Reg Edwards wrote:
Antenna conductors ARE transmission lines and the same sort of
calculations apply.

You're right, Reg. Without reflections from the ends of a
dipole, the feedpoint impedance would be hundreds of ohms.
A standing wave antenna is like a lossy transmission line
where the loss is to radiation. And the SWR on a 1/2WL dipole
standing wave antenna is quite high - in the neighborhood of
20:1.
--
73, Cecil http://www.qsl.net/w5dxp


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In message , Cecil Moore
writes
Reg Edwards wrote:
Antenna conductors ARE transmission lines and the same sort of
calculations apply.

You're right, Reg. Without reflections from the ends of a
dipole, the feedpoint impedance would be hundreds of ohms.
A standing wave antenna is like a lossy transmission line
where the loss is to radiation. And the SWR on a 1/2WL dipole
standing wave antenna is quite high - in the neighborhood of
20:1.

Just a quick question.
What is the impedance at the centre of an infinitely long dipole (in
free space)?
Ian.
--

Ian Jackson wrote:
Just a quick question.
What is the impedance at the centre of an infinitely long dipole (in
free space)?

Same as a terminated dipole in an anechoic chamber? 600-800 ohms?
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:
Ian Jackson wrote:

Just a quick question.
What is the impedance at the centre of an infinitely long dipole (in
free space)?


Same as a terminated dipole in an anechoic chamber? 600-800 ohms?

Not at zero Hz.

ac6xg

In message , Jim Kelley
writes


Cecil Moore wrote:
Ian Jackson wrote:

Just a quick question.
What is the impedance at the centre of an infinitely long dipole (in
free space)?
Same as a terminated dipole in an anechoic chamber? 600-800 ohms?

Not at zero Hz.

ac6xg


What is a 'terminated dipole'?
And why 600-800 ohms?

No, Mine will be just a normal dipole (but long). I'll be in a
spaceship, miles from anywhere, and I intend to put out a really long
antenna so I can work the universe on all amateur bands. (I don't care
about the polar diagram - there's bound to be someone out there in one
of the major lobes).

I intend to throw out an infinitely long wire either side of the ship,
and use a balanced tuner connected directly to the antenna (no feeder
required). Because of weight restrictions, I can only take one tuner,
and I want to make sure that the one I do take will cope with the feed
impedance of the antenna.

I think the impedance will be the same at all frequencies (maybe even at
'zero Hz'). But what will it be?

Ian.

--

Ian Jackson wrote:
What is a 'terminated dipole'?

In an anechoic chamber, the dipole has a resistor to chassis at
both ends. When the value of the resistor equals the characteristic
impedance of the dipole, the dipole becomes a traveling wave
antenna instead of a standing wave antenna. Your infinite dipole
in free space will be a traveling wave antenna, i.e. no reflections.

And why 600-800 ohms?

That's a ballpark Z0 for traveling wave antennas not located
near ground. It's the Z0 of a 1/2WL dipole during the transient
state before the arrival of the first reflections from the ends.

The resonant feedpoint impedance of a center-fed 1/2WL dipole is
(the difference between the values of the forward voltage and the
reflected voltage) divided by (the sum of the values of the forward
current and reflected current). Note: the two voltages are out of
phase and the two currents are in phase.

This is the (B) definition of "impedance" in the "IEEE Dictionary",
i.e. impedance as a *result*, not a cause. Destructive wave interference
at the feedpoint of a center-fed 1/2WL dipole is what causes a Z0-match
to a low resistive value, e.g. 50 ohms or 70 ohms.
--
73, Cecil http://www.qsl.net/w5dxp


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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.

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.
--

Ian Jackson wrote:

In message , Jim Kelley
writes



Cecil Moore wrote:

Ian Jackson wrote:

Just a quick question.
What is the impedance at the centre of an infinitely long dipole (in
free space)?

Same as a terminated dipole in an anechoic chamber? 600-800 ohms?


Not at zero Hz.

Now#1, an infinitely long wire has an infinite self inductance.
Now#2, an infinitely long wire has an infinite self capacitance.
Now#3, frequency does not enter into the equation

Zo = SQRT[L(infinite)/C(infinite)]

Now, from my old college days, infinity divided by infinity is
undefined. The SQRT of an undefined value is by definition undefined.

Conclusion: the impedance of an infinitely long wire in free space is
not the SAME as a terminated dipole in an anechoic chamber.

NOTE: an infinitely long wire in free space cannot be terminated. A
terminated wire in an anechoic chamber is not the same as a free space
model.

A terminated wire in in an anechoic chamber is a standard math model
with Zo varying as the ratio of the wire diameter and the distance to
the 'space cloth/cones' [which are not perfect rf absorbers ... hence
some reflection]. Also, the termination destroys the symmetry.



ac6xg


What is a 'terminated dipole'?
And why 600-800 ohms?

No, Mine will be just a normal dipole (but long). I'll be in a
spaceship, miles from anywhere, and I intend to put out a really long
antenna so I can work the universe on all amateur bands. (I don't care
about the polar diagram - there's bound to be someone out there in one
of the major lobes).

I intend to throw out an infinitely long wire either side of the ship,
and use a balanced tuner connected directly to the antenna (no feeder
required). Because of weight restrictions, I can only take one tuner,
and I want to make sure that the one I do take will cope with the feed
impedance of the antenna.

I think the impedance will be the same at all frequencies (maybe even at
'zero Hz'). But what will it be?

Ian.

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.