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Re: [RFI] Ultrasonic Dish

To: "rfi@contesting.com Reflector" <rfi@contesting.com>
Subject: Re: [RFI] Ultrasonic Dish
From: N1BUG <paul@n1bug.com>
Date: Wed, 02 Jan 2013 07:59:44 -0500
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
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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