RADIOACTIVITIES Newsletter of the Argonne Amateur Radio Club
Volume XLVI, Number 7	July, 2005

Communications in Caveman Radio by W9MKV adds more information using 160 meters cave to surface rated as successful, but higher frequencies are not. Using a technology magnetic induction very loosely coupling transformer action. Audio frequency magnetic fields penetrate geologic strata but the communication is short range because the magnetic dipole field strength decreases as the cube of the distance from the source, and conductive resistance will absorb the signal. The transmitter is a audio oscillator driving an amplifier which in turn drives a coil impedance matching is important for maximum coil current. A simple resonant coil hooked up to an audio amplifier will work for a receiver. Use crystal earphones because the magnetic type causes feedback. The circuit is a Q multiplier. The circuit is in negative feedback instead of being simply connected to the amplifiers input. The Q (regeneration) control taps some of the output and feeds it back to the non-inverting input. The amplifier forms a negative resistance, which cancels the resistance of the coil. As the Q control is advanced, sensitivity and selectivity get higher and higher until the circuit goes into oscillation (infinite Q). Since it will oscillate, the circuit can also be used as a very low power transmitter.

A simple resonant coil hooked up to an audio amplifier will work for a receiver. Use crystal earphones receiver gain can’t increase after a certain point. No amount of shielding can stop feedback. The interference comes from 60Hz AC power and harmonics also thunder storms are a problem. The best frequencies to use are mid range audio between 50 and 60 Hz. 3500Hz is used and 3276.8 would be easy to generate from a 32.768Khz wristwatch crystal. 3276.8Hz falls between the power line harmonics. Antennas maximize the magnetic moment of the coils. Magnetic moment is the ampere-turns multiplied by the coils area. Just to double the range means a 8-fold increase in magnetic moment, other factors being constant. Self-resonance limits the number of turns a coil may have. A 8-fold increase calls for 64-fold power increase or much larger wire. The route to long range is the largest diameter manageable. Ferrite cores can create temperature instability, micro phonics, and magnetic saturation. In 1984 the following author was waiting for technology that allows communication through the entire earth on modulated beams of neutrinos. Wishful thinking maybe nanotechnology would work better. Appearing in Feb ’84 73 Magazine.

QEX Sep/Oct 1999 ran an article “Underground HF Antennas” by KM5KG. This article looks at buried aerials for clandestine sites, underground military and civilian emergency sites that cannot rely on the survival of an above ground aerial structure. He states buried aerials operate within an air space or they are insulated from direct earth contact. KM5KG states receiving aerials are terminated with an impedance of 10,000 ohms. This will not always produce the most received signal voltage at aerial terminals. A well-designed wave dipole buried, as depth increases signal resistance is about 2db per meter, and now the self-impedance increases because the soil clearly affects the impedance. This effect is to shorten the dipole.

A wavelength at any frequency is (300/lf) where h is a factor on conductivity plus dielectric constant of the medium thus the dipole looks too long. Example: A 20-meter band dipole in air looks as though it were the 5-meter band or 10 meter long 14MHz dipole is about 2.6 meters long in the dirt. The signal drop in this medium is 2.2db/m units are decibels per meter. Our 14MHz-receiving aerial is 10 meters deep that’s a signal drop of 22db. This is like a one kilowatt dropping to 6 watts or 3 S-unit drop in strength. If you had 2 buried identical dipoles 100 meters apart, it would be better served by using a twisted pair of wire than radio. Low frequencies like 160 and 1750 meters yield better signals. The navy uses 15KHz VLF to talk to submarines.

We will get the last word from ARRL Antenna Compendium Volume One by W0YBF in “Subsurface Antennas and the Amateur”. Results show at 5MHz signal loss is 1.87db per meter of travel reflection loss adds another 4db, so 40% is reflected and 60% is refracted or the refracted wave is down by 2db. For obtaining more gain it is much simpler to get gain in these antennas by reducing losses done by paralleling dipoles or to design a buried Vee or Rhombic aerial for burying. Inserting capacitors in the wire or wires could improve the phase velocity to simulate a wire above ground. Impedance changes with frequency and soil moisture. See my article in Radio Activities January 2005 “Control Current Aerial”.

Some sky wave communications are possible on 160 meters during times of low atmospheric noise. It is suggested a narrow insulated loop of lengths given earlier can some of the ground loss supplies higher Q and works best near its resonant frequency.

Will VHF aerials work buried? The answer is yes. Placed just centimeters below for hiding. Listen all you foxes.

Contacts have been made using a full insulated horizontal dipole and a variety of small tuned loops and dipoles mounted a couple centimeters deep produced vertical polarization an angle of ten degrees above the surface that were 14 or 15db weaker than tests from a quarter wave monopole on the ground. Cosmic noise being the single competition, it’s been estimated such a buried aerial using one watt of power contacts with aircraft was possible at 50,000’ out to a distance of 70 to 90 miles.

W0YBF says the slot antenna just below the surface under a plastic lid under the aerial & wire mesh lined cavity and arrays of resonant slots might offer great gain. A 2 meter handheld 20’ in a plastic bay submerged in seawater has transmitted a signal heard on the surface.