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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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"Cecil Moore" wrote in message
...
Reg Edwards wrote:

Take a close-wound wire helix of diameter D metres and having N = 500
turns
per meter. Wire diameter approx 2mm.

All formulae available from Terman, Kraus, ARRL, etc., etc.

C = 55.5 / (Ln( 2 * H / D ) - 1) picofarads per meter.

L = Square( N * Pi * D ) / 10 microhenrys per meter.

TransLine impedance, Zo = Sqrt( L / C ) ohms.

Propagation Velocity = 1 / Sqrt( L * C ) metres per second.

Take a length of H =1.5 metres of this helix and use it as a short
vertical
antenna above a good ground.

It will resonate as a 1/4-wave vertical at 3.5 MHz.

Zo = 3243 ohms.

Velocity factor = 0.0701

Radiation Resistance = 0.176 ohms.

etc., etc.,

It's so simple you can't believe it. ;o)

Thanks Reg, this one is a keeper. If you made the helix long enough to
cause a phase reversal in the current, would the world come to an end?
--
73, Cecil

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

I forgot to say it will also have a 'cosine' current distribution between
base and top.

The antenna 'end effect' can be accounted for if necessary.

When the helix is not close wound, the increase in velocity factor and the
reduction in Zo can be accounted for.

Axial mode radiation cannot be distinguished from normal mode but this is of
negligible consequence until helix circumference approaches 1/4 wavelength
which it never does in the present context.
----
Reg.