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.
James n5gui