OK.....this might be a source for a metal parabola.
As a summer job when I was in high school I worked at a metal spinning
shop. They made parabolic reflectors for lighting fixtures.
What is metal spinning? Basically you chuck a wooden pattern in a BIG
lathe. The wooden pattern had a 1/4" diameter pin sticking out of the
center pointing towards the tail stock. Then you place a metal disk
with a 1/4" hole in the center of the disk and sandwich the disk against
the pattern with the live-center tail stock. Then a tool (lubricated
piece of oak) is used in the tool rest to "bend" the disk around the
pattern. The pattern has to be the proper (parabolic) shape.
Where am I going with this? The place I worked at had literally
HUNDREDS of different shaped patterns in all shapes and sizes. Chances
are very good that you could find the proper size and shape for this
application. So look up "metal spinning" in the local phone book.
There may be a place nearby.
Then all you have to do is sweet talk the production manger and tell him
what you're doing....he just might help you out. Maybe they would
entertain a small group order from a ham club?
By the way....even if you own a lathe don't even think about trying to
do this at home. If you can imagine an 18" diameter (or larger) solid
hunk of wood spinning at several hundred RPM attached to a thin and
SHARP edged disk of metal spinning at the same speed (think of how fast
the edge of that disk is spinning) and you get the picture. Think meat
slicer like in a deli. Most of the old guys that worked there were
missing part or whole pieces of one or more fingers.
Someone mentioned a metal wok. That might work too but go to a
commercial cooking supply store, preferable if you have a Chinese
neighborhood nearby, and look for either a wok or a wok lid. I have a
spun aluminum wok lid that would probably be just the thing to use.
On 1/2/2013 6:59 AM, N1BUG wrote:
I have been working on the "dish problem". It is not as simple as it
may seem, if one is looking for optimum performance. Aren't we all? ;-)
It is very important to match the shape (depth) of the dish to the
detector used. Virtually every available dish, including the Edmund
Scientific ones, are too deep for optimum capture, given the beamwidth
of commonly available transducers. If you study the pattern of various
transducers you will find there are some that have a beamwidth of
about 50 to 60 degrees in one "solid" lobe with no nulls or side
lobes. Others have a narrower front lobe with deep nulls on either
side, then another lobe at each side, for a total beamwidth around 70
to 80 degrees. The latter type would probably be best for the deep
dishes commonly available, but will not make use of the entire dish
surface no matter what you do. The now obsolete transducer used in the
W1TRC design was of this type.
The type with one 50 to 60 degree lobe approaches ideal use of the
dish **if** you use a flat enough dish. You want a dish with f/D in
0.7 to 0.75 range for these. That amounts to about one inch depth (rim
to center) for a 12 inch diameter dish, or two inches for a 24 inch
dish. Most dishes are not anywhere near that flat! One exception is
the 12 inch dish sold by Midnight Science. It is optimized for this
application but for the price I don't think much of its quality or
surface accuracy. Poor surface accuracy (not adhering to strict
parabolic shape) reduces efficiency and can create unwanted pick up
from directions off to the side of where it is being aimed.
One final comment on the two variations of transducer. Typically the
larger diameter transducers have the 50 t0 60 degree pattern. The
smaller ones tend to have the split lobe with overall wider beamwidth.
Note there is another trade off with smaller diameter transducers: it
is more critical to get them exactly at the focal point of the dish,
and dish accuracy needs to be better to focus the collected ultrasound
into a smaller "spot".
The fact it results in a bulkier unit aside (due to the transducer
needing to be mounted farther out in front of the dish), I believe the
larger transducers with solid 50 to 60 degree lobe are the better bet,
if a truly suitable dish can be found.
Green Power Science has dishes that are actually too flat at around
0.9 f/D! These might not be too bad, as it would mean the outer
portion of the dish would be the area not effectively used. That would
provide better rejection of noise from behind the dish (bugs, traffic,
etc.). However, from looking at videos of these they appear to be very
flexible and would probably need a solid rim support added. They are
also coated with a highly reflective surface, as they are intended to
be used as solar collectors. That would have to be removed or the dish
painted. I have not worked out the math to see how much the effective
diameter would be reduced by the too flat shape. Bear in mind the
transducer would be a *long* way out in front of the dish (it has to
be at the focal point, which gets further away from dish as the dish
gets flatter or higher f/D).
I'm still looking for a dish 18 to 24 inches in diameter with f/D
around 0.75 and good rigidity. I have not found anything. I may get
frustrated enough to try spin casting a parabolic mold and making my
own dish. It's not a trivial project. Maintaining parabolic shape is
very important (else we are back to the same problem of poor
efficiency). Spin casting is about the only practical method of
ensuring shape accuracy I can think of, but I am open to other
suggestions!
Obviously receiver sensitivity is another important factor. I bought
the Midnight Science RX2 and was not happy. The new RX3 is much more
sensitive. I have no idea how it compares with the expensive
commercial units. I bought all the parts to build the W1TRC receiver,
intending to compare it with my RX3 on the test range, but simply
haven't had time and energy to do it (yet). I wish I could get my
hands on a Radar Engineers 250 for side by side comparison on the
range, but that seems highly unlikely.
73,
Paul, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
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--
73,
Gary K9GS
Greater Milwaukee DX Association: http://www.gmdxa.org
Society of Midwest Contesters: http://www.w9smc.com
CW Ops #1032 http://www.cwops.org
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