[TowerTalk] Equation wanted

[L. B. Cebik]

Re:  characteristic impedance formula.

The formula in question is one of several formulations found in different
engineering references.  Jack Belrose has worked with them all.

All are based on treating the vertical element on analogy with a
transmission line.  The analogy presumes that a thin wire element is
sufficiently close to a conical section with the point at the feedpoint to
make the calculation accurate enough for some practical applications,
especially at LF and VLF.  I have worked with the equations at HF and find
that with constant diameter elements, they grow increasing inaccurate
above 3 MHz.  The last article in my Communications Quarterly series on
small beams will delve into this in some detail.  It will appear (most
likely) in the Fall issue.

The proposed element tapers in the opposite direction, with the wide
diameter at the feedpoint.  This will likely make the formulas even more
inaccurate.  Since the point of the equation is to return a characteristic
impedance that is itself of no great use, but to find in succeeding steps
the remaining capacitive reactance (and to translate that into a dimension
for a top hat), an easier and more accurate method might be to model the
proposed antenna and check the resultant feedpoint impedance for
capacitive reactance, if shorter than 1/4 wl (for a vert or 1/4 wl per
side for a dipole).

There are some articles on very short verticals using revisions of the
classic equarion(s) in past issues of Antenna Compendium, but my stock is
not at hand for precise reference.

Hope this is useful.

-73-

LB, W4RNL



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