Amateur Radio Direction Finding (ARDF)
Robert Vickers G3ORI offers an introduction to a Radio Sport with a difference.
Robert Vickers G3ORI offers an introduction to a Radio Sport with a difference.
The chances are you have heard of ARDF but what sort of picture does that conjure up? Quite possibly it’s something that your local club does a few times a year just for its own members, with a single ‘hidden fox’ to be found and mostly using 2m band handhelds pressed into service for both the fox and the hunters. Or maybe you think of direction finding on Topband (160m), continuing a tradition that has been established in the UK for over 80 years. However, you might not think of the ARDF contests that are conducted according to a standard set of rules and formats, adopted by the IARU (International Amateur Radio Union) and organised in many countries around the world. There is a group of enthusiasts in the UK who not only organise such contests in the UK several times a year but also make ‘holiday’ trips overseas and send a team to compete against other countries each year for the Region 1 (effectively European) or World Championships. In the last few years, a number of prestigious medals, including Golds, have been brought proudly back home by UK radio amateurs.
The Classic Format
The formats of these ARDF contests are rather different to those of the local UK events mentioned above. In recent years they have diversified but they are defined by a number of factors. The bands used are 2m and 80m, and the participants take part entirely on foot. This limits the range covered and each contest is contained within the area defined by a single, very detailed map – usually obtained through a local orienteering club and typically using a scale of between 1:15,000 and 1:4,000. The transmitters operate unattended (although in major competitions there may be referees located nearby). There are a number of ‘foxes’ to be found – usually five although more are deployed in certain events. There is an underlying ethos of ‘fairness in accordance with the rules’ so it is important that they are found by direction finding skill rather than luck, which dictates that it is not a game of ‘hide and seek’. This, however, does not detract from the main reason for taking part in these events – they are jolly good fun!
In the Classic format, there are five ‘foxes’ situated in the competition terrain, which might be a sizeable area of forest. The foxes are low-power transmitters with non-directional antennas. While the actual equipment may be concealed to protect it from unwanted interference, it is accompanied by a distinctive orange and white marker ‘kite’ and a means for the competitors to register their visit. Nowadays, this is normally accomplished with an electronic recording device, in conjunction with a ‘chip’ carried by the competitor.
All foxes transmit on the same frequency but operate in a five-minute cycle. So, the first fox comes on the air at the precise start of each five-minute period, transmits for one minute and then shuts down. At this point, number two starts up and sends for one minute and so on. Each identifies itself in Morse code with the letters MO followed by a number of dots, so the first sends MOE (one dot) and the subsequent ones send MOI, MOS, MOH and MO5 (which have 2, 3, 4 and 5 dots respectively). With this arrangement, the letters MO provide five dashes to facilitate taking a bearing and by counting the number of dots, the identity of the fox can be ascertained without the need for knowledge of Morse code.
Individuals are allowed to start at five-minute intervals so that everybody starts at the same point in the cycle. The objective is to find as many foxes as possible within an overall time limit – usually around two hours. The optimum distance from start to finish via all five foxes might be six or seven kilometres but those in older or very young age classes are not expected to attempt this. Instead, shorter courses can be arranged by specifying that only four, or three particular foxes need to be hunted.
The Competitor’s Strategy
Since the competitor starts with a blank map indicating only the start and finish locations, and the foxes can be hunted in any order, it becomes clear that the most important thing to do is to try to establish the approximate positions of those required, so that an overall plan of campaign can be drawn up. Competitors don’t want to waste time and effort going back and forth across the area simply because they failed to work out the optimum route at an early stage!
There are rules that state that there are no foxes within 750m of the start, nor within 400m of each other or the finish. A novice competitor might wish to devote the first five-minute cycle to taking bearings (and noting signal strengths) of all foxes and plotting these on the map before deciding on the best direction to set off. A seasoned expert will probably take into account the size and shape of the area and quickly head off, taking bearings as each fox comes on the air. Whatever the approach, an early decision is required to determine which fox to head for first. However, it is important to constantly review the bearings and signal strengths of all foxes, since with experience you soon realise that the strategy often needs to be reviewed in the light of circumstances.
At this point, it is worth mentioning that if the band in use is 80m, the bearings can be relied on to be accurate and can therefore be plotted on the map with some confidence. On 2m, however, the situation is very different since the signals are subject to multiple reflections, especially in hilly terrain, and are also attenuated by hills. It is important to use the map to appreciate the terrain and make adjustments. Not only can reflections be deceptive but a fox nearby in the next valley may appear much weaker than a distant one on a hilltop. It is the ability to take these factors into account that constitutes the most important challenge for the competitor – a real ‘radio’ skill!
The Equipment Used
The transmitters deployed for the foxes are basically simple in concept, portable and constructed to survive outdoors in all weathers, although it is customary to wrap them in a bin liner, or similar, to keep the rain out and to make them less obvious to mischievous prying eyes. For 80m, the signals are straightforward CW (A1A) and for Classic events the power is 1 to 5W delivered to an 8m vertical wire, up a convenient tree. A counterpoise wire of similar length is also laid out. An extendable pole is often used to get the wire up the tree and it is convenient to attach a semi-flexible hook to the top so that it can easily be pulled down at the end of the event.
Events on 2m normally run up to 1W ERP, and a variety of antennas are used. Probably most typical are crossed dipoles, mounted at a height of 2 to 3m. The signal is CW (A2A), amplitude modulated with an audio tone.
The more interesting aspect of the transmitters is the programming of the operation. PICs are used to set the timing and the identity of each transmitter. They must be synchronised such that each one starts sending precisely at its allocated slot in the five-minute cycle. Usually, a delay is pre-set so that they can be set out several hours in advance of coming on the air for the start of the competition.
Several enthusiasts have designed receivers for use in ARDF. From time to time, some have been produced in kit form, as well as fully constructed. Like the transmitters, most are fairly straightforward designs but built to cope with the outdoor conditions in which they will be used. An appropriate antenna is incorporated into the receiver. For 2m, the Yagi design is very popular but HB9CV antennas are also found. Opinion is divided about the choice of antenna for 80m, as ferrite rods and loops are both deployed. A vertical ‘Sense’ antenna is also needed for 80m. A compass is required for navigation and for taking bearings, and this is often attached to the receiver.
Having found all the required foxes (or as many as possible within the time limit), the competitor heads for the finish, which is pre-marked on the map. Usually, there is a ‘Beacon’ sending MO continuously to help in collecting competitors into the finish area, which is situated a short distance from the finish line itself. The route from beacon to finish line is marked as a taped funnel. Having registered at the finish, all that remains is to relish the applause of the spectators and to download the timing chip into the computer, which finally prints out the competitor’s results. And then, of course, a lively discussion about how it went, followed by some well-earned refreshment and, finally, the prizegiving!
Is that All?
You might be asking at this point, “I've got the picture but is that all there is to it?” Most definitely, no! This article can only give a brief description of this fascinating sport. There is a lot more information at:
and by following the links. Even better, if you can get along to one of the events listed under the ‘Events’ link, you will find a friendly welcome and loan receivers can be provided for beginners (but please contact the organiser beforehand to earmark one for you). A new dimension of radio awaits you in the forest!
This article was featured in the July 2018 issue of Practical Wireless