On Sat, 22 Nov 2014 10:19:36 -0600, John S
wrote:

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.

It's called a PIFA antenna.
http://www.qsl.net/va3iul/Antenna/PIFA/PIFA_Planar_Inverted_F_Antenna.pdf
http://www.antenna-theory.com/antennas/patches/pifa.php
etc...

They come in a wide variety of mutations, including one which you've
described. They can also be designed using a slot instead of a wire.
Most commonly, they're found on 2.4Ghz laptop antennas, aircraft
antennas, low profile bus/railroad/taxi antennas, etc. Some random
examples:
https://www.tessco.com/products/displayProductInfo.do?sku=309440
https://www.tessco.com/products/displayProductInfo.do?sku=72370
http://www.sti-co.com/antenna-products/low-profile-antennas/rugged-aluminum



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558