When you first decide to join in the fascinating pursuit of Amateur Radio Direction Finding (ARDF) you may be confused by the array of equipment and information available, both in magazines and on the Web.
There is a lot of jargon bandied about, and quite a bit secrecy maintained by those who wish to keep a competitive edge over their fellow hunters.
However there is a lot of fun to be had using simple equipment, most of which can be home constructed in a few evenings.
Building your own simple set-up can give you a sense of pride as well as being an educational exercise.
ARDF is a sport for all ages and technical abilities; it is a test of hunter and machine against the 'wily fox'.
I hope the information contained within this section will help you understand the basic concepts of transmitter hunting, give guidance on how to build some simple hunting kit, but mainly, prompt you to 'have a go'!
Remember you don't have to be a radio Amateur to go transmitter hunting, there are no exams or licence fees… just lots of fun!
Concepts:
Transmitter hunting is the sport of homing in on a hidden transmitter known as the 'T' (turkey) or 'fox'.
Radio 'fox hunts' usually take the form of a competition as to who can find the hidden transmitter first. There are often points awarded for 1st 2nd 3rd places etc. with a prize or trophy to be had at the end of the season.
ARDF isn't all about fierce competition though; transmitter hunting combines the skills of navigation, observation and perseverance. Building equipment, honing your navigational skills and developing a hunting technique can be a satisfying pastime in itself.
Radio foxhunts are organised such that the 'fox' transmits for a set period of time interrupted with longer gaps.
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A typical scenario would be for the 'fox' to transmit for 1 minute in every 5, synchronising his transmissions to an agreed standard. In this way, hunters using simple equipment can predict when a transmission is about to start and be out of their vehicle ready to DF. |
The silent 4 minute period between the transmissions is used to travel to a new location ready to take the next bearing. Using a map, a compass and triangulation, a picture will build up in the hunters mind as to the location of the transmitter.
The 'Fox' is usually a radio amateur with his transmitter, hiding somewhere within the designated hunting area. He will have chosen a 'den' with good ground cover to hide himself physically, and surrounding terrain to cause his signals to be confusing to the hunters.
Taking notice of your surroundings and the way the radio signal is affected by the terrain, will provide essential information toward the development of your hunting technique.
Even using simple equipment you will be in with a chance of finding the 'fox', the main obstacle to novice hunters is lack of determination.
Minimum equipment:
For a basic radio 'foxhunt' you will need a suitable portable radio receiver, a home-made directional aerial and a map.
A friendly word of advice:
It is far better to start with some simple equipment and then progress to a more elaborate set-up as you gain experience. Most of the top hunters have evolved along these lines rather than jumping in at the deep end.
Ask hunters how their system performs and try to see how it would suit your hunting style. When everyone meets up at the finish, you can gain some good information on equipment performance and operating techniques if you keep your ears open.
If you are thinking of a hi-tech solution, the last thing you want to do is go out and buy a commercial hunting system and then find that it is user unfriendly. Bide your time, do your market research and then decide, it could save you some heartache!
Radio:
Most local radio hunts take place on the 2 Metre Amateur band around 145MHz FM. You could use an Amateur band handheld transceiver, or more likely, a general purpose 'radio scanner' tuned to the transmitter's frequency.
It is not very cost effective to build radios for this frequency, particularly as there is a good trade in cheap scanners available. If at all possible, get a radio with some form of signal strength indicator (bargraph or meter). Being able to determine the relative strength of the signal not only gives you information about direction, but how close you are getting too, especially when used with an attenuator.
Once you have bought or borrowed a suitable radio you will then need some form of directional aerial.
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Aerial:
Why all this talk about a directional aerial? My scanner comes with a nice aerial already fitted!
The problem with the springy 'rubber duck' or telescopic aerials supplied with radios is that they are omni-directional. i.e. They will receive signals from all directions at the same time.
Transmitter hunting requires that we somehow screen or shield the aerial such that it only receives signals from one direction. Then, holding the radio and rotating yourself slowly through 360 degrees, you can detect the direction of the strongest signal.
It's bit like looking down a pair of binoculars; your field of view is narrowed and yet enhanced in that one direction.
To demonstrate this, try folding the aerial on your FM transistor radio along the top of the set and tune it to a weak broadcast station. Now rotate the set and you should find a position where the signal is strong and a position where it is very weak. These positions will be at right angles to one another.
The simplest way to restrict the reception area of your radio is to place it next to your body, rotate yourself 'on the spot' and listen to the strong and weak signals. The weakest signal should be received when the hidden transmitter is directly behind you.
"For you are shielding the set".
A simple directional aerial is described in the simple equipment section. This aerial (like the binoculars mentioned earlier) actually magnifies the signal, helping your radio to 'hear' distant signals more easily.
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What about the situation where the signal is too strong?
As you approach the area where the transmitter is hidden, the signals will be very much stronger than at the start. Your radio will be overloaded by this strong signal and the signal strength indicator will be stuck at maximum preventing you from determining any directional information. The 'fox' will seem to be everywhere.
In this situation you will have to use either an attenuator or signal shifter.
A bit like putting on sunglasses when you step out of the house into bright sunshine, an attenuator reduces the strength of the transmitters signal reaching your radio.
The signal 'shifter' is an ingenious bit of equipment that allows you to tune your radio to a different channel from the fox. It converts the transmitter's signal to this new frequency and attenuates it all at the same time. With one or the other of these simple to make devices you can overcome most of your strong signal problems.
Where am I?
Finally we come to the question of the map. Depending on how large your hunting area is you will have to deal with the problem of folding maps. There are some tips on map solutions in my basic equipment section. Basically you will need to make your map durable, especially if you want to draw bearings on it. Laminating is a good idea, as is cutting the map up into more manageable sized pieces.
Starting out:
To find out if there are hunts in your area you could contact your local Amateur radio club, they may even have a website with a calendar of events. If you are already a member of a radio club then why not suggest they have a fun evening, hunting a hidden transmitter. If you are a beginner then see if you can 'ride along' with an experienced team to see how they operate. They may appreciate an extra pair of eyes once they are in the immediate area of the hidden transmitter.
Beginners Only | Techniques
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