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  #41   Report Post  
Old September 17th 05, 11:35 PM
Cecil Moore
 
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Ham op wrote:
NOTE: an infinitely long wire in free space cannot be terminated.


Doesn't need to be terminated. Reflections are eliminated
without terminations.

A terminated wire in an anechoic chamber is not the same as a
free space model.


Not exactly the same but close enough for government work.
Reflections from the ends and from the walls are reduced
to a negligible value. Been there, done that.
--
73, Cecil http://www.qsl.net/w5dxp


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Old September 18th 05, 08:18 AM
pezSV7BAXdag
 
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| "Reg Edwards" wrote in message
| ...
|
| You had better go back to your old college days
| and start again with new teachers.
|
| The input impedance of an infinite dipole
| is as I have already simply mathematically described.
| It changes a little with frequency and wire diameter.
|
| Anything less than infinite length will be altogether different.
|
| Now you can stop trying to pull our legs.
| ----
| Reg.

Dear Mr. Reg Edwards,

This is an interesting question indeed!

Well,
my wife yinSV7DMCdag has developed sometime an AVI movie

[ in ZIP form:
[ http://antennas.ee.duth.gr/ftp/visua...s/fu010100.zip
[ 850 KB

which deals among other properties of a filamentary dipole,
with its Radiation Resistance relative to the Input Terminals (Driving Point)

[ Please take a look if you like
[ in the part D of the movie,
[ at the bottom right of the screen

This is actually the Input Resistance Rinp
of a filamentary dipole of perfect conductivity in free space.

The ratio of dipole length L to wavelength lambda takes values up to 10,
but someone may maintain arguably that either
the limit of Rinp exists in general as infinity or does not exist at all.

Yours Sincerely,

pezSV7BAXdag


  #43   Report Post  
Old September 18th 05, 08:29 AM
Ian Jackson
 
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In message , Reg
Edwards writes
What is the impedance at the centre of an infinitely long dipole

(in
free space)?


===============================
Its not very different from -

Zin = 120 * Ln( Wavelength / d ) ohms.

where d = conductor diameter, both measured in metres.

Thus, at wavelength = 80 metres with 14 gauge copper wire, input
impedance = 1300 ohms approx.

If you don't believe me, just measure it.
----
Reg.



Are you sure it's as high as that, Reg? I once did a Smith Chart plot of
the impedance at the centre of a dipole, the valued being taken from a
table 'compiled by Wu' (LK Wu?). These only catered for a lengths up to
a few wavelengths. As the plot progressed round and round the Smith
Chart, it seemed to be heading for something around 350 to 400 ohms.

I've just done a search on 'Wu+dipole+impedance', and one of the results
is
http://www.fars.k6ya.org/docs/antenn...nce-models.pdf
I'll have a read of it today.

Cheers,
Ian.
--

  #44   Report Post  
Old September 18th 05, 10:30 AM
pezSV7BAXdag
 
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| "Ian Jackson"
| wrote in message ...
| [...]
| it seemed to be heading for something around 350 to 400 ohms.
| [...]
| Ian.

120*pi maybe ...

pezSV7BAXdag


  #45   Report Post  
Old September 18th 05, 02:35 PM
Cecil Moore
 
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Ian Jackson wrote:
Are you sure it's as high as that, Reg? I once did a Smith Chart plot of
the impedance at the centre of a dipole, the valued being taken from a
table 'compiled by Wu' (LK Wu?). These only catered for a lengths up to
a few wavelengths. As the plot progressed round and round the Smith
Chart, it seemed to be heading for something around 350 to 400 ohms.


Maybe 377 ohms? Remember that any finite length dipole is a standing
wave antenna and the feedpoint impedance is (Vfor+Vref)/(Ifor+Iref)
where Vfor is the forward voltage phasor, Vref is the reflected
voltage phasor, Ifor is the forward current phasor, and Iref is
the reflected current phasor.

For a 1/2WL resonant dipole the feedpoint impedance is low:
R = (|Vfor|-|Vref|)/(|Ifor|+|Iref|) ~ 73 ohms

For a 1WL (anti)resonant dipole the feedpoint impedance is high:
R = (|Vfor|+|Vref|)/(|Ifor|-|Iref|) ~ 5200 ohms (EZNEC)

An infinite dipole would not be a standing wave antenna. It would
be a traveling wave antenna (as in a terminated rhombic). So the
feedpoint impedance of an infinite dipole would be Vfor/Ifor=Z0.
Since the reflections modify the feedpoint impedance, we might
suspect that Vfor/Ifor falls between the feedpoint impedance for
a 1/2WL dipole and a one WL dipole. Seems to me, the Z0 of the
dipole, i.e. Vfor/Ifor, must be in the ballpark of the square
root of the product of those two feedpoint impedances.
--
73, Cecil http://www.qsl.net/w5dxp

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  #46   Report Post  
Old September 18th 05, 02:47 PM
Richard Fry
 
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"Cecil Moore"
For a 1/2WL resonant dipole the feedpoint impedance is low:
R = (|Vfor|-|Vref|)/(|Ifor|+|Iref|) ~ 73 ohms

_________________

The 73 ohm radiation resistance value applies to a physical 1/2-wave,
thin-wire, linear dipole in free space, however a reactance term of + j42.5
ohms also applies to such a configuration (Kraus 3rd Edition, p. 182).

