Designed by Graham Naylor

Development and testing in collaboration with SSSI and ADRASEC 38


Graham NAYLOR, a physicist at the European Synchrotron in Grenoble, is a member of the Isere cave rescue team and works in collaboration with the ADRASEC 38 on the development of new cave communication systems. He is the third from the left.

He is explaining the operation of the system NICOLA to gendarmes of the PGHM and to the Lieutenant ROMARY of the technical department of the gendarmerie nationale at Rosny-sous-Bois during test in the cuves de Sassenage (near Grenoble) with regards to future use by the gendarmes of the PGHM.

  1. Technical characteristics
  2. Principle : Emission/Reception SSB super heterodyne

    Euro Norm : I-ETS 300 330 Class 3

    Emission frequency : 86.95 Khz

    Intermediate frequency : 455 Khz

    Mixing and BFO : Derived from quartz oscuillator

    Front-end filtering : Preselection with low pass filter and two stage LC filter

    IF filters : Ceramic filter + high gain ampl + mechanical filter.

    Modulation : Single side band.

    Microphone: electret

    Power into antenna : 3 Watts

    Current loop : 0.1 A when matched

    Adjustment : A 3 position switch to adapt the via the transformer the impedance of the ground to the impedance of the output stage.

    Supply:  12V to 15V

    Consumption at 12V : 0.06A on reception and 0.9A on emission

    Antenna : Large virtual loop constituted by the two electrodes connected to earth spaced by 40-80m

    Design : Mixed surface mount and through hole.

    Performance : Through rock transmission 1200 m maximum dependant on the local geological conditions, 500 m generally useful.

    Size : 150 X 80 X 50 mm

    Weight : 0.250 Kg without batterries

Block diagram



The unit is shown here housed in a protective box for use underground. It is recommended that it is transported in a seperate waterproof container to avoid excessive contamination with mud and water. The microphone is hard wired to avoid any problems with connectors, but a connector is provided to allow future expansion for example the addition of remote control equipment or the transmision of digital messages.

The system has been tested on several foreign expeditions to Spain, Mexico and China aswell as in France. The compact unit should be suitable for transport in most commonly used waterproof caving or cave diving containers. As of March 2000 these devices have been used on about 20 days of rescues.

The device shown above has a supply lead to allow the connection of an external battery. Two push terminals allow the connection of the antenna wires. A volume knob is provided and a three position rotary switch for the transmission power. There a three indicators: a red/green LED indicates emission reception; a red flashing LED indicates the presence of a carrier signal (useful for indicating that some-one is calling when the volume knob is reduced) and an orange led indicating the current running into the antenna (shows emission strength). A seven pin connector allows the connection of a spare headset, a spare microphone, remote control from VHF, connection of a SWT, connection of a beacon or connection of an interface  (eg DSP) for the transmission of digital signals.

The device can be supplied from batterries providing between 12 and 15V. This is a view of the inside of the device showing the main low profile circuit board. All the connections for the front panel are made via a seperate PCB attached to the front panel. There is even space in the box for 10 AA batteries (giving about 2-3 hours of intensive use with Duracell Ultras).

The output stage uses a series of 4 low profile circuit board mounted transformers to adapt the impedance to the ground. The primaries are connected in parallel and either 1,2 or 4 of the transformer secondaries are connected in series to the output, depending on the position of the rotary switch.
This design represents about a year of evenings and week-ends. Several prototypes have been built and the current circuit board design is the fourth version. The loudspeaker is in the handset.



View of the surface mount side of the main board.

Protection of the device in a waterproof box.

2 - Construction of the device

  • The production of a series of about 50 circuit boards is anticipated by mid april 2000. The boards can be assembled in these numbers for about 1850 F per board. There should also be some bare boards for hand assembly in small quantities in the future. The AD607 is however a 20 pin device with a 0.65mm pitch between pins. Hand soldering this device requires a little practice but a well made board should be able to be solderred by a competent technician in an afternoon. Around 1500 F is required to complete and box the device with a good quality handset and good quality front panel components. A few hours are required for the final assembly and trimming of the presets.
  • 2 - Operation :

    1. Wires are run out in each direction from the device to convenient points at which a good ground contact can be made. A ground connection can be made underground using a length of about 10m of electric fence tape in a pool or mud. On the surface  metal stakes (or large metal tent pegs) will work well. The symmetry of the antenna is not important but aim to have a seperation between the grounding points of about 1/6th the distance over which communication is required.
    2. Once the ground contacts have been made the battery should be connected causing the LED to light red.
    3. The three position switch should be turned to 1. Blow into the microphone while squeezing the transmit switch. Raise the position of the switch until the orange LED lights.
    4. Correct adaptation to the ground should now be achieved. With a good grounding connection this will be on position 1, with normal grounding it will be on position 2 but if you have to switch to position 3 then this indicates a poor ground connection or perhaps a broken wire.
    5. Once the voice connection has been made it may be possible (if the reception is good at the far end) to drop the switch position in order to conserve the batteries.

    There should be some co-ordination with the surface team to agree the location and time of such transmissions. It is up to the surface party to be ready. The underground team may well try and transmit well before or after the agreed time. It may be necessary for the surface team to displace themselves if the signal is weak and or if determined by the strata. A loop aerial is sometime useful while moving to find the strongest signal.

    Using earthed antenna wires provided they are of significant length will normally give a much stronger signal than a loop. It is more important to get a good transmission strength form the underground to the surface due to the sometimes high levels of background noise on the surface. Achieving good ground connections is important for getting the best transmitted signal strength. It is important that no-one is holding the antenna wires (especially the ends and the terminals) whilst a transmission is being made, due to the several hundred volts induced on the output (Position 3 gives the highest output voltage).

    The reception underground is often much better underground than on the surface. This is due to the high level of atmospheric disturbance in the LF band. Lightning strikes produce strong signals in the LF band which will propogate up to a 1000km due to the ground effect. Undergound these parasitic disturbances may be significantly attenuated (unless the cave is quite shallow). These noise sources will vary enormously from one day to the next and form day to night, due to the drastic changes in propogation conditions for these perturbing sources.

    The Loran C transmitter also causes considerable disturbance in the region 80-130kHz. At the 87kHz frequency currently used in the system Nicola, this means that although over 1km is sometimes possible, in practice it is best to limit the distance to 500m to ensure reasonable reception is achieved on the surface. This distance is normally sufficent for even the deepest caves in France due to the fact the often the surface above the deepest part of the cave is often well below the level of the entrance. In the case where large distances are to be attempted then it is often useful to consider the geology so as to avoid passing through more comnductive strata. Sometimes finding the outcrop on the surface in shich the underground party is situated is sometimes beneficial. Once a contact is made, if the signal is weak it is often worth moving to improve the signal strength (as indicated above by maximising the received signal strength with a loop on the surface while the underground team connect their radio to an automatic beacon).