Getting Started in ARDF

Taking Bearings

de Dale Hunt WB6BYU

It is good to learn to take bearings with your receiver before you go out on your first hunt. Bearings are directions, in azimuth, from a specific location. By taking bearings to a particular radio transmitter from multiple locations, you can get a much better idea about where the transmitter is located. Taking good bearings is one hte most important fundamental skills in ARDF. Each receiver is a little bit different, but there are two basic approaches to taking bearings.

Most two meter receivers will be attached to a multi-element directional antenna. The most popular antenna designs are yagis, but log periodic dipole arrays and and phased dipoles are also used. These antennas are more sensistive in one direction than the others. Holding the antenna in front of you, rotate yourself, keeping the antenna in the horizontal plane. When the receiver indicates the strongest signal strength, the antenna is pointing in the direction to the transmitter. Alternatively, you can rotate until the receiver indicates the weakest signal strength, in which case the antenna will be pointing in the opposite direction of the transmitter. Depending on the equipment you are using, signal strength may be indicated by a meter on the receiver, by a change in an audio tone you hear in your headphones, or simply by the received signal getting louder.

The signal strength indicator on a receiver usually only works over a limited range of signal strengths. If a signal is too weak, the signal strength indicator may be unable to register anything in any direction. If the signal is too strong, the signal strength indicator may indicate its maximum in every direction. For this reason, ARDF receivers have a receiver gain control that can be adjusted so that the received signal is in a useful range for the type of indicator used. Advanced ARDF receivers have automatic gain control circuitry.

Start by listening to the desired signal on your receiver. Make sure it really is the signal you want to find! Try adjusting the gain control: the signal should be very weak with the control one direction, and stronger (possibly much too strong) with the control all the way the other direction. Adjust the gain control so the received signal is a comfortable listening level in the receiver. Now turn slowly in a circle with the antenna. The signal strength should peak in one direction and be weaker in the other directions. (You may need to readjust the gain while circling.) It is good practice to stop and turn all the way around in a circle while you are learning to take bearings to make sure you are really listening to the strongest peak in the antenna pattern.

Most eighty meter receivers will require an entirely different method for taking bearings. Instead of looking for the strongest signal, you will be looking for a direction where the received signal is as weak as possible. Again, tune in the signal and adjust the gain for a comfortable level. Now turn in a circle: there will be one or two points where the received signal is much weaker. This is the point you are looking for. The null can be very sharp, and it takes some practice to find it, especially when the signal is keyed on and off.

A magnetic loop antenna or a ferrite rod antenna will have two sharp nulls in the pattern, in opposite directions. We you have found a sharp null in received signal strength, you know that the transmitter must be in one of two directions. We must know which of these two directions is the direction to the transmitter, or we will go in the wrong direction. Most eighty meter receivers have a small vertical antenna called a "sense antenna" that is used to change the pattern. When the sense antenna is active, the antenna pattern will have only one null, and a peak in the opposite direction. The null (and the peak) are not as sharp as those without the sense antenna, but some people find they are good enough for hunting, and they always use the sense antenna. In this case, you can take a bearing using either the peak or the null, as long as you know which is which.

The common method of taking bearing uses both patterns. First, find the null direction using the bi-directional pattern. Then, turn the receiver sideways and enable the sense antenna (usually, this means holding down a momentary-contact switch button.) Consider how loud the received signal is, and then turn the receiver 180 degrees, and notice whether the signal strength gets much stronger or much weaker. With the sense antenna enabled, the receiver will be much more sensitive in one direction than the other (and this direction is usually marked on the receiver or the magnetic loop antenna.) When the direction of strongest signal strength matches the direction of greatest receiver sensitivity, then that is the direction in which the transmitter is located.

Once the correct direction is found, you can walk towards the transmitter using just the sharp bidirectional null. Because the nulls in the bi-directional pattern are sharper, you can get more accurate bearings. The risk you run not using the sense antenna is that you might run right past the transmitter while it is not transmitting, and not realize that the transmitter you think is directly in front of you is really directly behind you. Using the sense antenna once in a while can help you check to make sure you are still going in the correct direction.