The dipole length needs to be shorted by several percent in order to zero
out the reactance term, at which time (according to Kraus), the resistance
term will be about 65 ohms.

RF

  #47   Report Post  
Old September 18th 05, 04:08 PM
Ian Jackson
 
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In message , Cecil Moore
writes
Ian Jackson wrote:
Are you sure it's as high as that, Reg? I once did a Smith Chart plot
of the impedance at the centre of a dipole, the valued being taken
from a table 'compiled by Wu' (LK Wu?). These only catered for a
lengths up to a few wavelengths. As the plot progressed round and
round the Smith Chart, it seemed to be heading for something around
350 to 400 ohms.


Maybe 377 ohms? Remember that any finite length dipole is a standing
wave antenna and the feedpoint impedance is (Vfor+Vref)/(Ifor+Iref)
where Vfor is the forward voltage phasor, Vref is the reflected
voltage phasor, Ifor is the forward current phasor, and Iref is
the reflected current phasor.

For a 1/2WL resonant dipole the feedpoint impedance is low:
R = (|Vfor|-|Vref|)/(|Ifor|+|Iref|) ~ 73 ohms

For a 1WL (anti)resonant dipole the feedpoint impedance is high:
R = (|Vfor|+|Vref|)/(|Ifor|-|Iref|) ~ 5200 ohms (EZNEC)

An infinite dipole would not be a standing wave antenna. It would
be a traveling wave antenna (as in a terminated rhombic). So the
feedpoint impedance of an infinite dipole would be Vfor/Ifor=Z0.
Since the reflections modify the feedpoint impedance, we might
suspect that Vfor/Ifor falls between the feedpoint impedance for
a 1/2WL dipole and a one WL dipole. Seems to me, the Z0 of the
dipole, i.e. Vfor/Ifor, must be in the ballpark of the square
root of the product of those two feedpoint impedances.


Yes, I did think of 377 ohms (which I understand is 'the impedance of
free space'), but I'm no expert in these matters.

As you indicate, the impedance must lie somewhere between 73 and 5200
ohms. You suggest that this might be something like the square root of
the product of those two feedpoint impedances (the geometric mean),
which gives 616 ohms. However, you would see 600 ohms simply by looking
into an infinite length of 600 ohm feeder, which has parallel,
non-radiating conductors. If the length of the feeder was relatively
short (compared with infinity!!), pulling the conductors apart would
increase the impedance (probably to a lot more than 616 ohms). The
question is, 'when does radiation start to influence the impedance?'

If you look at K6OIK's paper at
http://www.fars.k6ya.org/docs/antenn...nce-models.pdf
and look at, for example, page 22, you can see how the feed impedance at
odd halfwaves increases, and at even halfwaves, decreases. I only found
this paper this morning, and haven't had time to look to see which (if
any) of the many formulas was used to obtain the plot. It must be
possible to get close to the infinity condition by entering values for a
very, very long dipole.

Cheers,
Ian.

--

  #48   Report Post  
Old September 18th 05, 09:50 PM
Roy Lewallen
 
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In King and Harrrison's _Antennas and Waves_, they show a plot of
calculated antenna feedpoint impedance as X vs R up to about 5
wavelengths. Antenna wire radius is 0.008496 wavelength. The Z of an
infinite length antenna is indicated by locating the centers of the
circles and noting that the center converges. The point of convergence
for this particular wire radius is about 250 - j170 ohms.

In the chapter on experimental measurements, there's a plot of the
calculated admittance of an antenna of radius 0.000635 wavelength up to
about 10 wavelengths. Superimposed are measured values from another
source which show very good agreement. The theoretical values converge
at 214 - j189 ohms, and the measured values at 218 - j174 ohms.

Dervivation takes about a chapter of very heavy math, and numerical
results were obtained with a computer.

Roy Lewallen, W7EL
  #49   Report Post  
Old September 18th 05, 09:57 PM
Richard Harrison
 
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Ian Jackson wrote:
"Maybe 377 ohms?"

Arnold B. Bailey in "TV and Other Receiving Antennas" shows his
calculations of radiation resistance at the current maximum point for a
center-fed thin dipole at its various resonances:

1st--------------------------------72 ohms
2nd------------------------------200 ohms
3rd-------------------------------102 ohms
4th-------------------------------260 ohms
5th-------------------------------117 ohms
6th-------------------------------295 ohms
7th-------------------------------127 ohms
8th-------------------------------321 ohms
9th-------------------------------135 ohms
10th-----------------------------340 ohms

He also shows the current distributions and feed point resistances which
I can`t and won`t.

The input resistance of the center-fed antenna at resonance never equals
the surge resistance in value, but is related to it.

Best regards, Richard Harrison, KB5WZI

  #50   Report Post  
Old September 19th 05, 04:12 AM
Cecil Moore
 
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Roy Lewallen wrote:
The theoretical values converge
at 214 - j189 ohms, and the measured values at 218 - j174 ohms.


Free space? As a data point, I pushed EZNEC to the limit on 40m
with a 9000 ft. dipole. Resonant feedpoint resistance at
7.152 is 390 ohms. Anti-resonant feedpoint resistance at 7.092
is 1980 ohms. It appears that EZNEC would converge to something
in between those two values for an infinite dipole in free space.
I ran into the segment limit at 66 wavelengths.
--
73, Cecil http://www.qsl.net/w5dxp


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