On 11/22/2014 4:09 PM, Wayne wrote:


"John S" wrote in message ...

On 11/22/2014 10:19 AM, 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.

# No, Guys, nothing that I have read so far is the thing I have in mind.

# Picture this...

# A folded unipole on a large sheet of conductive material. Let one of the
# vertical elements be connected to the sheet. The other is insulated from
# the sheet and has the generator attached.

# Now, this configuration has a high feed impedance, even when resonant.
# However, if you now bend the antenna at an appropriate point vertically
# so that the top portion leans horizontally to the sheet, the antenna
# gets lower to the sheet and the impedance lowers.

# I have an EZNEC file that I can share if anyone is interested. I will
# also continue to search my books for the example.

Ahhh....the "folded" part didn't sink in when I responded.

I suspect my original comments still apply.
With two conductors, the current is half in each conductor, thus a
doubling of radiation resistance at the feed point in one conductor.
So by carefully selecting the height at which the vertical part is bent,
there might be a good match for coax.

I'll play with that configuration a bit on EZNEC.

If you have AutoEZ to go with EZNEC, it is easy to find the bending point.