The HANDI-Finder^® is a HANdheld DIrection Finder antenna retrofit attachment 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.

Thus it also works in receiving and DFing AM Aircraft Transmissions. It has been designed for bare-bone functionality as well as low power consumption, simplicity, and economy!

Overall, it is a basic, inexpensive way to explore the concept of electronic direction finding, and provides something relatively compact to keep readily available for locating sources of interference.  It provides a convenient basis for further experimentation by those who want to try more elaborate antenna arrays.

Soldering, general assembly skills, and patience ARE required, but it makes a great club project for the basis of also teaching those valuable skills!

Fabrication can be straight-forward, but takes time.   Overall, expect the process of unpacking, checking the parts, and assembling the basic board with care, to take about two hours.   The subsequent steps can easily take longer than one might expect.  Two open-loop antennas (not supplied) are made from coat hanger wire or welding/brazing rod, bent into halves of a "bow-tie" shape and mounted directly to the foil side circuit board, as illustrated in the "layout" drawing.

The 50-Ohm coax downlead (not supplied) 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.  --What IS a problem is "multipath",  --multiple reflections of the original-source signal off structures and terrain!

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.

Finally, a handle (not supplied) must be fabricated and added. This can be anything from a wooden paint stirrer to a nicely molded paint roller handle, modified to attach to the flat circuit board, as partially seen in the photos.

"Build the HANDI-Finder" appeared as an article by R. A. Leskovec K8DTS, 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.  If you are unable to buy the kit, you can emulate 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! This 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
Tel: 330-225-7373, Rick, K8SCI   www.noard.com


Click here for prices and order info.
Sales are limited to the US and Canada only.

Unfortunately, due to outrageous shipping costs,
we have to require an ADDED charge for Canada.

Only complete packaged kits may be ordered.

Problems with defective or missing parts
are promptly addressed.