire diameter. Assumes round wires, and air dielectric. You can
rearrange that: if you want x ohms and have d diameter, D =
d*exp(x/120)/2, where exp(x/120) is "e to the x/120 power". If you
don't have a calculator that handles that sort of math, this table
will probably get you close enough:

D/d impedance, ohms
1.9 150
2.8 200
6.2 300
22 450
75 600

It does not matter what the units (inches, cm, feet) are as long as you
use the same. Say .1 inches in diameter and 3 inches spacing or 1 foot
in diameter and 30 foot spacing although that is an awful example but
could come into play with VLF or even power line frequencies.

The units cancel each other out and it is just a ratio of diameter and
spacing.