[RFI] Ultrasonic Line Noise Detectors (Rick Karlquist)

Richard (Rick) Karlquist richard at karlquist.com
Wed Apr 28 08:22:14 PDT 2010

I threw together a "proof of concept" unit.  Not
ready for a construction article, which I don't
currently have time to write.  Key takeaways:

1.  The transducer is very narrow band around
40 kHz.  The transducer impedance is something
like 1k ohms.

2.  The transducer is the same series as cited in
QST, but is the widest available beamwidth.  The
QST author used the highest gain transducer, which
has the narrowest beamwidth.

3.  A low noise op amp preamplifies that transducer.
Select an op amp that has good noise for a 1k
impedance transducer.

4.  An envelope detector detects the 60 Hz modulation
on the 40 kHz pseudo-"carrier".  It is so narrow
band, it acts like that.  In QST, they down convert
to audio enmass, because the design was for bird
calls.  Big difference in these applications.

5.  A high power green laser pen ($100 at Pacificon)
can be seen in full sunlight when illuminating the
power pole from the ground.  Mount it coaxially
in front of the transducer.

6.  Even the widest beamwidth transducer can't fully
illuminate the short focus length dish, which is
optimized as a solar furnace, not a telescope.  You
would be better off buying the larger Edmund
Scientific dish and cutting off the outer 1/3 or so
of it.  I have one of these larger dishes, shipped
by Edmund by mistake ("just keep it").  Still trying
to figure out a way to cut it neatly without butchering

Rick N6RK

G. White wrote:
> RE: Rick N6RK and the revised detector...
> How about posting your design on your webiste, Rick? I am just abt ready to start the QST detector, but your post has intersted me. After checking out your professional website, you obviously know what you are talking about!...so can you provide some cct drawings?
> Greg
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