Bill wrote:
Telamon wrote:

In article , Bill wrote:


Telamon wrote:



For average earth conductivity and a 22 gauge wire the height above
ground for 500 ohms impedance would be less than 5 foot and most
likely you would want it around 2 to 3 feet off the ground.


At what frequency did you calculate this?
-Bill



The impedance of the wire is not dependent on frequency.

Z= 138 * log (4* height / wire diameter)

Don't confuse a physical property of the wire with reactance.

Well, you're correct, but. There's more to the antenna than the natural
impedance of the wire alone. You have to look at the 'feedpoint'
impedance which is totally different and thats where you'll find the
reactance which cannot be ignored in actual practice.
Z=R+jX
Thats where frequency gets into the picture and gives you a number to
work with when matching the antenna to your radio.
This is Smith Chart 101...(which I never did too well with)


The fixed *characteristic* impedance of the wire is key to understanding
the feedpoint impedance. If you choose Z0 of your Smith chart to be
equal to the characteristic impedance of the wire antenna, you'll find
that the feedpoint impedance makes a spiral about the center of the
chart as the frequency is varied. This means that the characteristic
impedance (the center point of the spiral) is the best *frequency
independent* match to the wire.

Since for practical configurations the formula Telamon quoted above
yields characteristic impedances in the range of 300-700 ohms, many
receivers have ~500 ohm inputs and many of us use 9:1 matching
transformers when using coax feed.

See http://anarc.org/naswa/badx/antennas/SWL_longwire.html

-jpd