The HANDI-Finder® is a HANdheld DIrection Finder
which can be used to localize both AM and FM carrier-based sources
using a single connection to the antenna input of an FM receiver tuned to the
frequency of interest in the range of 45 to 450 MHz.
It has been designed for bare-bone functionality as
well as low power consumption, simplicity, and economy!
Because it is both an easy-to-build kit for the beginner
and a convenient basis for further experimentation by those with more experience,
it makes a great club project!
Overall, it is a quick, inexpensive way to implement
the concept of direction finding, and provides something
relatively compact to keep readily available for locating
sources of interference.
Except for adding a handle, fabrication is minimal.
Two open-loop antennas are made from coat hanger wire bent into halves of a "bow-tie"
shape and mounted directly to the circuit board, as illustrated in the "layout" drawing.
The coax downlead is connected to an FM receiver and detects
the carrier regardless of whether the transmission mode is AM or FM.
The unit is switched on (UP) and rotated for a null in the audio tone
that it adds to the audio coming out the receiver.
The signal direction is perpendicular to the
plane of the antennas. There is 180-degree ambiguity,
but this is not a problem in actual use because
multiple "bearings" must be taken anyway
in order to establish a "fix" on the location.
Once close in, the operator works toward the general
sense of the direction, and again the ambiguity is
not a problem.
The 3-position switch is moved DOWN to stop the tone
but still keeps the antenna activated for monitoring and standby purposes,
--or it is moved to the CENTER
position to shut off the unit. Refer to the extensive discussion
in the manual which describes construction variations, and the concepts behind the design.
"Build the HANDI-Finder" appeared as an article in QST Magazine, May, 1993.
All the information in that article and much more is included here!
By clicking on the appropriate boxes in this web page,
all the drawings and text that come with the kit can be printed out or downloaded from
PDF files. You can copy the layout and build one on perfboard! All parts
are available from sources like Digi-Key and Mouser.
It should be noted that this is an experimenter's kit that starts out simple, but can get more complex
as one expands on the principle to get better performance. Generically, this is known as a TDOA,
Time-Difference-Of-Arrival detector. Time/phase-difference detectors
of this sort are susceptable
to multipath, which is often overwhelming! The simple configuration can be quite acceptable on
lakes or flat open land. However operation
in the vicinity of hills, buildings, or dense forest can be quite problematic due to multipath.
Here, a combination of techniques such as using the circuit in combination with two spaced beam antennas
can be much more effective, but also much more bulky! Also note that since this scheme detects a carrier-based
signal source, it cannot be used to localize random noise sources!
HANDI-Finder® Experimenter's Kits
may be obtained by contacting:
Northern Ohio Amateur Radio Development
P. O. Box 271
Brunswick, OH 44212-0271 USA
for prices and order info
Only complete, packaged kits may be ordered.
Problems with defective or missing parts are promptly addressed.