[amsat-bb] Re: DIY: Parabolic mirror

James Whitfield n5gui at cox.net
Fri Aug 15 08:28:56 PDT 2008

Finally.  I was wondering if this thread would pass from the grousing and
"It'll never work" stage into something more like "How can I take this idea
and adapt it to something I want."

The end product of the two videos was a curved mirror, presumeably intended
to be a part of a solar energy collector.  The material used was a
reflective film, which needed to be mounted on a properly shaped and
smoothed surface, which was made of fiberglass/resin.  Its shape was
determined by form, in this case made of concrete, which was cast in the
concave end of a used water tank.

Can this be applied to another project......Maybe with a little thought and
a lot of adaptation:

A few months ago, I had the idea to demonstrate PSK31 communications at a
Boy Scout event, only I wasn't going to use radios.  My idea was to use a
speaker/microphone mounted at the focus of a paraboloid reflector to send
the sound a few hundred yards to another similar setup.  I didn't have the
time to construct the project for the Scout event, largely because I would
need to build a parabolic surface 30 to 60 inches in diameter.  But I
haven't dismissed the idea entirely.

Some of you might be interested in a parabolic radio wave reflector.  Buying
one may be outside your budget, so you might consider trying to build one.

The videos suggest a way to build a curved surface, that is common to the
sound and radio projects suggested above.  Parabololic would be the
mathematically ideal shape.  In practice, as was discussed in previous
posting, the surface needs a smoothness better than a quarter wavelength at
the frequency in use.  The shape is often spheroid, the distinction and the
actual choice of shape should be another discussion.  The point being, it is
possible to define the desired shape and the allowable errors.

Would a fiberglass shell built up on a concrete form meet the shape and
error requirements?  Perhaps.  For my sound project, the wavelengths under
consideration start at about 4 inches and get longer. ( 3,000 Hz at 1,000
feet per second, and down to about 250 Hz, typical of  PSK31 output from a
sound card.  I would get better results using 6KHz or higher frequency sound
waves. )  I can be pretty confident that the shape and smoothness
requirement could be met with the DIY manufacturing technique.  Would it
work for a radio reflector -- choose a frequency, calculate the shape and
smoothness, then make your own judgement call.  If you wanted to use it for
an optical telescope, not a good idea.  It was after all, originally
designed for a solar cooker, shaped and smoothed to put the sun's rays on a
cooking pot, not star images on film.

What about the reflector material?  For a sound reflector, I don't need
shinny plastic to be added, the fiberglass would be reflective of sound by
itself.  Painting might help smooth the surface, particularly if I had to
fill bubbles or sand down other flaws.  For radio waves, I seriously doubt
the plastic film would be adequate.  You might be able to use aluminum foil
or tape.

Thinking differently, you might consider metal screen as the first layer of
material in the construction of the paraboloid shape.  Then add resin and
layers of fiberglass to strengthen and set the shape.  (  No.  I have never
tried it.  I have no idea if it is worth trying. )  It may be possible to
put a conductive coating on the shape, then electro-plate copper onto the
surface in a thickness that will reflect radio waves.  Maybe someone else
has better ideas on how to make a radio reflective surface to be added to
the curved surface.

Substitute some materials.  I briefly considered paper machet or a wood and
stryofoam form instead of the concrete suggested in the video.  The concrete
form is probably more durable and stable, but it requires that you have a
mold that you can pour it into.  That means another chance for "copy
errors".  Might be well worth the trouble if you intend to make several.

I also considered paper machet instead of fiberglass for the shell material.
It would have been something that the Scouts could build.  The problem is
that it shrinks as it dries, so the shape would not be very precise.  After
doing some research on construction, I found out that they used to make
boats and canoes out of paper and glue ( waterproofed with paint and
varnish ).  The end product was probably not as light or as durable as the
wooden boats produced at the same time, but certainly less expensive.  They
had their niche.  From that, I seriously considered paper/glue composite my
primary choice for construction - again, mostly so that it could be
constructed by Scouts with available, low hazard, materials.

Does any of this have any relevance building a parabolic reflector that can
be used in satellite communications?  If it gives you ideas, or challenges
you to find a better way.....More likely you will just shake you head
murmuring about that crazy old geezer and his crack pot ideas.  I tip my hat
to those of you who are my elder, and wish health and long life to all.


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