wrote in message ...

Wayne wrote:


"John S" wrote in message ...

On 11/23/2014 11:16 AM, 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...
snip

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

If it isn't too much trouble, I'd like to see the EZNEC file. My email
address on the post is correct.


# No trouble at all, I think. Never tried this before, so let me know if
# it is not successful. By the way, it is for my frequency of interest,
# 434MHz. If that is a problem, I can scale it for you.

Got it, thanks.

It is as I had envisioned originally, but without the second wire.

Scaling it to 7 MHz, the 3:1 SWR bandwidth is about 400 KHz, and the
antenna
is about 12 feet high.
Over "real" ground, there is some pattern skew. And the feedpoint is not
a
bad match for 50 ohm cable.

This is an interesting antenna solution if antenna height is a
consideration.

# I just ran the thing through the optimizer at 7.15 MHz and got dimensions
# (in wavelengths) of .116 high, .132 to the feeder stub, and .14 for the
# length of the radiator AFTER the stub, or a total of .272 wavelengths
# long.

# For very good, average, and very poor ground I got:

# Very good
# 1.9 dBi @ 25 degrees

# Average
# .3 dBi @ 31 degrees

# Very poor
# -1.1 dBi @ 35 degrees


# The SWR was less than 1.2:1 across the entire band for all cases.

# I get basically omniadirectional with a slight skew of greater gain
# in the direction oposite the radiator direction.

# At about 38 feet long it should fit in most suburban lots, and at about
# 16 feet tall it is above everyone's head.

# One would probably want to put at least some short radials at the feed
# point.

Interesting. I might look into this a bit.

My situation: small lot, 2 story house that is taller than the surrounding
trees on a sloping lot. There is no good place to string any kind of
dipole.
But...
The metal roofed patio cover is about 30 feet wide and 12 feet deep.

I have a 16 foot whip mounted on the patio cover, and use a tuner in the
shack for multiband operation.
I'm considering a remote ATU mounted at the base for easier matching.

The folded unipole might be an alternative for 7 MHz.