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Re: [RFI] Ultrasonic Dish

To: "rfi@contesting.com Reflector" <rfi@contesting.com>
Subject: Re: [RFI] Ultrasonic Dish
From: dalej <dj2001x@comcast.net>
Date: Wed, 2 Jan 2013 13:00:10 -0600
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
The two squirrel baffles I have are:  

Perky Pet brand dimensions are 16" diameter 3" deep and the hole is 2 3/4" 
clear plastic

The other one is a North Stars industry brand and it is 15 1/2" diameter 5" 
deep and the hole is 1 1/2"  it is a solid color dark grey.  

They don't do very well against the squirrels, but maybe better for noise 
finding.

Dale, k9vuj



On 02, Jan 2013, at 9:40, dalej <dj2001x@comcast.net> wrote:

> 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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> 
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