ARDF Equipment Design Considerations
de Dale Hunt WB6BYU
The basic electronic equipment needed for ARDF includes one or more
transmitters to locate, and a hand-held direction finding receiver for
each participant. ARDF equipment does not need to be very expensive or
complicated. Many ARDF enthusiasts begin with simple equipment
when they first get interested, then improve their equipment later.
Several factors are important in selecting a receiver for ARDF. The
ideal receiver would:
- Have a directional antenna;
- Have a signal strength indicator;
- Have a wide-range RF gain control, so it is able to take bearings
close to a transmitter;
- Be convenient to carry by hand;
- Allow a person to take bearings while walking (or running) without
looking at the receiver;
- and not interfere with other receivers on the same frequency.
In many cases, a general-purpose receiver can be used with an external,
directional antenna and an attenuator, although an integrated ARDF receiver
is usually more convenient.
Almost all ARDF receivers are designed to work
with headphones so other competitors nearby in the woods can not hear your
receiver. A voltage-controlled audio oscillator is commonly used for an audio
"S"-meter to indicate the received signal strength by the tone pitch. The
RF gain control knob gets the most use, so it is mounted in a convenient
place, often where it can be operated by a finger or thumb of the hand that
holds the receiver. Power consumption should be low, to maximize battery
life. Usually, the receiver and directional antenna are attached together
to make a single piece of hand-held equipment.
80 Meter ARDF Receivers
Most simple CW or SSB receiver circuits for 80 meters will work for ARDF.
Because the output from a loop or ferrite rod antenna is very low, an RF
amplifier stage is recommended. Controlling the gain of this stage may give
enough attenuation, but in some cases an additional stage must be controlled
as well. Dual-gate MOSFETs or cascade stages are convenient for gain
The simplest receivers are direct-conversion, consisting of an RF amplifier,
VFO, mixer, and audio amplifier. Simple receivers for practice or beginners
can use a crystal oscillator that matches the transmitter frequency.
Although direct-conversion receivers are easy to build, and are capable of
good performance, they can have problems because the receiver will pick up
the local oscillator signal. For best results, it is best to:
- Use a metal case;
- Use as little audio gain as is practical;
- Minimize common signal paths (especially between the audio
output stage and the earlier stages in the receiver;) and
- Use balanced mixer stages and cascode amplifiers to reduce the
oscillator signal radiated from the loop antenna.
There are several small commercial shortwave receivers that cover the
80 meter band. Some shortwave receivers use a ferrite rod antenna for
the lower frequencies, and this may work for ARDF with the addition of
a sense antenna. If the receiver accepts a coax antenna input, it may
be usable by building a loop antenna for it. In either case, the AGC
(automatic gain control) on the receiver may need to be turned off in
order to hear the nulls in the pattern. The receiver also may overload
when close to the transmitter - if this is a problem, the antenna can
include a mixer circuit to allow the receiver to be used on a different
frequency from the transmitter. Such circuits are described in the
section on two meter receivers.
80 Meter Antennas
Because of their small size, loop antennas are used for 80 meter ARDF
receivers. There are two types: an air-core loop, usually 10cm to 30cm
in diameter, or a coil wound on a ferrite rod (perhaps 1cm in diameter,
and 5 to 15cm long, like those used in pocket AM broadcast band radios.)
The primary winding is tuned to resonance with a variable capacitor, and
a second winding with fewer turns matches the low impedance input of the
receiver. Such a loop antenna is bi-directional, with sharp nulls along
the axis of the coil. When the coil is oriented for minimum signal
strength, the axis of the coil points to the transmitter. An additional
sense antenna is added to resolve the bi-directional ambiguity.
Two Meter ARDF Receivers
Many people start ARDF using a two meter FM hand-held receiver. This
works when the signal is weak, but most FM receivers have limited
dynamic range, and it is difficult to take bearings on stronger signals,
because the S-meter may read full scale in all directions. Using an
active attenuator between the antenna and the receiver avoids
this problem. An active attenuator allows the signal from the antenna
to be attenuated until it is in the usable range of the S-meter, while
converting the signal to a different frequency in the band, so the
receiver does not pick up the transmitter signal directly through the
Receivers designed for two meter ARDF usually have an AM detector, which
is designed to recognize differences in signal strength (unlike an FM
demodulator). Common circuits use single or double conversion and a
wide-range gain control. The detector output can drive a voltage-controlled
oscillator, which gives a better indication than listening for the
strongest signal by ear. For convenience, the receiver usually is mounted
on the boom of the antenna, or even built inside the boom.
The selectivity required will depend to a large extent on how heavily used
the two meter band is in the local area. When there are many strong
repeaters, DX Clusters, and other signals, the selectivity may need to be
15 kHz, with no in-band spurious responses worse than -60 dB from the
desired signal. Where there are fewer signals in the band, a wider
bandwidth or poorer image response may be acceptable.
For accurate bearings, especially on strong signals, the receiver should
be well shielded.
Two Meter Antennas
The most popular directional antenna for two meter ARDF is a three-element
yagi. Because ARDF often requires walking or running through forest
undergrowth, rigid elements are often damaged. The best elements are
stiff enough to take good bearings, but will bend out of the way when
they hit a tree branch, then return to their original shape. Some
types of spring steel, or plastic tubes or rods with conductive wires
attached to them, will do this, but the most common material to use is
a steel tape measure. The elements are cut from a metal tape measure
blade, with extra sections in the center for stiffening. A five meter
long tape measure should make one three-element yagi.
Other popular antennas are the two-element HB9CV design phased array,
which is capable of good front-to-back ratio, or yagis of two to five
elements. The larger the antenna, the sharper the bearings are, but
also the more cumbersome it is to use in the forest. This is a matter
of personal preference. The most important antenna characteristic is
the pattern - there should be no lobes that are within 15 dB from the
main lobe to avoid confusion. Using a balun at the feedpoint will help
to prevent pattern shift due to pickup on the coax cable to the receiver.
An ARDF transmitter can be operated by hand during a competition, but it
is more convenient to use an automatic controller to turn it on and off
at the proper times. Usually, the transmitter, controller, and battery
are assembled together in a box with an external antenna. This makes
it easy to set up the transmitter in the woods.
Transmitter antennas should be omni-directional. On 80 meters, this is
commonly a vertical wire antenna with ground radials. On two meters,
with horizontal polarization (standard for international competitions)
a turnstile or halo is used, but a simple dipole with the ends bent is
adequate for less formal competitions. A quarter wave whip attached to
the transmitter is sufficient when using vertical polarization.