On Nov 27, 5:08*pm, Jeff Liebermann wrote:
On Fri, 27 Nov 2009 16:29:22 -0800 (PST), "

wrote:
I fiddled with the AMOS 1090 and didn't like the big dip in the
vertical axis. No amount of running the optimizer could get rid of it.

http://802.11junk.com/jeffl/antennas/AMOS-5-1090MHz/
for the curious. *Yeah, the side lobes are not going to go away with
the 5 element model. *However, if you add elements, such as the 7
element model:
http://802.11junk.com/jeffl/antennas/AMOS-7/
the side lobes are somewhat reduced. *With 40dB difference between the
major lobe and any of the side lobes, I don't think you'll have a
problem.

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

No, it's not the side lobes as much as the gain dips as you sweep
elevation, i.e. starting from the horizon and going to vertical. Mode-
s is very different from your typical point to point comms. You need
coverage at all elevations. Perhaps I'm not explaining this well.
Maybe vertical plane is a better term. With mode-s, the targets are
3D. All around you, plus up and down. It is like satellite reception,
only worse since you want to cover horizon to horizon.

Think of the Lindenblad antenna, but with more gain.
http://www.amsat.org/amsat/articles/w6shp/lindy.html
The FAA has played with these, as shown in the article, but I've never
seen one at an airport. The Lindy has the advantage in that it's
circularly polarized. The mode-s transmitter is on the belly of the
bird, so you would think it's vertically polarized, but unless you
want a lot of angry passengers, you tend to fly level to the earth.
So distant plane's antenna would appear to be tilted from the
distance observer.

There are stacked Lindenblads, but I'm pretty sure that can't be done
strictly passive. There is also that variant of the J-pole that has
circular polarization, which broadcast FM sometimes uses in stacks.

All that said, my gut feeling is parallel the 3 element inverted Amos
will probably do the trick. The stock antenna, which is 3dbi at the
horizon, does 200nm. A receiver is at best only a few thousand above
average terrain. The last time I did the math, I got a line of sight
to the highest flying plane of about 330m. Using a square law
relationship, that means a linear increase of about 2.7., or 4.3db.
OK, make it about 7dbi.

http://yu1aw.ba-karlsruhe.de/invertamos.pdf
Figure 12 shows nearly 11db for the 3 dipole inverted amos.