On a New Nonlinear Effect in the Ionosphere

In a whole series of radio-communication problems it is necessary to take nonlinear effects in the ionosphere into account. Such a nonlinear effect is the Luxembourg–Gorky effect \(^{1,2}\)—the mutual influence of radio waves from different transmitters and, generally speaking, of different frequencies. As a result, a radio transmission coming from a powerful station with carrier frequency \(\omega_1\) is also heard in the reception of another station operating at frequency \(\omega_2\) \((\omega_1 \ne \omega_2)\).

Recently another nonlinear effect \(^{3,4}\) has been discovered—the decrease in the modulation depth of a radio wave upon its reflection from the ionosphere. This effect is resonant in character and is observed for carrier frequencies close to the gyromagnetic frequency of electrons in the ionosphere,

\[ \omega_{\mathrm{H}}=\frac{eH}{mc}. \]

The author proposes for it the term “autodemodulation,” or “gyrodemodulation,” since the phenomenon is connected with the gyrotropic properties of the electron plasma.

The phenomenon of autodemodulation was discovered under the following experimental conditions. The transmitter used was the Florence-2 radio station with a power of 3 kW. The operating wave of the radio station was changed in steps in the interval from 215 to 280 meters (i.e., between frequencies of 1.07 and 2.3 MHz); every 5 minutes the wavelength was changed by 5 meters. The gyromagnetic frequency of the ionosphere in Central Italy is estimated at approximately 1200 kHz (250 m), i.e., it lies in the middle of the operating range of the radio station. Five-minute dashes were transmitted, modulated at a constant frequency of 230 Hz with a modulation depth of \(80 \pm 2\%\). To check the constancy of the modulation depth at the transmitting station, a small antenna was installed, the signal from which was fed to an oscillograph.

Reception of the signals was carried out in Turin (distance 320 km) with a receiver of the “Safar” or BC314 type. After the intermediate-frequency stages of the receiver an oscillograph was connected. The modulation depth was estimated

by scanning on the oscilloscope screen; it is equal to the ratio of the half-difference between the maximum and minimum ordinates of the envelope to the half-sum of these ordinates. The results of the first two series of experiments are shown in

Fig. 1.

Fig. 1. Because of unavoidable fluctuations in reception strength due to atmospheric interference, the authors estimate the error of their initial results at approximately 10%.

Fig. 2.

Then the receiver was transferred to Naples (distance 410 km). The increased accuracy of the measurements made it possible to obtain a curve of the dependence of the modulation depth on the modulating frequency (Fig. 2).

Fig. 3.

Finally, in the third and last series of experiments, a special receiver was constructed for measuring the modulation depth. Some results of these experiments are shown in Fig. 3. Photograph b is an oscillogram of the unmodulated signal; photograph a is an oscillogram of the modulated signal emitted by the transmitter. Photographs c—h show oscillograms of the signal at the receiving point, corresponding in all cases to the same modulation depth of the transmitter signal shown in photograph a. The difference in modulation depth between photograph a and photographs c—h characterizes the phenomenon of automodulation.

The author gives no concrete explanation of the experimental results he obtained, pointing only to the quite obvious circumstance that automodulation is due to nonlinear effects in the ionosphere.

The phenomenon of automodulation is of substantial interest from various points of view. It can serve for investigating the properties of the ionosphere. In addition, in practical terms, the phenomenon of automodulation must also be taken into account when choosing the operating-frequency range of radio stations: the carrier frequency must be sufficiently far removed from the gyromagnetic frequency at the given location.

M. P.

CITED LITERATURE

1. V. L. Ginzburg, Theory of the Propagation of Radio Waves in the Ionosphere, Gostekhizdat, 1949.
2. V. L. Ginzburg, Izv. AN SSSR (ser. fiz.), 12, 293 (1948).
3. M. Cutolo, Nuovo Cimento, 9, 695 (1952).
4. M. Cutolo, Nature, 167, 315 (1951).