"John S" wrote in message ...

A special purpose antenna for maybe just below the 6M band. For railroad
cars (a large metal ground plane). The antenna must be short enough to pass
through tunnels and have a 50 ohm feed impedance.

I saw this antenna in a book but I can no longer remember which book and,
although I've searched, I can't seem to find a reference. It was probably
from the 1950's. Anyway...

Imagine a folded unipole over a large sheet of metal. It will probably have
a high feed resistance of 100 or so ohms. But, if it is bent over 90
degrees starting a short distance above the ground plane, it can be
adjusted to match a 50 ohm feed and with no imaginary component. This will
satisfy not only the feed impedance but also the short height requirement.

Other than the really nice ground plane of a railroad car's roof and using
a frequency proportional to the plane, there is no obvious reason this
cannot be use in other situations.

Is that not really cool? Comments welcome, of course.

This sounds like a straightened out version of the DDRR antenna. QST did an
article on it some years ago, but used some substandard construction
techniques.

It can be built for HF. It is essentially a quarter wave vertical bent
over. The bent part can be straight or formed into a circle.
IIRC the configuration you describe was used for presidential communications
during the 1950s and was attached to campaign trains.

The antenna also has a history on naval vessels. Some hams have used it as
a means of having a HF vertical shorter than their fence to avoid homeowner
association rules.

I built a 2 meter version of the antenna in the 1970s. The antenna was
constructed from 1/4 inch copper tubing formed into a ring, with the
grounded straight vertical section being 2 inches long. It was tuned to
resonance with a 1/2 inch disk attached to the open end. A screw moved the
disk to form a variable capacitor.

The antenna worked very well with local repeaters, and it allowed my Chevy
van to have a top mounted antenna and still fit in the garage.