Topband: Extreme directivity

[Michael Tope]

----- Original Message ----- 
From: "Ford Peterson" <ford at highmarks.com>
To: <topband at contesting.com>
Sent: Sunday, July 22, 2007 1:35 PM
Subject: Topband: Extreme directivity


> 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
>

The only reason the ultrasonic system works is that there is a non-linear
interaction between the ear and air around it that effectively demodulates
the signal. The important thing to understand is that the energy propagates
from the speaker to the listener as a modulated ultrasonic wave, so the
properties of air in the ultrasonic regime dictate the propagation
characteristics.

Your proposal to modulate the 432 MHz signal with 1.8 MHz would
produce a very wide 432 MHz signal (~2 MHz wide if you used SSB).
There would be no energy propagating at 1.8 MHz. You would gain
the benefit of the high directivity, but the propagation characteristics
would be strictly those of the 70CM medium. Topband propagation
characteristics would not apply at all.

73, Mike W4EF.......................................................