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

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.

--
Telamon
Ventura, California

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)


-Bill

"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.

Frank Dresser

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

dxAce wrote:


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


I agree.

Adios,
Bill

dxAce wrote:


Yes, why try to belabour the point?

I recognized Telemon's antenna formula as something very much like the
transmission line formula. I'm not sure how it applies to resonant
receiving/transmitting end fed wires. If it does, I'd like to learn
something.

But, generally, I don't see much point in trying to caluclate a receiving
antenna's impedance.


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.

That's right.


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

dxAce


Not only that, but the rocket scientists stay stuck on the ground!

Frank Dresser

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