On 01/02/2013 12:32 PM, Lee K7TJR wrote:
Virtually anything that is roughly the desired shape will work
I have to take issue with this statement as I spent several weeks a couple
of years ago building an ultrasonic detector. I too thought that a snow dish
would work, however when compared to a real parabolic curve it was
nearly useless. YMMV
I agree 100% with Lee. A parabola is a very specific curve derived
from a mathematical formula. "Close" shapes may work to some extent
but will almost certainly be very inefficient compared to a proper
parabolic surface of the same size. Even the best of ultrasonic
detectors is none too sensitive for this application, so anyone
serious about using them for power line noise source verification
would be well advised to optimize in every way possible. Before
using any object as a dish I would suggest taking at least 10
*accurate* measurements of the surface along a radial from edge to
center and verifying that it is a parabola using the proper
formula**. Also, as I said earlier, to live up to its full potential
it needs to be the proper depth or flatness of parabolic surface for
the characteristics of the detector being used. Guessing games will
likely produce perfectly horrible performance in *most* cases.
** a possibly easier alternative would be to measure the dish depth,
then plot/cut a parabola out of stiff paper, cardboard, or whatever
and then place that in the dish to compare the curve.
73,
Paul N1BUG
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