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.