James Newtons Massmind wrote:

>>This is not about what the balloons carry. Mylar balloons 
>>reflect radar (and
>>light) like chaff does. A bunch of balloons floating over a 
>>city in darkness can take up any shape. Even the shape of a 
>>pighead imho <g>.
>>
>>Peter P.
> 
> 
> 
> I've just had this picture of a huge build: On a scale even larger than the
> "Helium Raft" done over at Mythbusters. A truly massive frame of bamboo
> lifted by a ring of mylar helium balloons with a few extra in the middle... 
> 
> ...arranged in the shape of a smiley face.
> 
> So the local air traffic control or military radar operator is sitting there
> minding his business when off from the left comes floating this blip...
> 
> Given the load capability, we could put LED's and coin cells or just little
> mirrors on the bottom.

Well, your BEST 'mirror' for RADAR signals is a corner cube reflector.
http://roswellproof.homestead.com/RAWIN_construction.html

Easy to fabricate out of stiff foil. Suspend several from a single balloon,
With all strings weighing the same, and all balloons with same lift power,
you could launch from a large field near an airport (just not TOO close) and
pull it off with zero wind conditions. (All balloons would rise at same rate).

Now the PIC version of this would have a magnetron tuned to the airport frequency,
and using the interrogate pulse, fire off a response at the appropriate time after
the pulse, and progressively with the scan. (Think propeller clock where the sweep
is the beam sweep, and the LEDs are times after interrogate pulse).
Most dishes have quite poor off axis sidelobes so it would be easy to WRITE stuff
on a radar controllers screen, if you wanted to.
And most airport radar domes are quite far AWAY from the airport so that they
can see the runways clearly (outside the 'dead time' of the interrogation).

You could probably get away from needing a magnetron by setting up a feed horn to
collect the interrogation pulse up close, then send it back with switched delays
achieved with loops of coax or commercial delay lines (like used in old DRAM controllers).
PIN diode switches controlled by a PIC of course.

Feed horn >--|
8 way split  |---- Delay line 1---- Shorting PIN switch--- Terminator.
              |---- Delay line 2---- Shorting PIN switch--- Terminator.
              |---- Delay line 3---- Shorting PIN switch--- Terminator.
              |---- Delay line 4---- Shorting PIN switch--- Terminator.
              |---- Delay line 5---- Shorting PIN switch--- Terminator.
              |---- Delay line 6---- Shorting PIN switch--- Terminator.
              |---- Delay line 7---- Shorting PIN switch--- Terminator.
              |---- Delay line 8---- Shorting PIN switch--- Terminator.

When the PIN switch is closed (shorting line), you get a reflection,
delayed by 2x delay, which goes back out the feed horn.

When the switch is open, the signal is absorbed by the terminator, so no reflection.

With a good splitter there is little extraneous reflection.

Frequency independent. Switches see only a couple of milliwatts, and only when
they're making a 'pixel'.
Pulse rates are on a few hundred hertz so the PIC can easily keep up with changing
the switch patterns for each pulse (or clusters of pulses).

Have fun....

Robert



-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist