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An introduction to Amateur Radio Direction Finding

Out and about with a directional antenna

Amateur radio direction finding (ARDF), foxhunting or hidden transmitter hunting is a fun activity where people compete to be the first to find a hidden radio transmitter. They do this by using receivers with directional antennas to hone in on the transmitted signal.

Apart from the excitement of the hunt itself, those who like making small receivers and directional antennas will enjoy it for the challenge of building equipment that is rugged, reliable and does not give false readings. Nevertheless, constructional ability is not required to enjoy the activity - many beginners use hand-held VHF transceivers or scanners to receive signals from the 'fox' transmitter.

Regular foxhunts are held by local radio clubs or foxhunting groups. Participants may go on their own or be part of a team. Hunts are conducted either on foot or in vehicles. An amateur transmitting licence is not necessary to participate.

A variant of foxhunting is Amateur Radio Direction Finding (ARDF). This is a rapidly growing international sport and calls for a degree of physical fitness not possessed by many amateurs. Details on ARDF are presented elsewhere in this article.

Bands to use

Most foxhunts use the two metre (144 MHz) band. The national ARDF frequency is 145.300 MHz, though some groups still use other frequencies. There is also some ARDF activity on 3.5 MHz. Low power transmitting and receiving equipment for this band is very easy to build. Compact directional receiving antennas for 3.5 MHz are also interesting projects. Because most amateurs already own a portable VHF scanner or transceiver, this article concentrates on foxhunting on the two metre band.

Rules

Except for ARDF, which is an international sport (see end of article), local foxhunt groups set their own rules. These may include things like driving carefully and requiring that the transmitter be within a certain distance of the starting point. Other rules are fairly informal.

The person setting the fox goes off and hides the transmitter. Meanwhile, participants ('hounds') gather at the starting point. They may monitor a local repeater for liaison purposes. When the transmitter has been hidden, the fox setter switches it on and announces that the fox is transmitting and that the hunt has started. Hounds first need to know which direction to travel. They madly swing their beams around until they can get a bearing on the fox's signal. They may then consult a map and start heading in the direction of the signal.

The first individual or team that finds the fox is the winner. Those who have seen the fox transmitter walk away from it to avoid giving clues to following teams. The transmitter is turned off when the last hound finds the fox or announces on the liaison frequency that they have given up. The winning station or team is then entitled to set the next fox. Either another hunt is run or participants may socialise at a participant's house or cafe.

The 'Fox'

The transmitter used in the foxhunt must be compact and rugged. Its frequency should be stable (crystal control is ideal) and be able to run for several hours off a nicad or sealed lead acid battery. RF output powers as low as 20 milliwatts are satisfactory for pedestrian-based events of a few hundred metres. Higher powers (1 to 5 watts) are better for longer hunts. A fox transmitter with two or three RF output power settings is desirable to save power and/or fool the competitors.

Because many people will be using FM receivers, it is desirable that the fox's signal is frequency modulated with an audio tone. This tone can be keyed to transmit a Morse ID. Programmable ICs are often used to send the Morse. However, 20 second digital message recorders are so cheap nowadays that these are the logical choice for those wishing to build a Morse or voice ID for a transmitter.

Antennas for fox transmitters can be almost anything. A quarter wavelength piece of wire is recommended for beginners. However more experienced groups have used fences, bridges or sheds as antennas. The use of directional beam antennas can also be worthwhile. This is because they can fool competitors into thinking that they are very close to the hidden transmitter. Also competitors can be given misleading bearings by orienting the beam so that it bounces the signal off a large building or hill some distance away. Other interesting effects can be had by experimenting with the antenna's polarisation. Effort should be made to camouflage the antenna and feedline to make finding the fox harder. For example, a tree branch and fencing wire can be made into a yagi antenna that is almost invisible when concealed in a tree. Similarly, a wire antenna could be dunked in a lake or river.

Receiving equipment

This is a matter for the individual competitor. The equipment used depends on whether the hunt is vehicle-based or pedestrian-based.

Competitors in vehicle-based hunts typically have some sort of steerable antenna mounted on the car. Some keen hunters have bored a hole through the roof of their vehicle to allow for a rotatable pole for the antenna. Others use an antenna on the roof rack or a vertical piece of dowelling protruding through a passenger window. This last suggestion is preferred for those without beam heading indicators installed for reasons explained later.

A two or three element quad or yagi is the most common choice for competitors. This should be optimised for maximum front-to-back ratio rather than forward gain. A sharp null off the back or side can be very useful in direction finding.

It is important to know the direction that the car-mounted beam is heading. Some people use remote-control motors and indicators. However, this is not necessary for the beginner. A simple approach that works well is to have a nail knocked in to the side of the antenna support dowel that faces the direction at which the antenna is aimed. This method can course only be used where the antenna support dowel protrudes through the passenger window.

Inside the vehicle is a switchable RF attenuator. This is used when the signal from the fox is very strong but you still need to get a bearing. Descriptions of suitable attenuators appear in the standard handbooks. Good quality construction is important to reduce signal leakage.

Lastly there is the receiver. This should have an s-meter so that it is possible to get an indication of the strength of the received signal. SSB receive capability may also be desirable. A reasonably small multimode two metre transceiver (eg Yaesu FT290R) is ideal for this application. An FT-817 would also be suitable though watch its high receive current consumption. Alternatively, a home-made receiver with a variable tone output to indicate received signal strength could be used instead.

