Topband: Extreme directivity

[Ford Peterson]

Bear with me on this.  I am coloring outside the lines again.  But I think
it's fascinating to consider on Topband.  

A few months ago, Scientific American ran an article about extremely
directional speakers.  The application was for an art museum.  An ultrasonic
speaker was mounted in the ceiling above each painting.  Visitors walking
below the speaker would hear the chatter regarding what they were looking
at.  People standing only a few feet away would hear nothing.  Dozens of
messages could be delivered to multiple audiences in the same room.  I'm
very curious to know if the same approach could be done on the low bands.

The principle was simple.  An ultra-sonic speaker can be made very small and
directional by fixing the speaker to one end of a long tube, not unlike a
toilet paper roll--the longer the tube, the narrower the beamwidth.  The
trick was to modulate the ultrasound at sonic rates.  The directivity of the
ultra-sound speaker was intact.  And the ear would hear the sonic modulation
directly.  Step outside the speaker's ultrasonic beam width and you would
hear nothing.

Extrapolate this to HF.  Suppose you have a very directional Yagi at say
432MHz.  Let's make it a long-john style and give it 25 elements or
more-make it a laser on 70cm. Let's go crazy and make it a 4 x 4 array of
these.  While this is a huge antenna by 70cm standards, it's a very small
profile by Topband standards.  

Forgetting about the legality of the experiment for the moment, modulate
that 432 carrier at 1.832MHz.  This gadget is going to throw a WHOPPING spur
on Topband.  You are going to hear that spur without-a-doubt.

The question is: will the modulation radiate well?  Will the beam form or
not?  Will the wave front propagate so as to conform to the 70cm antenna's
beam pattern?  Or will it propagate like you were loading a coat-hanger on
Topband?

Ford-N0FP