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By Mike Beer (NSS# 16938)

Nielson's Well is a shaft cave (315 feet deep) with a large room (approximately 120 feet wide x 170 feet long x 80 feet high) at the bottom. These characteristics make verbal or whistle communication difficult, particularly with people in the large room. The sound tends to fill the room, rather than be directed up or down the shaft.

At the point I became involved with the exploration of Nielson's Well, one of the other cavers (Jim Nicholls) had decided to try to use a set of FM radio headsets that he had purchased for surface work. When he used them on the entrance rappel, they worked well for about the first 200 feet. However they then faded abruptly and were useless in the Big Room.

I had an experience in Papoose Cave in Northern Idaho where FM radio communication had been possible in and near a short pit during a mock rescue. The mock rescue organizers said that the FM radios worked in the cave because of the presence of a field telephone wire. On the next trip to Nielson's Well, I took a spool of 28 gauge, insulated magnet wire, and lowered the end down alongside of the first rappeller. The end of the wire was left hanging into the Big Room by about 20 feet or so.

The results the wire produced were much better than those experienced in the mock rescue. Not only did radio communication remain clear while on rappel, but it was possible to communicate from anywhere on the floor of the Big Room to the surface. There were some areas in the Big Room that had minor fading, but moving a few feet would get out of the fade area. This provided excellent communication for rope management, and quite a psychological boost for the people waiting to ascend since they could easily converse with the surface crew.

This experience made me wonder about the differences and similarities between the two times that I have used FM radio in a cave. I believe that I may understand what is happening, and would appreciate hearing from other people about good (and bad) experiences in other caves to help refine my understanding. What follows is somewhat speculative since I have made no quantitative measurements, but it provides an operating model that will serve for at least this discussion.


The Nielson's Well entrance is a large surface opening of the cave's shaft. It is approximately 30 by 50 feet in area. The first 100 feet of the shaft are nearly this dimension, with the next 100 feet tapering gently to about 8 feet in diameter until the shaft intersects the ceiling of the Big Room at about the 200-foot level. The shaft enters the Big Room at one end. The radios were consumer-grade headset versions with voice-activated keying, operating at about 50 Megahertz (MHz). The input sensitivity of the front end is not known to me. They were identical in outward appearance to the ones that had been used in Papoose Cave.


The 50 MHz operation places the radio's wavelength at about 20 feet in free space. This is a small enough wavelength, relative to the size of the opening of Nielson's Well, for the shaft to act a waveguide, until the shaft diameter becomes less than one wavelength (assuming TE and TM propagation in a fundamental mode).

I believe that this explains why the radios could not work at all without the wire in the narrow portion of the shaft. The walls of the cave are wet and may have sufficient conductivity to provide a lousy wave guide that does not attenuate too much. The 200 feet to the fade out point in the shaft is about 10 wavelengths, and if the situation were extremely attenuative, communication would be difficult this far down the shaft. The diameter reducing to less than a wavelength at 200 feet would cause the radio wave to be reflected back up the shaft. These type of reflections often occur at less than one wavelength.


Dangling the wire down the shaft changes the situation quite a lot. I believe that wire in the shaft causes the shaft and wire to act as a transmission line. 50 MHz is not far from the frequency of broadcast television. Cable companies' coaxial cable and TV antenna leads work quite well at transmission line dimensions much smaller than the 8 foot diameter of Nielson's Well shaft. Calculating a characteristic impedance for such a transmission line is difficult due to the uncertainties in material and dimension.

Transmission lines must be properly terminated to get much energy down them. In retrospect, we probably provided this termination by dangling the wire in the shaft and into the Big Room. I assume that the signal couples from radio to wire through a combination of near field antenna and transmission line radiative loss, setting up a standing wave on the wire. I also suspect that the room acts as a cavity resonator driven by the portion of the wire hanging out of the ceiling. The room's dimensions are quite a bit larger than a wavelength, and I suspect that some sort of standing wave could form between the walls and ceiling. Presence of a standing wave would also explain the fade areas that seemed to occur locally in the Big Room.


While virtually any wire will work for this type of transmission line, there are several things that should be considered

In no case should the wire be of enough strength that it cannot be broken if a caver should become tangled in it or endangered by it in any way.

The initial experiments done in Nielson's Well used solid copper, insulated wire (magnet wire). This wire is prone to kinking and nicking. The kinks and nicks provided weak points at which the wire would break when snagged during retrieval or by a rappeller. We finally settled on 22-gauge stranded wire covered with a plastic insulation. This wire provided several advantages. The heavier insulation reduces chances of nicking the conductor. The multiple strands reduce the possibility of kinking or nicking and failing all of the strands simultaneously. This made a more robust conductor. Plastic wire insulation makes a larger diameter wire and is usually brightly colored. Both color and increased diameter make the wire easier to see, reducing the chances of its being accidentally snagged by cavers. A color selection also allows color changes at each splice, so it is possible to determine how far along the wire the caver is while rappelling or ascending


In Nielson's Well, the wire was strung down a pit. This creates the problem of having the wire and rope in close proximity. To avoid tangling them we lowered the wire down the pit with the first rappeller. A small lead weight (a couple ounces) kept the end of the wire from recoiling. The weight would be lowered so that it was about 10 feet below the rappeller. Any tangles that developed could easily be cleared since there would be very little wire below the rappeller and the rappeller would be descending into any tangle, rather than trying to straighten out a mess overhead. It is also a good idea to have the next rappeller down the wire tuck the wire behind ledges or into cracks to further separate the rope and wire.

There is also a problem of removing the wire from the pit. The last climber should clear the wire from any ledges or cracks where it has been tucked. If the wire hangs cleanly in the pit, removing the wire as the last climber ascends is a good idea, this time keeping the end of the wire about 10 feet above the climber. It is best to have a spool of large enough diameter so that one revolution of the spool retrieves at least a foot of wire (4 inches minimum spool diameter). Smaller spools than this are quite tedious to rewind.


It is possible to communicate with FM radio in caves. Based on the discussion above, it is possible to give guidelines that will either prove or disprove the discussion's assumptions.

If one has a choice of frequencies, choose the highest one possible. This shortens the wavelength, allowing use of simple reflections or wave guide phenomena. Don't expect long distances with this technique, since reflections and wave guides are usually quite lousy.

The transmission line principle seems to work well for pits, particularly those with large rooms at the bottom. It is a good idea to get the end of the wire out into a large room away from the walls, to form a standing wave in the room. It would be an interesting experiment to vary the amount of wire dangling into the room and its position in the room in an attempt to "tune" to the room's dimensions or to move a fade area.

Accurately predicting the performance of FM radio inside caves is at this time more art than science. I encourage others to try it. In situations such as deep pits or large rooms, it can provide a convenient and echo-free means of communication. Where other wires already have been strung through an area, it can provide another communication channel for minimal investment of effort.

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