On Wed, 02 Apr 2008 20:33:34 -0600, msg <msg@_cybertheque.org_> wrote:
>Jeff Liebermann wrote:
>
><snip>
>
>> The airplane mounted video camera I helped throw together had about
>> a 1000 meter range. However, we used a 24dBi dish antenna on the
>> ground, that tracked the aircraft (both automagically and manually).
>Did you build a nutating feed for the ground station, or perhaps a
>phased array with electronic nutation?
>Michael
Nope. There were numerous models of tracking antenna. That was my
job. The problem was that the spread spectrum signal does not have a
carrier, which made it difficult to throw together an easy tracking
circuit. The ability of the airplane to fly close and perpendicular
to the antenna made tracking even more difficult. Worse, the 24dBi
dish has a -3dB beamwidth of about 7 degrees. What seemed like an
easy project turned into a real mess.
One scheme that came really close to working required 4 additional
receivers. The center feed mount was extended beyond the feed and
made from an RF absorber (i.e. PVC pipe fill of water). 4 antennas
were arranged in 4 quadrants on each side of the center feed pipe
extension. With the signal source dead ahead, the signal levels at
all 4 receivers is (allegedly) identical. If the antenna were offset
in one direction, the center pipe casts an RF "shadow" on the one of
the 4 antennas. A differential amplifier runs a gimbal mounted pair
of motors to correct the direction.
This worked well fairly for tracking under ideal conditions, but had
acquisition problems that drove me into overtime and later panic.
There were just too many reflections that screwed up the direction. If
I had a deep solid dish (large f/D ratio), instead of a barbeque grill
dish, the reflections would have been minimized and it might have
worked better. However, once the direction antennas were hit by a
reflection, the motor would swerve the antenna radically, losing lock,
and not easily recovering.
The radio link was via 802.11b wi-fi so unlike the ATV link, there was
a transmitter involved at the dish. When it transmitted, the
directional receivers were instantly overloaded. I worked around this
problem by temporarily disabling the servos in transmit.
I eventually gave up on the RF approach.
There were several other attempts to build a tracker. I finally threw
together an optical system that worked. It was similar to the RF
tracker, but was immune to all but the most disgusting reflections.
The aircraft carried several green LED's that pulsed at about 100 Hz.
The 4 antennas were replaced by 4 security camera lenses and photo
transistors. The center pipe was replaced by a fiberglass tube, which
blocks light, but passes RF.
This system worked much better, especially at night. It crapped out
when pointed into the sun, when some dingbat shines his flashlight on
the antenna, and when I took a flash photo. Range was limited to
about 300 meters. I never did nail down the servo loop damping
factor, so it tended to either crawl across the traverse, or twitch
badly as it moved. Neither seemed to bother the RF data link, but it
sure made everyone around the antenna nervous. Like the RF solution,
it worked for tracking and sucked for acquisition.
I had thought of using a nutating (conical scan) feed (straight out of
the WWII SCR-584 radar):
<http://www.carnarvonspace.com/wiki/index.php?title=SCR-584_Technical_Description>
However, I didn't have time to machine the required components and the
corresponding control system. When I was much younger, there were
tons of those feeds available in the WWII surplus stores, but those
are long gone today.
I have some other ideas on how to do a wi-fi tracking antenna.
However, I keep seeing high skool and college project proposals that
involve the construction of such systems. I don't wanna ruin it for
the students.
--
Jeff Liebermann
jeffl@cruzio.com
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558