Breathtaking Ingenuity, By Capt. Allen R. Lord, Travis Composite Squadron 22

(From Spring 2006 Eagle Call Magazine)

Allen Lord’s Direction Finder-in-an-Altoids-Tin Gives
Ground Teams an Affordable and (OK, We’ll Bite)
“Curiously Strong” Shirt-Pocket Solution

TRAVIS AFB—Conventional direction-finding
(DF) receivers suffer from several shortcomings, not
the least of which is their cost. Many senior members
and cadets who would love to embark on missions
have to beg, borrow or steal—well, hopefully not
steal—standard issue L-Tronics L-Pers, often in
short supply due to squadrons’ tendencies to destroy
them through rough handling, blow up the antenna
switch-box with VHF radios, or corrode the units by
letting batteries rot inside.

The L-per nonetheless remains Civil Air Patrol’s
preeminent tool for emergency-locator transmitter
(ELT) radio direction finding (DF). Recently, LTronics
came out with its new DF unit and it appears
to be a fantastic piece of gear, with all the features we
need now and the ability to add new features—such
as 406 MHz data decoders—later. It looks to become
as much a CAP standard for hunting ELTs as the old
L-per, which has worked perfectly for our purposes
for decades.

But at $750, they’re also priced out of reach for
most CAP members, even many squadrons. So I set
out to design a simple, low-cost a direction-finding
radio receiver that could be carried in a pocket yet
achieves sufficient dynamic range (the ability to differentiate
between loud and soft signals) and selectivity
to eliminate off-frequency transmissions. It
would also need to deliver clear audio and sport an
easily readable signal-strength meter. It should be
able to closely localize a signal and so determine
which particular aircraft out of hundreds on an airfield
has the activated ELT.

This is not a trivial task: The signal from an ELT,
though relatively puny in the world of radio transmitters,
can overwhelm (“swamp”) the L-per’s highly
sensitive receiver. Moreover, the L-per can be
“desensitized” by aircraft or tower transmitters using
nearby frequencies. (Seniors members might liken
this to becoming selectively deaf in noisy environments,
such as restaurants.) Most ground teams can
testify to having heard voice traffic coming through
their L-per. And this can happen intermittently,
which can be even more frustrating.

My initial experiments were dismal failures.
First I tried to build a souped-up crystal radio and it
was easily “swamped,” and couldn’t provide a suitable
signal-strength display. I realized that I needed
to use a Narrow Band FM (NBFM) receiver to
accomplish all of my design goals. I contacted one of
my suppliers in Britain, a manufacturer of radio
products for telemetry. I asked them if they could
build a radio-frequency (RF) “front-end” module for
my design. As it happens, another U.S. Government
agency (which shall remain nameless) had already
asked for modules capable of receiving the 121.5
MHz distress signal. Eureka! They sent me a few
samples, and I began to experiment.

At first, I considered building a receiver system
much like the L-per using a “Switched Antenna,
Time-Difference-of-Arrival” method (c.f. Joe

Leggio’s http://home.att.net/~jleggio/projects/rdf/
tdoa2.htm). But as I was trying to keep the size and
expense to a bare minimum, I decided to stick with a
basic signal-strength-based design, which usually
functions in one of two ways. Most commonly, a
highly directional Yagi-type antenna is connected to
a receiver and when pointed toward the source the
signal strength increases, indicating the direction of
the signal’s origination.

Unfortunately, directional antennae intended for
Civil Air Patrol ELT missions (121.5 Mhz frequency)
require elements that are about four feet
long (see Saman Seneviratne’s “In Search of the Perfect
Sticks,” Eagle Call, Winter, 2005). It is a little
ungainly. Even first-generation L-per’s induce our
mothers’ worst fear (“You’ll put your eye out!”).
What ground team member hasn’t poked his teammates
with his “sticks” while trying to get in and out
of vehicles?

