Take your ruler along to a store where they sell bird feeders. You will find
squirrel baffles, they are a parabola shape and come in clear plastic and not
clear, they have a hole in the middle. I have one in the garage and I can get
some measurements if you like.
Dale, k9vuj
On 02, Jan 2013, at 6:59, N1BUG <paul@n1bug.com> 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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