Hi! Thanks for the reply. (1) If I was using WiFi, I'd definitly put signal boosters on both ends, not just the server end. (2) The robot would be intitialy controlled by me.  However, after I get that all working, I would add a GPS and a few sensors to have it drive itself around. (3) There are some other 2.4 ghz users, but they aren't nearby, so it isn't a problem. (4) The trees here aren't really big ones.  There're basicly large shrubs. (5) To start, there wouldn't be any weapon.  However, I may add one later. I like your idea about splitting up the communications channels.  If it seems like I can't get WiFi to work, I think I'll do that. Just ask if you have any questions. Thanks! -- robowifi brought to you by http://www.wifi-forum.com/

robowifi wrote:

Weapons? You're not sure about how to get this thing to work at all and
you're thinking of arming it?

On Sat, 05 Mar 2005 15:08:54 GMT, robowifi
<robowifi.1lfeya@WiFi-Forum_dot_com> wrote:

Ok.  Let's play with the numbers for 802.11g.  A typical amplifier
will raise the tx power from about +15dBm to perhaps +23dBm (250mw).
You can go higher but that will limit the size of the antenna (to
remain legal).  So, I'll assume reasonable size omnidirectional
antennas at both ends.  You don't really need the ultimate in
performance, so if you fix the data rate to the slowest OFDM
modulation at 6Mbits/sec, the receiver sensitivity will be about
-88dbm.  Therefore, we have:
  TX power =  +23dBm  (starting from the robot end)
  TX coax loss = 3dBm
  TX antenna gain = +8dBi (omni about 2ft long)
  Distance = unknown
  RX antenna gain = +12dBi (omni about 4ft long)
  RX coax loss = 3dBm
  RX sensitivity = -88dBm
  Desired fade margin = 20dB
Plugging into:
  http://www.terabeam.com/support/calculations/som.php
and doing the trial and error thing with the distance, I get about 1
mile or 5280 feet for a 20dB fade margin.  So far, so good. 

However, this is under ideal conditions and doesn't include any losses
from antenna misalignment and foliage (bush) attenuation.  These are
difficult to calculate but can be estimated (or guessed).  I'll throw
in 10dB loss for worst case antenna misalignment and another 10dB for
a single large bush in the way.  That will effectively increase the
fade margin requirement to 40dB.  Doing the trial and error thing
again yields a range of about 0.1 mile or about 500ft.  That's what
about a 30 degree tilt on the robot antenna, plus one nominal bush
will do to your range.

Hmmm, maybe a lower gain antenna on the robot, with a larger vertical
radiation pattern, and with a much larger tolerance to tilt error will
help.  Replacing the robot antenna with a 5.5dBi omni results in a
range of about 0.8 miles (4200 ft).  The bush will still eat 10dB of
signal, but the 30 degree tilt error is on -3dB instead of -10dB for a
combine fade margin target of 33dB.  That yields a range of about 0.19
miles or 1000ft.  Much more better.  That might even work if there are
no other signal loss errors involved.

Of course, if you use multiple store and forward repeaters or a mesh
network, the requirements are no longer for LOS from one position, but
from overlapping areas.

Methinks you'll find that the required software and control system are
quite different between a remote controlled vehicle and a
semi-autonomous robot.  If this is really your plan, I suggest you
plan ahead and add provisions for both.  There's nothing more
disgusting that "adding a GPS" navigation and finding you've run out
of ports or computing horsepower.  Robots tend to consume ports for
sensors and control outputs at an alarming rate.  Plan ahead.

What I find interesting is that you apparently have the ability to
visualize and plan a potentially complex robot motion algorithm, but
claim that you find it difficult to design a tracking antenna.  If you
have sufficient computing horsepower and some simple 2-axis motor
control, you can easily build in an antenna track that position a
patch, panel, or biquad antenna in the desired direction.  Actually,
with a decent GPS, you don't even need the signal sensing part.  You
know the base antenna position and you know the robots position.  The
antenna direction can then be easily calculated.

If you can see through them a little, they won't block much signal.
If they're a solid wall of shrubs, the signal will be blocked
completely.  If you can raise the antenna high enough to go over the
shrubs, you win.  However, my worry is about non-penetrable
obstructions such as automobiles, houses, and hills.  With 2.4Ghz,
line of sight is a fundamental requirement.  That's why I suggested a
much lower control frequency, which will either penetrate these
obstructions, or bounce around them.

A friend built an autonomous robot with a water pump as a weapon.  It
was suppose to be some kind of fire fighting robot.  However, his wife
converted it into a house plant watering robot and is currently
working on getting it to wash windows or the kitchen floor.  What a
waste of creative weaponry.

yep.

How much money do you have to spend on this?  How much time do you
think it will take to build?  I'm just curious to see if you can guess
these to within an order of magnitude.

Best of luck.

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