Signal-strength units call for a combination of
field techniques. In “Body Shielding” the ground
team member places his body between the source of
the transmission and the receiver. This is a practice
utilized frequently with the L-per receiver. Since the
body effectively absorbs radio waves, when it is
between the transmitter (ELT) and the receiver (your
DF unit) it partially blocks the signal, allowing the
ground-team member to narrow in on the direction.
At close range, the switched antenna array is then
disconnected and a rubber duck antenna (sometimes
no antenna at all) is attached.

The second method to reinforce the bodyshielding
technique is “near field proximity,”
wherein the signal gets stronger as you get nearer the
transmitter and suddenly even stronger as you enter
the “near field” within one wavelength (with 121.5
signals, that’s about eight feet).

My design has three principal building blocks:
An RF receiver section, a signal strength section, and
an audio demodulator section. The RF section is a
narrow-band FM receiver module. Some hams may
have noted that ELT’s are amplitude modulated
(AM), but fear not. FM receivers will hear them just fine.

FM receivers work a little differently than AM
receivers. AM receivers can hear weak signals with
lots of noise in the background. That’s good in a
direction finder for detecting an ELT on the fringe.
FM receivers, in contrast, exhibit a phenomenon
called capture: They either pick up the transmission
perfectly or not at all. But since my design is
really intended for close-range detection, this is
not a problem.

The display in my design acts more like an AM
receiver. You can detect a signal using the bar graph
LED display without actually hearing it. This is especially
useful capability since a high percentage of
ELT failures result in a carrier-only signal, without
the characteristic swept tone. In the absence of a signal,
you will hear a hissing sound, atmospheric and
electronic noise. As a carrier-only signal becomes
stronger, the receiver will become “quieter,” a phenomenon
called (you guessed it) “quieting”. And,
given the urban direction finding task, it’s not
uncommon to be on airfield, surrounded by moving
propellers.

The signal-strength display
section has a total of ten LEDs in
a bar graph array. The receiver
has an 80dB dynamic range
on its RSSI (Received Signal
Strength Indicator). In theory that
means an 8dB difference in signal
between segments, assuming
good linearity. (I’m sorry to have
subjected you to that, but I knew
someone would ask.) In practice,
the first or second LED will usually
light up just from noise. The
last LED will not light until you
are almost touching the antenna—
unless of course someone is keying
up their radio on 121.5.

The demodulator section
drives a speaker or headphone,
allowing you to hear the audio
information transmitted, which is
very helpful. Hearing the swept
tone of an ELT or EPIRB allows
you to confirm an actual distress

device activation. Hearing a carrier only implies an
ELT failure, often the dying breaths of an ELT as
the battery dies. Hearing a conversation means that
someone is sitting on his aircraft band push-to-talk
switch. “Digital” sounds indicate a microprocessor
or other electronic device inadvertently emitting on
the distress frequency. (CD players, DVD players,
and a number of computer peripherals have done
this—and by now we’ve all heard about CAP’s nondistress
Find of a big-screen television.)

I designed the circuit footprint with the idea of
disguising it in an unpretentious Altoids tin, although
you can certainly put it in a nice metal project box
marked “TOP SECRET” if you wish. (In any case,
always use a metal box for radio receivers to keep
them from picking up and generating noise.) Because
it uses a standard FM receiver, it can work with
off-the-shelf radio direction finding kits such
as the Ramsey Fox or Doppler kits (see
ramseyelectronics.com).

I expect to sell my design as a finished and tested
printed circuit board. Buyers would provide the case,
an on-off switch, an antenna connector (BNC, RCA,
etcetera), a small speaker or headphone jack, doublesided
tape for mounting the PC board, and a nine-
volt battery. The PC board has a built in volume
control, and a pin-out section with connections for
ground, audio, power, and RSSI.

I’m selling the device as a kit to keep the unit
affordable. A fully assembled DF device would
require an FCC approval costing thousands of dollars
and thwarting my objective to put a cheap, effective
receiver into the hands of search-and-rescuers. Selling
the device as a kit eliminates that problem, since
experimenters can build any kind of receiver they
wish, so long as it does not cause interference. And
as the RF modules were designed for sale in the U.S.
and Great Britain, they already pass FCC muster. So
sit back, enjoy a mint, and expect to see an “Altoids
ELT Receiver” on E-bay soon.