"We're right in the centre now," Dave said, struggling with the map in the wind, while I examined the view from the hilltop.

There was nothing to see in any direction. Finally, I looked up. I could see nothing but low dark cloud fitfully reflecting the distant orange streetlights of Rochdale and Oldham. In any case, the wind blowing the drizzle more or less horizontally over the hilltop made it near-impossible to see very much in any direction.

I was beginning to wonder if we had been wasting our time, that all we had to look forward to was a mile walk in the wind and rain. We might not even make it to the pub before closing time. I thought back the sequence of events and just how we had got here.

*

I have to admit that I am a Radio Amateur - a Ham Radio enthusiast if you insist, although generally we don't really care for the term. If pressed, I would also have to agree that this is a rather anorak-y hobby, rather like train-spotting (for which I have no enthusiasm whatsoever) - although I was certain very glad of the heavyweight waterproof cagoule I was currently wearing.

Actually, amateur radio is not all sitting in damp wooden shacks at the bottom of the garden, applying soldering irons to mysterious electronic components and speaking to distant strangers over the airwaves. In fact, there is a strong social aspect, and I am a keen member of the local radio society. The club organises a whole variety of different events, including regular weekly meetings in the community centre.

This particular fixture was a radio Direction-Finding contest, and was one of the more frequent evening activities. Now, these foxhunts have their roots in the Second World War. The scenario is that spies are communicating their intelligence using a hidden transmitter. Our task, as the Good Guys, is to track down the agents and apprehend them, using only the sporadic transmissions for guidance.

A small team - usually two people - sets out early in the evening to erect a suitable aerial in some out-of-the-way but publicly-accessible area, preferably somewhere with trees or bushes to conceal the operators. They take a portable transmitter and a twelve-volt car battery to run the thing. I've done this myself a time or two, and I know it is worth taking an old sleeping bag and plenty of warm clothing, since you need to remain quiet and motionless for extended periods, hidden in the undergrowth. It was a cold November evening, and I certainly hoped the operators had remembers their winter woollies.

On this occasion, the 'fox' transmitter was being operated by two blokes from the club I knew quite well: Alan, known as "The Good Doctor", partially because he really does have a PhD and partially because he looks a little like a younger Isaac Asimov, and John, known as 'JS' to distinguish him from the numerous other Johns in the club.

The participants gather in their cars at a designated starting point, which might be ten or twenty miles from the hidden transmitter. Competitors are usually in teams of three: a radio operator, a driver and a map reader. The first team to locate the hidden transmitter is of course the winner, although finding the 'fox' at all is often a challenge for newcomers to the exercise.

The hidden operators make an initial transmission at a pre-designated time and using a specific frequency. They then transmit at certain defined times over the next half-hour, and then at random intervals no more than ten minutes apart. The transmissions get longer towards the end, with a final continuous transmission, to make it easier for less experienced, or just less lucky, teams to successfully locate the transmitter. All this is carefully timed so that it is possible to track down the transmitter, get the hidden operators to sign the entry form as proof, and still meet in the pub afterwards for a steak-and-chips supper and a pint or two of beer.

Originally, these events used short wave frequencies, just like the wartime Secret Agents or Resistance Fighters, depending on your point of view. These days, though, the ready availability of reliable and miniaturised kit - made in Japan as a rule - means that every Ham I have ever met would be carrying a two-metre portable transceiver in their pocket.

Two Metre RDF aerial Now, 'two metres' does not refer to the size of the equipment, but rather to the wavelength of the radio signals. This also defines the size of the antenna used. For foxhunts, the aerial is shaped like a capital letter 'H' and is invariably homemade from aluminium rods and tubes, a variety of odd bits of plastic and copious quantities of sticky tape.

The homebrew device has long arms about a metre long (a 'half-wave dipole') set on a crosspiece, the bar of the H, about half a metre long. There is a compass fixed to the crossbeam handle, and a short cable which runs to a portable receiver (kept in an inside coat pocket to keep it dry) with headphones (usually ex-army surplus, since they are likely to get wet and muddy).

The two dipoles are phased so that the reception pattern is heart-shaped: good reception all the way around the points of the compass, except for one direction - a 'null' - so that the signal disappears when pointed at. You will just have to imagine a car park full of anorak-wearing nerds carrying silly aerials swivelling around frenziedly trying to get an initial bearing on a distant transmitter.

The bearings are plotted on 1-in-50000 series Ordinance Survey map of the area, which are usually carefully covered in transparent self-adhesive plastic - yes, the famous Blue Peter 'sticky-backed plastic'. This makes the map still readable in the inevitable rain and, more importantly, it allows marks and bearings to be made using those dry-wipe pens intended for whiteboards.

The technique is to take one bearing from the official start, then leap in the car and drive manically (I mean, 'with all due care and attention', of course) to a selected point to one side of the expected direction. The objective is to take several bearings from different places and plot these on the map. You normally expect to find the lines crossing to form a triangle, since there is likely to be a few degrees error in any reading. The hidden station is probably somewhere in the triangle, so you drive closer - to within a mile or so, and take more readings.

All was going according to plan until Dave and I took the third reading. We had screeched to a halt at the side of the road when I had heard the latest transmission, and I leapt out of the car to take another bearing. Dave unfolded the map and plotted the direction I had shouted out. The second and third lines diverged. Now, this is of itself not entirely unusual; when you've been hurried or careless, or not held the beam steady while reading the compass, it is easy enough to get it quite wrong.

We still could infer some idea where the transmitter was hidden and, after a certain amount of huddled discussion, Dave suggested that we drive as far as we could before the next transmission and see if we could get a better fix.

We tried this, and again, but whatever readings we took, there seemed to be an area of the map which radio signals just would not propagate through. The transmissions seemed to be coming from two points simultaneously, separated by a distance of perhaps a mile.

Introduction Part 2