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.

Receivers

Several factors are important in selecting a receiver for ARDF. The ideal receiver would:

1. Have a directional antenna;
2. Have a signal strength indicator;
3. Have a wide-range RF gain control, so it is able to take bearings close to a transmitter;
4. Be convenient to carry by hand;
5. Allow a person to take bearings while walking (or running) without looking at the receiver;
6. 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 control.

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:

1. Use a metal case;
2. Use as little audio gain as is practical;
3. Minimize common signal paths (especially between the audio output stage and the earlier stages in the receiver;) and
4. 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 case.

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.

ARDF Transmitters

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.