Mine communications and telemechanics systems, such as mine rescue communications, mine announcement systems, communications with the engineers of conductor-rail and battery locomotives, communications with a moving cage, etc., are systems of mobile communications and telemechanics in which the best method is frequency and frequency-phase modulation.Other conditions being equal, frequency modulation gives superior radiotelephone range and quality. This is because in frequency modulation the amplitude of the modulated waves does not vary, and consequently the mean power is equal to the maximum power of the generator. This is all the more important because in a mine the radio transmitters of the systems must be low-power and "intrinsically safe" from sparks which might cause explosions.Service and mass-production requirements for the standardization of components and the simplification of the design and construction of various mine radio systems oblige the designers to make extensive use of previously developed components and subsystems. In the Control Systems Laboratory of the Institute of Mining of the Siberian Branch of the Academy of sciences of the USSR we have developed an FlVl transmitter circuit for mine communications and telemechanics systems.The transmitter has the following technical characteristics: carrier frequency 100 kHz; output power 4 W; frequency instability 5 9 10 -4 for AUfeed = + llYr] o and t = + 10-50"C; nonuniformity of frequency characteristic less than 3 dB in the 300-3000 Hz range; equivalent antenna resistance Req u = 80-280 ~; frequency deviation + 3 kHz; nonlinear distortion less than 6% at 400 Hz. Figure 1 shows the block circuit of the transmitter. It consists of four stages: a microphone amplifier, a frequency-modulated master oscillator, an externally excited IF oscillator and an externally excited push-pull output generator based on a common-emitter circuit. The output generator circuit reduces even-harmonic emission and yields the required power under load. The output stage makes use of medium-power germanium transistors T4, Ts, of type P602. To improve the heat balance, the output-stage transistors are fixed to a common heat sink via an insulating mica sheet.The IF oscillator is based on a common-emitter circuit with a P15 transistor with transformer coupling, since it is necessary to ensure not only a given excitation power but also a given excitation voltage amplitude to the out-

UDC 622.002 ~le rocks opened up by a coal mine constitute an inhomogeneous medium, owing to the large number of seams with differing electrical and physical properties. In some cases, rough calculations of a theoretical or engineering nature can be limited to a homogeneous model of the propagating medium. The energy losses incurred in transmission through a conductorless channel in a medium with the structure of a real rock mass are due to losses in the side rock strata, the electrical properties of which are similar owing to dependence on the moisture, temperature, and rock pressure. As an example of the use of a homogeneous model of the solid rock in an engineering calculation, we can take a conductorless link between different horizons across the stratification.The emission, propagation, and reception of an electromagnetic field in a homogeneous semiconducting medium have been quite exhaustively studied: the starting-point was the field of elementary antennas in a homogeneous medium with arbitrary characteristics. The general expression for Hertzian oscillators of the electrical and magnetic typos, used for transmitting longitudinally polarized electromagnetic fields,can be written as Me=~ -~t' [1 + j k r --k 2 r t) e -/k" sin O, (1) where /~I 0 is the longitudinal component of the electric field of an electric antenna or the magnetic field of a magnetic loop, i ~ is the dipole moment of the antenna, k is the wave number of the medium, r is the distance, and 0 is the angle between the axis of the antenna and the direction to the reception point.In the long-and superlong-wave ranges, an elementary oscillator of the Hertzian electrical type consists of a grounded electric dipole. End loads level out the current distribution along the antenna. The moment of the grounded dipole can be found from the equation:where I e and l are the current and length of the dipole, ~ is the complex dielectric permittivity of the medium, and w = 2r] is the frequency of the radiated field.In conduetorless mine communications systems use is made of loop antennas of the magnetic type, with a dipole moment ofwhere Ip is the current in the loop, and n and S are the number of turns and area of the loop.The electrical conductivity of the main mass of rocks in coal pits is o = 10-1-10 -3 mho/m [2, 3]. The dielectric permittivity is in the range (10-40)e 0. With such rock characteristics, the displacement currents in the Institute of Mining, Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk. Khar'kov Institute of Radioetectmnics.