parnass@ihuxz.ATT.COM (Bob Parnass, AJ9S) (04/15/89)
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I BUILT AN ULTRASONIC RECEIVER
by Bob Parnass, AJ9S
Both midwestern farmers and radio enthusiasts suffered
during last summer's record drought. Many of us were
plagued with power line interference, as the wooden power
poles shrunk without moisture, allowing the power line
hardware to become loose and electricity to arc. We
could hear the "eggs scrambling" noises throughout the
shortwave spectrum, even up through 220 MHz in places.
I spent a few days driving around the neighborhood with
an Icom R-7000 VHF/UHF receiver on the front seat trying
to pinpoint the offending power line fixtures. I even
walked part of the area with a portable scanner, attenua-
tor, and home made loop antenna.
It was a challenge. The fun of the hunt outweighed the
annoyance of the interference. Although I was able to
locate a few offending transformers, the power company
didn't return my call.
Vowing to build another gadget to help pinpoint power
line leaks for the next time this happens, I set my
sights on building a device to detect the ultrasonic
energy emitted by leaky lines. William Nelson's
Interference Handbook (Copyright 1981 by Radio Publica-
tions, Inc.) shows an old Hewlett Packard ultrasonic
detector, useful for finding corona discharge in the 30
kHz to 50 kHz range (pg. 117). Autumn rains came, vanqu-
ishing the noise problem and cooling plans for an ultra-
sonic detector.
Article Revives Interest
A magazine article rekindled interest in ultrasonics.
February 1989 Popular Electronics featured a construction
article on an ultrasonic receiver. This was a
superheterodyne circuit, with a tuneable local oscilla-
tor. I decided to build this unit and ordered a kit of
parts from Krystal Kits.
The kit consists of the printed circuit board, potentiom-
eters, ICs, resistors, capacitors, PVC pipe and end caps
(used as an enclosure). I bought a 3-3/4" Radio Shack
piezo tweeter (40-1382) for use as an ultrasonic micro-
phone, and supplied my own IC sockets, as none were fur-
nished.
This Krystal Kit is nothing like a Heathkit. There are
no instructions, just a reprint of the magazine article
with a few hand drawn corrections. Mechanical work, like
laying out and drilling holes for the enclosure, remains
the responsibility of the builder.
The printed circuit board eased construction, but was
only of fair quality. There were extra splashes of
copper, which required removal with an X-Acto knife. The
holes for the mylar capacitors and several resistors were
spaced too closely, forcing contorted bending of some
component leads.
The oscillator was supposed to tune 15 kHz - 35 kHz, but
mine would only tune 12 kHz - 21 kHz using the specified
components. I substituted a 2200 ohm resistor for R5
(originally 15K) which expanded the tuning range to 15
kHz - 66 kHz.
Only young listeners can hear pure sounds near or above
20 kHz. Using my ultrasonic receiver, I have listened to
sounds I could otherwise not hear from these emitters:
1. televisions
2. older ultrasonic TV remote control transmitter
3. computer terminals
4. video cassette recorder
5. metal detector searchcoil
Sound sources radiate almost equally well in all direc-
tions when the wavelength is large compared with the
source. At higher frequencies, and smaller wavelengths,
the radiation becomes more directional.
Although the receiver is sensitive, there are a few
shortcomings: Moderately loud noises in the sonic range
can leak through the receiver and be heard. Since the
receiver is not shielded, it can detect some electromag-
netic fields as well as acoustic signals.
In conventional radio receivers, the local oscillator
generates a fairly clean sinusoidal wave. By contrast,
the ultrasonic receiver uses a square wave oscillator,
which produces strong harmonics. Thus, you can hear har-
monics of the same sound at more than one place on the
tuning dial.
I am awaiting the arrival of better weather so I can try
listening for power line corona and the ultrasonic sounds
emitted by insects.
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Bob Parnass AJ9S, AT&T Bell Laboratories - att!ihuxz!parnass - (312)979-5414