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.


--
Jim Pennino