Ian Jackson wrote:
"What is the impedance at the centre of an infinitely long dipole in
free space?

It is the antenna`s Zo. This depends on the size of the conductor used
to make the dipoole.

Arnold B. Bailey has already worked all this out and presents it in his
1950 edition from Rider`s of "TV and Other Receiving Antennas".

Like the Zo of a transmission line, antenna Zo has nothing to do with
reflections and terminations. When you first apply power, energy must
flow into the antenna at some definite voltage to current ratio. This is
the surge impedance or Zo. If the antenna or line is uniform and
infinitely long, the energy sent away is never heard from again. Zo is
the only impedance anywhere.

Page 345 gives the surge impedance in ohms for a balanced antenna as:

Zo = 276 log 1/P

P is the circumference of the antenna rod, or periphery, expressed as a
fraction of the free-space wavelength (see page 342) This may sound
goofy but Bailey has his reasons.

Bailey`s graph on page 345 gives dipole impedances from 70 ohms to 680
ohms for rod peripheries from 1 wavelength down to 0.00001 wavelength

If you have no reflections or standing waves, the impedance you
calculate should be the Zo.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Page 345 gives the surge impedance in ohms for a balanced antenna as:

Zo = 276 log 1/P

Bailey`s graph on page 345 gives dipole impedances from 70 ohms to 680
ohms for rod peripheries from 1 wavelength down to 0.00001 wavelength

Is the graph impedance looking into 1/2 of the dipole?
276 log 1/0.00001 = 1380 ohms, just about double the 680 ohm value.
--
73, Cecil, http://www.qsl.net/w5dxp

The impedance looking into the feedpoint of an infinite dipole is
TWICE Zo.

Zo + Zo = 2*Zo.

The formula for Zo doesn't seem right. When the circumference of the
antenna rod is one wavelength, Zo = 0.

And when the circumference is greater than one wavelength, Zo becomes
negative.

For an 18 gauge wire, at a frequency of 183 GHz, something funny
happens.
----
Reg.

"Cecil Moore" bravely wrote to "All" (29 Sep 05 15:25:42)
--- on the heady topic of " 73 Ohms, How do you get it?"

CM From: Cecil Moore
CM Xref: core-easynews rec.radio.amateur.antenna:217594

CM Asimov wrote:
Cecil, an infinitely long antenna is simply an impedance transformation
between different mediums. i.e. wire to free space.

CM We know one of the impedances to be 377 ohms.
CM Question is, what is the other impedance?


I think it is whatever you want it to be because it is a transformer.
Varying Rs would only affect the pattern.

A*s*i*m*o*v

.... Thank Thor Friday Nears!

On Fri, 30 Sep 2005 06:11:10 GMT, "Asimov"
wrote:

I think it is whatever you want it to be because it is a transformer.
Varying Rs would only affect the pattern.

In standard antenna parlance, the "pattern" is unaffected (aside from
magnitude, and then only by consequence of mismatch) by
transformation.

73's
Richard Clark, KB7QHC

Cecil, W5DXP wrote:
"Is the graph impedance looking into 1/2 of the dipole?"

I only quoted from the plot for tthe whole dipole in free-space.

The graph on page 345 has two traces:

Zo, or antenna average surge impedance, ZA, for a balanced, center-fed
dipole in free-space, which is found to be a function of the antenna
thickness=

ZA = 276 log 1/P

The other formula is also plotted. It is for a vertical rod against
ground. It has exactly 1/2 the resistance of the dipole.

For peripheries larger than 0.25 wavelength, Bailey notes that surge
impedance departs from the formula. For smaller peripheries, the plots
are almost straight lines on the scale used. Peripheries are plotted
with log spacings. Impedances are plotted with linear spacings.

Best regards, Richard Harrison, KB5WZI

Reg, G4FGQ wrote:
"When the circumference of the antenna rod is one wavelength, Zo = 0."

Bailey adrees with Reg. I was remiss in not quoting Bailey`s caveat. The
formula does not hold for circumferences greater than one-quarter
wavelength.

Bailey notes that uniform cross section conductors don`t have ubiform
impedances throughout their lengths. Zo is inversely proportional to
capacitance per unit length. Zo is lower at the antenna feedpoint than
at its conductors` middles. At the tips or open ends of antennas, Zo is
low. This is explained by the concentration of electric force lines at
the open end.

Variation of Zo along an antenna need not deter one from finding a
workable average of surge impedance. Bailey has determined this to be:

276 log 1/P, where P=circumference of the conductor in wavelength, for
circumferences of less than 1/4-wavelength.

For practical lengths of center-fed dipoles, the feedpoint impedance is
determined by combination of incident and reflected waves. Bailey has
worked out these for resonant lengths between 1/2 and 5 wavelengths. I
posted these long ago. But, for infinite length, Zo must prevail, as no
reflection will ever return.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Reg, G4FGQ wrote:
"When the circumference of the antenna rod is one wavelength, Zo =
0."

Bailey adrees with Reg. I was remiss in not quoting Bailey`s caveat.
The
formula does not hold for circumferences greater than one-quarter
wavelength.

Bailey notes that uniform cross section conductors don`t have
ubiform
impedances throughout their lengths. Zo is inversely proportional to
capacitance per unit length. Zo is lower at the antenna feedpoint
than
at its conductors` middles. At the tips or open ends of antennas, Zo
is
low. This is explained by the concentration of electric force lines
at
the open end.

Variation of Zo along an antenna need not deter one from finding a
workable average of surge impedance. Bailey has determined this to
be:

276 log 1/P, where P=circumference of the conductor in wavelength,
for
circumferences of less than 1/4-wavelength.

For practical lengths of center-fed dipoles, the feedpoint impedance
is
determined by combination of incident and reflected waves. Bailey
has
worked out these for resonant lengths between 1/2 and 5 wavelengths.
I
posted these long ago. But, for infinite length, Zo must prevail, as
no
reflection will ever return.

Best regards, Richard Harrison, KB5WZI

=====================================

Bailey, who I assume is a product of our universities, made a wild
guess and then worked backwards towards a sensible question. ;o)
----
Reg.