Re: NEC-LIST: HEMAC matching

At 11:26 AM -0400 4/17/07, D. B. Miron wrote:
>Do you mean helix rather than toroid? I imagine a helical coil
>wound around a tree trunk, spiraling up, would give better coupling
>than a toroid. Have you, or someone else, done this?

He means toroidal. This has been done, and it has worked. The last
name of the person who told us about it on this list escapes me at
the moment. IIRC, his first name was George; he developed the
technique during the Vietnam war while working for the U S Army,
possibly as a contractor (as a staff member of Stanford
Telecommunications Inc.?); and he used it successfully in Vietnam.

After reading his report on this list, I used NEC-4 to simulate an
air-core toroidal winding around the trunk of a tree rooted in soil.
IIRC, the technique worked plausibly. I do not recall the electrical
conductivity that I assumed for the tree trunk, or for the ground for
that matter. Nor do I recall NEC-4's computed efficiency of the tree
antenna.

Think of the tree trunk as a single turn through, along the axis of,
the toroidal winding and you will see why a toroidal winding is just
what's required to excite the tree as a vertical antenna.

Another method of exciting the tree is to drive two metal spikes into
the trunk, one near the ground and the other at a height above ground
chosen so the impedance seen looking into the terminal-pair formed by
the spikes is, e.g., 50 ohms. Then you connect a coaxial feedline
directly to the spikes. (I would put ferrite common-mode chokes on
this coax.)

-Chuck W1HIS

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