The Radiation Efficiency of a Small Loop Antenna

This is the final version. Available from British Cave Research Association via the link in this recordFor cave radio applications, we are not normally interested in the radiation from a loop antenna because the distance over which we are working is small (relative to a wavelength) and so only near-field effects need to be considered. However, it is still interesting to consider the radiation field, and to express the radiation efficiency in terms of the specific aperture where, just as for near-field operation, the number of turns on the antenna does not affect the result. David Gibson explains radiation resistance, summarises the equations and shows how the skin effect in the wire and the proximity effect between turns of the antenna winding can be taken into account

Remote Data Logging with Cell-Phones and the Internet

This is the final version of the article. Available from BCRA via the URL in this record.By means of the GSM cell-phone network and the Internet, data can be delivered from remote locations to anywhere in the world. Using this existing infrastructure can be an advantage over the more traditional point-to-point radio telemetry. An obvious use in speleology is the remote monitoring of long-term cave-based experiments or water levels and the progress of flood pulses. David Gibson outlines the principles

Novel Cave Radio Antenna uses Small Ceramic Tiles

This is the final version of the article. Available from BCRA via the URL in this record.Induction loop antennas are often inefficient due to skin and proximity effects, self-capacitance and tuning losses. However, an electric field antenna will generate a magnetic field which, in some circumstances, can exceed the field strength available from a loop antenna of similar mass and power dissipation. The main difficulty is in achieving efficiency at a small size. David Gibson explains how this may be achieved using high-permittivity ceramic tiles

Stereoscopic Vision with Reduced Definition in One Eye

This is the final version of the article. Available from BCRA via the URL in this record.Practical experiments with stereo pairs show that an adequate stereoscopic image can be formed by the human brain even when one of the images is considerably defocussed or pixelated. In a digital data transmission, the reduced-definition image can have a bandwidth (pixels x palette size) as low as 1/48th that of the full-definition image. David Gibson suggests that this could have useful implications in the transmission of stereoscopic images over reduced bandwidth channels

How Earth-Current Antennas Really Work

This is the final version of the article. Available from BCRA via the URL in this record.With cave radio equipment, there has been a trend away from the use of induction loop antennas to the use of so-called earth-current antennas, i.e. long wires grounded at both ends. Both the HeyPhone and Nicola system use this type of antenna. However, the popular explanation for how this antenna works is fallacious. The antenna does not operate by allowing the current to flow in a 'big loop' in the ground, nor is it a 'conduction mode' of operation. In fact, it does not depend, fundamentally, on current flow in the ground at all. The fact that the popular explanation is wrong is important because, if we do not understand how the antenna works, it is difficult to know the best way to use it, nor how to design a better one. In this short note, David Gibson outlines a more useful model - that of the Grounded Horizontal Electric Dipole - but without the mathematical justification, which will be given in a future article

The Inductance of a Wire Hoop

This is the final version. Available from British Cave Research Association via the link in this recordMany textbooks and webpages quote a formula for the inductance of a thin wire hoop, but few actually give the derivation. David Gibson had been confused by formulas that appeared to differ by a factor of two but has eventually decided where the subtle difference lies. This leads to an interesting possibility for the design of a wideband loop antenna with a lower Q-factor than is normally achievable

Determining the Photographic Guide Number of an LED

This is the final version of the article. Available from BCRA via the URL in this record.White LEDs are increasingly being used as sources of illumination for cave photography, both as modelling lamps and as alternatives to flashguns. Clearly, a simple way to determine the effectiveness of such illumination - the guide number - is to calibrate the light source in a series of practical tests. However, we can also determine the guide number theoretically. David Gibson goes through the steps in this exercise, which brings together several aspects of physics and photometry that the reader may have forgotten

Using AMSS on BBC Radio 4 to Send Data to Subsurface Locations

This is the final version of the article. Available from BCRA via the URL in this record.AMSS (AM Signalling System) was originally developed by the BBC to transmit data, using a subcarrier, on a long-wave broadcast transmission. Similar in concept to the more well-known RDS (Radio Data System) that operates on VHF stations, AMSS was standardised in 2006 by ETSI as an extension to the Digital Radio Mondiale system. Because long-wave broadcasts can be received underground, AMSS may, as David Gibson explains, provide a convenient method of broadcasting emergency data to miners trapped underground

Radio Propagation in Standedge Tunnel

This is the final version of the article. Available from BCRA via the URL in this record.Back in 2003, CREG was invited by Network Rail to investigate radio propagation in Standedge tunnel. During these tests, some unexpected results were obtained. Various hypotheses were discussed, but the work was never followed up. David Gibson asserts that a lack of ‘experimenter’s diligence’ in the original work prevents us from drawing any useful conclusions, and he warns of the danger of making a ‘wishful’ hypothesis

Using Dispersion to Measure Ground Conductivity

This is the final version of the article. Available from BCRA via the URL in this record.Several methods of remotely measuring ground conductivity are known, which depend on measuring the amplitude and phase of a received signal, relative to its transmitter. All of these methods require the separate transmission of a phase reference signal, with attendant difficulties. David Gibson proposes that by transmitting a suitably constructed signal, a phase measurement may be made without the need for a separate reference. This is achieved by making use of the phenomenon of dispersion, in which the phase velocity of a wave depends on its frequency. A related technique that makes relative measurements of signal amplitude is also described