The RE 250. I have one. The dish dia measures 18" and the depth is
4-3/8".
The transducer opening is just over 4-3/8". The opening of it's housing
is
level with the dish when viewed across the dish opening. Hope that
helps.
I'll repeat the gist of my previous post on the squirrel guard. It works
VERY well
as a US dish. It has pin point accuracy! It's easy to check the focus
of
any dish that has even a somewhat reflective surface. Clear plastic
dishes
are in that group. Set the dish up on a tripod in a darkened room. Set
up
a light source, such as a flashlight (AKA a "torch") say, 50 ft away and
aimed
toward the dish. View the concentrated light "falling" on your detector.
It
should fully illuminate it without blowing by the detector. It should
NOT be
a single point of light, but instead, illuminate the entire surface of
the detector.
Don't have a detector yet? Just use a piece of white paper and do
whatever physical measurements
you want. You can even do this at your local store to check the focal
length, etc.
I've done that. The dish, of course, must be pointed directly at a light
source.
Will this work outside, pointing at the sun? Yes, but it may set the
paper on fire!
Now a few words on the detector electronics. First off, don't bother
setting
up a dish to bench test. Whatever detector circuit you use should be
sensitive enough to hear (loudly!)
your fingers rubbing together at a distance of at least 10 feet. That's
with NO dish
attached. No kidding. You should hear virtually no "rushing" noise when
the
transducer/circuit is NOT receiving a US signal. IOW, the circuit should
have very good S/N ratio.
Do all of that and you'll have the most sensitive US detector you could
buy.
73,
Charlie, N0TT
On Sat, 8 May 2021 08:08:44 +0000 (UTC) Charlie Delta via RFI
<rfi@contesting.com> writes:
> Hi
> Can someone who owns a Radar Engineers model 250 or model 251
> Ultrasonic RFI receiver kindly tell me the dimensions of the dish?
> I am particularly interesting in the dish depth and the focal
> distance point. I understand that dish is 18 inches in diameter.
> The plan is to get a suitable parabolic dish molded by a local
> Dome skylight maker who thinks that he can mold a suitably
> accurate true parabolic dish.
> I am curious to know if the Radar engineers dish falls within the
> 0.5 to 0.6 F/D ratio? Since it works so well I thought I might as
> well start off with a set of dish dimensions that is proven to work
> around the 40khz Ultrasonic frequency.
> Since the beam-width is so sharp , I presume that the dish
> dimensions and the illumination from the dish of the ultrasonic mic
> will be critical. I have already played around with the W1TRC
> detector and various odd ball dishes and the results have not been
> that great since these dishes have a far from optimum F/D ratio.
> From playing around it seems that a deep dish with the focal point
> deep within the dish produces a very poor result and may even
> produce minor sidelobes that causes confusion. Depending on the
> strength of the signal, an ultrasonic signal can disappear because
> of the poor illumination of the microphone sensor. So it does
> appear that an optimum f/d ratio dish would produce a better result.
> Any input or comments would be welcome from anyone who has tried to
> build a similar project.
> (pick your mould VS mold spelling)
> 73CraigVK3OD
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>
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