Demonstration
For a classroom demonstration I have prepared two things. The
first is an interactive Maple program which will allow the students
to see how rays are reflected from the various surfaces to converge
at a point.
Secondly, I will demonstrate the concept of antenna gain with
a 12 inch parabolic reflector and an infrared heat source. I will
not go through, in class, all the steps listed in this paper,
however.
This is the paraboloid I found in
the physics lab. It is pretty
small.
I measured the width and depth to
perform the calculations to
determine the location of the focal
point.
This dish has an f/D ratio of .25.
I tried the dish as an ear. It
looks like it should work, but the
results were not impressive.
I will show calculations
using the wavelength of the sound I
tried (note tuning fork and box
transmitter), and the geometry of
this dish.
This dish should be more effective
at infrared frequencies, as I will
show with calculations prior to the
demonstration.
I am 20 feet in front of
the dish here, using a telephoto
lens. The heat is uncomfortable
here in front of the antenna.
I found another dish and
decided to use it as a receiving
antenna. I hoped a fly might wonder
near the focal point.
You can see the paper towel about
to ignite from the focused infrared
heat.
And we have ignition. Our
antenna works well as a transmitter
and as a receiver of infrared
radiation.
In class I will omit the
receiver part
of this demonstration. Instead, we will show the class the antenna
gain over the isotropic infrared radiator by allowing the students
to warm up some distance in front of the transmitting system.
Sumary