[Elecraft] dipole antenna efficiency

[David Woolley]

John Magliacane wrote:

> It's a traveling wave antenna where the helix is wound around a
> supporting vertical mast that serves as a reflector.

None of the references I found (including university course notes), when 
researching this at the weekend, described anything other than a 
dielectric core.  They did suggest a spectrum of behaviour from 
basically linear polarized, to circular polarized, depending on the pitch.

Also, travelling wave antennas normally radiate almost parallel to the 
length of antenna.

> capacitively loaded, gamma matched dipoles.  Some implementations of
> these antennas place a number of identical dipole rings in parallel
> with one another to reduce ohmic losses and improve efficiency. 

I think I can produce a fairly good qualitative argument that that might 
not work for radiating elements, even if there is an advantage for 
lumped inductors.

If the individual wires strongly interact, which will be the limiting 
case as you bring them close together, they will behave like the 
individual elements of the cross section of a single conductor, and 
therefor suffer a skin effect which will exclude current from the inner 
ones and the insides of outer ones.

If they are positioned so that they don't interact with each other, each 
will act as an independent radiator and have the radiation resistance it 
would have had in isolation.  It will also have the the same ohmic loss 
resistance.  So, although you reduce the ohmic resistance by paralleling 
them, you also reduce the radiation resistance, in proportion.

For a non-qualitative proof, you would need to demonstrate that there 
isn't some peak in performance, between these two extremes
> 
> These antennas may not be made from Litz wire, but the underlying
> concept of distributing RF current among closely placed parallel
> conductors in an effort reduce resistive losses is still the same.

Note that a fundamental part of the concept of Litz wire is that it is 
intertwined, so that no one strand is always outside or always inside.


-- 
David Woolley
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