The modern environment contains huge amounts of electrically conductive material, such as the steel reinforcing rods in modern concrete buildings. If the metalwork is separated by gaps significantly smaller than the wavelength, then the radio waves just will not propagate inside the building. At this frequency, for example, an excellent Faraday cage can be constructed using just chicken wire.

Large objects containing a lot of metal, such as the skyscrapers in cities, tend to reflect and refract radio signals, leading to dead spots, as well as re-radiation where the entire structure acts as an aerial and re-broadcasts the signal.

By this time, I had begun to suspect that the Good Doctor had devised some sneaky way of disguising the origin of his signals. It was not unheard-of, for example, for a hidden station to be deliberately sited close to overheard power cables, so that the radio signals appeared to emanate from multiple metal pylons along the line of the wires.

As we were taking more bearings, we hade been steadily converging on the dead zone bracketed by our measurements. I was convinced that the 'fox' team had deliberately identified some location where strong re-radiation produced the confusing signals we had detected. This was just the sort of cunning trick I expected from JS and the Good Doctor.

Even so, I was confused. A closer look at the map showed that the zone was located in remote area, well away from fixed, high-power transmitting sites (TV masts, for example) and the map showed no sign of power lines. It was just an area marked with the moors and fields of an ordinary East Lancashire hillside.

It was this nondescript point that Dave and I were currently heading for, alternatively walking and running up the hillside in increasingly poor weather, having abandoned the car at the end of a rutted track.

*

I dragged the headphones over my ears. Nothing - the receiver was not working. I took it from my pocket and noticed that the battery pack was loose again. I had cracked the plastic latch by dropping the thing on a previous foxhunt and it could have easily become dislodged during the stumbling hike over the moor. I jiggled it back into place; it re-seated with a click, and the receiver burst into life.

The Good Doctor was transmitting continuously now, nearing the end of the competition. I was getting a very strong signal but, as I waved the beam aerial around, I could not find a null in any direction. The transmitter could not be far away, but I had no idea of the direction.

Meanwhile, Dave was swinging his torch around, looking for telltale signs of movement in the dark mounds of heather and gorse bushes that dotted the area. An essential part of the foxhunter's equipment is a powerful torch, preferably with batteries which can be recharged from the car cigar lighter socket. The portable lanterns both Dave and I were carrying were heavy and cumbersome, but were as powerful as car headlights and lit up the countryside all around.

On a whim, I pointed the beam upwards. The signal in my headphones dropped off suddenly and, almost automatically, I stiffened into a position which indicated a clear direction. I was detecting a deep null from almost straight up, as if the hidden station was somehow hovering in mid-air over our heads.

Or, the thought rushed into my head unbidden, as if some large metal object was hanging over our heads, reflecting the signal from the transmitter.

"What are you doing?" Dave bellowed.

"The signal!" I shouted in reply, "It's coming from up there!"

As one, we swung our torches upwards. There was nothing to see, other than droplets of rain glittering in the wind. Just then, something happened. There was a great sense of movement immediately above us, somehow telegraphed by strange movements of the winds. The drizzle seemed to be moving upwards, swirling around in the light of the torches, rather than being blown uniformly sideways as it had been only a few moments before.

The movement was accompanied by hissing, rushing sounds, loud enough to be audible over the noise of the wind and rain beating on the hood of my anorak. These noises were joined shortly afterwards by a near-subsonic rumbling. It did not sound like any aircraft I had ever heard before.

For a few moments, we could not actually see anything out of the ordinary. But then, there was a disruption to the clouds above us. I could see strange movements lit by the distant street-lighting as some black bulk, impossibly huge pushed its way upwards, just visible as a contrast against the grey low-lying cloud base. Then it was gone.

What I think happened is this. Almost all radio receivers also transmit signals, although most people do not realise this. The super-heterodyne principle, used in pretty much all receivers, requires the use of a local oscillator. By design, these oscillators do not emit very strong signals, but they are nevertheless detectable over short distances. This is how those old TV detector vans used to work.

Whatever it was up there, it became aware of the receiver only when I reconnected the battery, and faint but detectable signals suddenly started coming from directly underneath it. We simply scared it off.

Thinking about it later, the flying object, whatever it was, possessed effective optical camouflage - a cloaking device, just like the Klingons in some old episodes of Star Trek. Actually, I understand that such technology is theoretically possible: the properties of certain molecules can be employed to bend light around an obstacle, perhaps, or the use of phased array optics to project a hologram of the view from all directions.

I can imagine that this technology could be made to work for kinds of electromagnetic radiation other than light, which probably explains why the object was not detected by radar. Modern radar uses millimetric waves (microwaves, as in the cookers). These obvious strong signals, with characteristics special to radar, could be managed by suitable countermeasures, absorbing or re-routing the microwave energy around the object.

On the other hand, VHF radio, like the two-metre wavelengths we were using, is populated by a large number of low-power transmitters, everything from police walkie-talkies to taxi companies. These devices are intended for local communications, and are in highly intermittent operation. There would be almost nothing to distinguish the occasional transmissions from our competition fox from any of the myriad of other radio signals in the vicinity.

So why did it run away? Perhaps we were just too close to allow the same cloaking to work, or maybe there was no need to hide itself from radio signals. But it must have easily detected our presence - we were not trying to hide ourselves, flashing our portable searchlights around - and simply decided to move away.

But what was it, really? A UFO, obviously, some kind of flying machine, but was it really an alien spacecraft, some visitors from another plant on their own inscrutable mission, hovering over an out-of-the-way place for more than an hour? I cannot imagine what the attraction of that particular spot was. Was it attempting a landing, or perhaps waiting to rendezvous with some other vehicle?

Or was the Unidentified Flying Object of entirely human origins, some kind of top-secret military aircraft on a night-time training mission over an obscure part of the English countryside? I have no way of finding out.

It was just dumb luck that the Good Doctor had chosen to locate his hidden transmitter not so far away from the mysterious object, whatever it was. The transmitter was actually located in a wooded valley, slightly shielded in the direction of the hilltop where Dave and I ended up, but with a clear propagation path towards the official start-point.

I made a few discreet enquiries at the pub later that evening. It turned out that we were the only team to head west for their second and third readings, the only team to notice anything unusual, and so we were the only team to observe this anomaly.

Back on that wet hillside, stunned and shocked by what we had seen, Dave and I looked at each other.

"What are we going we do?" I asked.

"I don't know," he replied thoughtfully, "But I don't think anyone is going to believe us, whatever we say."

Slowly and carefully, Dave started rubbing out the lines on the map.

Part 1 Afterword