Someone on another board I read posted a description of how to
construct a self-resonant, helically-wound vertical as an emergency
replacement for a 1/4-wave monopole for a MW broadcast station. His
conclusion was that its radiation resistance would be somewhere around
40 ohms, and could be driven at least at reduced power by a tx
expecting a 50 ohm load.

I responded as below, and thought it might be a topic for discussion
on rraa.
________

Such a configuration may be self-resonant, but that does not mean that
it
has the same radiation resistance as that of a linear monopole at its
1st
self-resonance (1/4-lambda).

According to John Kraus in "Antennas," 3rd edition, Chapter 8-22 --
the
radiation resistance of such a helical antenna is approximately the
same as
that of a linear monopole of the same overall height, and not to the
length
of the wire used to wind the helix.

The radiation resistance of a short monopole is approximately (h^2)/
312,
where h = height in electrical degrees. A 20-ft monopole is about
12-1/2
degrees at 1700 kHz, so the highest radiation resistance possible for
your
configuration for MW broadcast use is about half an ohm.

Kraus says in this chapter "The advantage of the helix over a straight
wire
or stub is that its inductance can resonate the antenna."

This means that a separate "loading coil" is not needed, which can
reduce
the fixed resistive losses in the antenna system. But it doesn't mean
that
a self-resonant, normal-mode helix has the radiation resistance and
radiation system efficiency of a linear, 1/4-wave monopole (other
things
equal).

RF