Safety First

The accident rate for cell phone users illustrates the danger of engaging in distracting activities while driving. Hunting transmitters is inherently distracting. Therefore it is best to hunt in pairs, or even groups of three or four. The driver should concentrate only on the road and the cars around him. The duties of taking signal measurements, navigating, and strategizing should be assumed by the other members of the hunt team. When it is necessary to hunt alone, always pull off the road and stop before taking signal measurements. Then plan your route before heading out for the next "triangulation point".

Beam Antennas

Beam antennas can be oriented for vertical or horizontal polarization, or anything in between. Quads are generally easiest to make polarization-adjustable, since only the feed point on the driven element needs to be moved to effect a polarization change. Some mobile transmitter hunters employ circularly-polarized antennas, which are more complex to build, and less sensitive for receiving linearly-polarized signals, but perform predictably regardless of signal polarization angle.

Other frequently-used antennas for transmitter hunting include the log periodic dipole array (LPDA, or LPDA#2, or the "Pounder" in reference [1]) and the HB9CV. These antennas resemble the yagi, but unlike the yagi which has but one driven element, the LPDA and HB9CV have all their elements driven. Some hunters find that the all-driven element design makes these antennas less prone to having their directional patterns distorted by nearby metallic objects. The LPDA has the added advantage of being very broadbanded. The HB9CV has the advantage of being small, and light weight, having but two elements.

Beam antennas are the instrument of choice for tracking weak signals. However, for all these antennas you'll also want to have an attenuator handy, for when it becomes necessary to knock a strong signal down to usable S-meter range. Beam antennas, whether built or bought, are generally the most economical DF tool.

Doppler Direction Finders

An effective DF tool that lets you take signal readings while in motion is a Doppler direction finder. Doppler direction finders switch quickly among three or more co-located vertical antennas, and feed the resulting phase-shifted signal to a phase detector (typically an FM radio receiver). The audio from the detector is processed to derive bearing information. The bearing information is then displayed numerically, or by illuminating one LED in an "azimuth ring" comprised of 16 or 32 LEDs placed in a circle.

Dopplers can be built from scratch using a printed circuit board (or PCB #2, or PCB #3, or PCB #4), from a kit (or kit #2), and are available as fully assembled and tested commercial units. (If you build your own, be sure to check out this improved Doppler antenna switch design.) Many Dopplers either include a serial port, or can have an interface added, so that bearings can be sent to a laptop computer running APRS software. 

Dopplers are especially adept at determining bearings on signals of very short duration. Another nice thing about a Doppler direction finder is that an attenuator is not essential: the stronger the signal the better the Doppler works. When hunting with a Doppler you'll still want to pay attention to signal strength, however, because signal strength gives the best indication of how close you are getting to the hidden transmitter. So you'll still want to carry an attenuator to keep your S-meter indicating your progress all the way to transmitter.

Doppler direction finders have some drawbacks. Dopplers are generally not as sensitive as a beam antenna for tracking weak signals. Also, Doppler antennas are comprised of vertical elements, which can lead to difficulties when the fox's signals are horizontally polarized.

Amplitude-Comparison Direction Finders

Like Dopplers, amplitude-comparison direction finders allow you to take accurate bearings while in motion. Amplitude-comparison DFs examine the signals being received by three co-located antennas: two bi-directional antennas oriented perpendicular to one another, and an omnidirectional sense antenna. The bi-directional antennas are usually Adcock dipoles, Adcock monopoles, or loop antennas. (When Adcock monopoles are used, the antenna visibly resembles a Doppler antenna.) The signal strengths received from all the antennas are processed and compared to derive bearing information. The bearing is then displayed either by a numerical read-out, a mechanical pointer, or by illuminating one LED in an "azimuth ring" comprised of 16 or 32 LEDs placed in a circle.

Amplitude-comparison direction finders are more challenging to construct than a Doppler DF. At least I assume this to be the case, since I've yet to find a schematic, or detailed design for an amplitude-comparison DF. Fully assembled and tested commercial units (ACDF#2) are available. Prices for these units can be expected to exceed those of Doppler direction finders by a factor of five, or more. Prices start around $10K. (However, I know of a fellow that picked up an old maritime unit at a garage sale for a few hundred dollars!)

The above manufacturers of amplitude-comparison DFs claim that they are equal to Dopplers when it comes to tracking signals of short duration, and offer superior sensitivity. But due to their antennas being comprised of vertical elements, most amplitude-comparison DFs share the Doppler's vulnerability to horizontally-polarized signals.

Summary

There is a variety of suitable commercial and homebrew equipment for mobile transmitter hunting. Whether you build a yagi for $50, or purchase a commercial $2000 Doppler direction finder, there is no infallible transmitter hunting equipment. The most important factor that will determine your success is experience. So, whatever you use, use it often!