|By far the most popular mobile DF setup
is the rotatable yagi or quad antenna. These beam antennas usually have
or four elements.
By running the mast through a window, or sunroof, the antenna can be rotated
from the dry, comfortable, interior of the car.
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 the "Pounder" in reference ) and the
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
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
circuit board (or PCB
#2, or PCB
#3, or PCB
#4), from a 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
design.) Many Dopplers either include a serial port, or can have an
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
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
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.
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!