Frank Dresser wrote:

"Bill" wrote in message
...
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)

Isn't that a transmission line equation? I found a similar one in Terman's
Radio Engineer's Handbook, which I posted on the alt.binaries.pictures.radio
newsgroup.

Or might the non-resonant formula apply only to terminated antennas such a
beverage antennas? A quick scan through the Terman book doesn't give a
formula for beverage antennas.


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

Thicker wires, at the same center to center distance, have more capacitance
to the other conductor.


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

The radiation resistance (or reception resistance, I suppose) of a wire
shorter than a half wavelength is very low but goes up as the length of the
wire goes up. The reactance goes down as the wire approaches half a wave
length. In effect, they add up to a high number for end fed wires.
Something like the same effect exists above half a wavelength. These
resistances and reactances can be calculated and measured, but there's
little practical reason to do either for reception. The reactance adds to
the resistance, and the impedance can safely be assumed to be "high" for end
fed wires.

Just to be complete, I'll mention that an antenna tuner or balun can more
efficiently match an antenna to a radio. It's not always worth the effort,
however.

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)


-Bill

I don't think I"ve looked at a Smith Chart in 25 years. The average SWL can
do just fine without getting into all the confounding technical details of
the hobby. I think Steve covered the most important part when he wrote that
it's the nature of end fed antennas to have a high impedance.

Yes, why try to belabour the point? He'll do just fine to plug the dang wire
into the 500 ohm input. If he wants or needs to do better he can improvise a
matching transformer, keep his antenna away from the house or whatever and then
feed the 50 ohm port.

This ain't rocket science, though a few minor details can enhance performance.

dxAce