On Nov 27, 12:52*pm, Jeff Liebermann wrote:
On Fri, 27 Nov 2009 11:20:48 -0800 (PST), "

wrote:
Those old schemes where you quickly switch between antennas work well
for voice signals. I've found data channels don't do so well.

A "Homer" works fine for continuous uni-directional data. *It doesn't
work well for half duplex such as Wi-Fi. *They're really awful for
situations where you hear multiple xmitters with the added bonus of
collisions (i.e. packet data). *There are workarounds, but all require
decoding the data first and using a computah to keep track of which
xmitter you're hearing.

I was
told that it has to do with how the data channels are nearly always at
full modulation. One of these days I'm going to hack two radios so
they share one local oscillator and see if I can DF based on phase
comparison of the last IF output.

It will work with one big catch. *You'll have to replace the IF
crystal filters with one that has a fairly stable group delay across
the IF bandwidth. *If you're working with data that has modulation
components all the way to the edge of the IF filter, you're going to
go through huge variations in phase shift between the carrier and the
filter skirts. *Building two matched receivers just adds to the
complexity. *OAR (Ocean Applied Research, now part of Cubic) once made
an Adcock antenna DF system that had 3 receivers for measuring the
phase and displaying the direction on a scope.
http://www.cubic.com/cda1/Prod_&_Serv/C4ISR_Prod_&_Sys/DF_Products/df...
http://www.cubic.com/cda1/pdf/aa1319.pdf

Personally, I think you would do better with multiple remote receivers
and measuring the TDOA (time difference of arrival) at each receiver.
Each burst of RF is time coded and packaged along with signal
strength, GPS phase, and decoded data, and sent off to a central
computah for processing. *The time codes are critical as that allows
storage, replay, and post processing. *Spewing RF around Area 51 is
probably a bad idea, so post processing make sense. *Collect the time
coded data on thumb drives, stuff the data into a computah on your way
out, compute, and replay the captured data later.

Drivel: *If you're thinking of doing it with a Doppler ADF, then
please read my previous rants before blundering onward:
http://www.qsl.net/n9zia/doppler_notes1.txt
http://www.qsl.net/n9zia/doppler_notes2.txt
http://802.11junk.com/jeffl/AN-SRD-22/
Those are 15 year old postings about a 35 year old product, but it's
still generally accurate.

Did you ever build and try the 1090 Mhz AMOS/Franklin antenna?

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

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.
I do appreciate the NEC file as it helped me understand the program.

After studying all the AMOS designs, the best plan would be to use the
3 element inverted AMOS. It is a natural 200 ohm antenna, so I assume
you could skip the combiner and the associated loss. That is, just
slap 4 in parallel to make 50 ohms. I'll probably be bugging you off-
line on this, but I want to hack more myself er since it builds
character. .-) [Of course it also wastes time, but I consider the
wasted time education.] The 3 element AMOS creates a near perfect
"blob" in the vertical. I could see the quad 3-element inverted AMOS
working for satellite use.

Regarding TDOA, I bought two Trimble (Datum) StarLoc disciplined 10Mhz
time references at Livermore. New old stock at $30 a pop. What I fool
that I didn't wipe the vendor out since he had three. But at the time
I didn't know if I could make it work. Anyway, I think TDOA using time
stamp and synchronization via GPS disciplined clocks would make TDOA
work well, even if after the fact. If I had three, I'd be TDOAing. As
it stands, I was nice enough to sell one at cost to a friend, so now I
need two more. I wasn't thinking TDOA at the time.

If you look at Plane Plotter, they have a MLAT for 1090. [Not every
plane reports position.] The big drawback is the MLAT position
reference is some other plane in the sky that is position reporting.
Some airports have a constant mode-s, so I suspect there may be a FAA
MLAT in the works. However, if they built the 1090 receivers with
capability to use GPS timing, it would work with a simple TDOA.

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 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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.