The equipment mentioned above is of course the ideal. However, do not be put off if all you have is an FM handheld transceiver - foxhunts have been won by stations using these as the receiver. Tuning off frequency is sometimes a useful technique to effectively attenuate the received signal.

In many cases, vehicles cannot be parked close to the fox's hiding spot. Alternatively, signals may be so strong as to render the vehicle-mounted direction-finding system ineffective. The solution is to use a hand-held 'snoop loop'. This consists of a hand-carried two or three element yagi, attenuator and simple receiver. This may either be a handheld transceiver, portable multimode transceiver, or home-made receiver. Especially important is effective shielding to prevent leakage into the receiver other than through the antenna connection.

Close up of bidirectional loop antenna Bag with transceiver

Pedestrian hunters are limited by the size and weight of equipment that can be carried, especially if the walk will be several kilometres. A compact multi-mode transceiver or homebrew receiver, attenuator, two-element yagi and map are all desirable for the pedestrian hunter. If the attenuator is built properly and the transceiver is well-shielded, such equipment can be used to locate the transmitter to within a metre.

Often the last hundred metres of a fox hunt can take much longer than travelling the several kilometres required to reach the general vicinity of the transmitter. This is particularly so if the transmitter, feedline and antenna are well hidden and signals are strong. Effective triangulation of the location of the fox (including searching up and down, using horizontal and vertical polarisation and careful observation) is important here. It is quite possible for a team to be the first to reach the general area but squander this advantage to later arrivals by having poor equipment and/or poor powers of observation. Always remember that the signal radiates from the antenna and not the fox transmitter. Thus all bearings will be towards the antenna. For this reason, the antenna is often the first part of the transmitting equipment located and you will need to follow the feedline along to find the transmitter itself.

Further guidance on direction finding equipment and techniques is given in the favourably reviewed books below:

 

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Hiding spots

To many, finding novel and unusual hiding spots is the best part of foxhunting. There is a peculiar pleasure in hiding a transmitter that takes other people several hours to find. If you join a foxhunting group or team, you will hear many anecdotes about past hunts where transmitters were hidden in strange places. The following are a few ideas for those whose turn it is to hide the transmitter.

* Buried underground (use a fence as an antenna)
* Under a bridge or underpass
* On a peninsula, near (or in) the water
* Under a skateboard (preferably in use)
* Near a sewage outflow
* In a bus or train carriage (not necessarily stationary!)
* Up a tree
* In a rubbish bin
* On a hill without vehicle access
* Shopping trolley in/near shopping centre
* Near a police station
* Close to a pager transmitter or broadcast station
* Inside a hollow log

Where possible foxhunts should be held where there are concentrations of people. Examples include main streets, shopping and restaurant areas. The reason for this is to increase the visibility of amateur radio and foxhunting in the general community. Pedestrian foxhunters are normally in a better position to answer questions from the public than those in vehicles.

The sport of Amateur Radio Direction Finding

Amateur Radio Direction Finding or ARDF is a form of orienteering which was started in 1933 by the Swiss Army. Since then it has become very popular throughout the world. International competitions are held every year, mostly in Europe, and a World Championship every two years.

International competitions are held over a 4 to 7 km course. A total of five transmitters are to be found within a set time period of about 120 - 140 minutes. The competitor with the fastest recorded time is declared the winner, provided all transmitters are located.

The only assistance given is a detailed map of the area with the start and finish only marked on the map. A compass is a necessary piece of equipment. For a team event the times of the members of the team are added together and once again the lowest time would be the winner.

ARDF requires competitors to have reliable equipment, be physically fit, be able to interpret beam headings and read maps. It combines electronic, map reading and physical skills in the one activity.

All the transmitters are on the same frequency but do not transmit all the time. Instead they are switched sequentially so that only one transmitter is on air at any time. Each transmitter comes on for one minute every five minutes. Transmitters send a simple Morse code signal so that competitors can identify each one.

At each transmitter there is a punch which is used to mark a card the competitor carries to show that the transmitters have been found. Transmitters can be found in any order.

There are several categories for the competitors. These are:

* Open Category
* Women's Category
* Junior Category (under 19 years)
* Old Timer Category (over 40 years)

Every five minutes a group of competitors start one from each of the categories above. Each category is required to find a different number of transmitters, so that following someone is not necessarily a good idea!

Two Amateur bands, 3.5 MHz and 144 MHz, are used. Receiving and transmitting equipment is readily available, however the transmitters must be controlled by a licensed amateur radio operator.

Abridged from material supplied by Wally Watkins VK4DO

Simpler approaches

The above has covered amateur radio direction finding on amateur frequencies with amateur gear. However short range hunts can be done with small FM transmitters ('bugs') and receivers, eg as a fun activity for youth. Many mobile phones have an FM receiver function which could potentially be used as the receiver. The video below demonstrates a 'hunt' over a few hundred metres of beach.

 

Acknowledgments

I would like to thank Wally Watkins VK4DO (SK), Ron Graham VK4BRG (SK) and Neil Pickford VK1KNP for their assistance in the preparation of this article.

An earlier version of this article appeared in Amateur Radio August 1998 with updates made since.

 

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