A method for implementing a receiving system based on a combined antenna

Buoy communication technology is developed on the basis of traditional wireless communication technology. Since it was applied to submarine communication, buoy communication has begun to be widely used in military communications of various countries. However, the current buoy communication basically still uses a single omnidirectional antenna to realize the reception of electromagnetic waves. Since the buoy is generally placed on the sea level, it is easy to be disturbed by waves and tides, which can easily cause the antenna to rotate, and sometimes even turn over or invert, and the traditional omnidirectional antenna cannot achieve reliable signal reception. In order to avoid this phenomenon, this paper proposes a design method for a combined antenna. The combined antenna includes two antenna units: one is a whip antenna, which can receive electromagnetic waves in the horizontal field; the other is a magnetic induction antenna, which can receive electromagnetic waves in the vertical electric field. By combining the two antennas and using frequency selection circuits and high-frequency amplification circuits, reliable signal reception can be achieved.

1 Combined antenna design

1.1 Whip Antenna

The whip antenna is also called a grounded monopole antenna. The monopole is perpendicular to the ground. The ground is assumed to be an ideal conductor. The influence of the ground can be replaced by its image, and the electromagnetic field exists only in the upper half space of the ground. The monopole ground-fed antenna can be equivalent to a dipole antenna. In buoy communication, the surface of the buoy barrel is generally assumed to be an ideal conductor. The whip antenna model is shown in Figure 1.

Compared with the dipole antenna, the whip antenna has a lobe direction that is tilted toward the traveling wave direction, the maximum radiation direction is biased by 25°, and the half-power beam width is reduced from 78° to 60°. At the same time, compared with the dipole antenna, the upper half-space directional function and directional pattern of the two antennas are the same, and the polarization characteristics, frequency band characteristics, etc. are the same. However, the input impedance of the whip antenna is half of that of the dipole antenna, mainly because the excitation voltage is halved while the excitation current remains unchanged. At the same time, the directivity coefficient of the whip antenna is twice that of the dipole antenna, and because the field strength remains unchanged and the radiation power is halved, that is, it only radiates in half space, so the loss resistance is large and the radiation efficiency is low. The calculation formula for the far-field component Eθ of the whip antenna is as follows:

The whip antenna used in this design has a length of H of 30cm and receives electromagnetic waves at a frequency of 1.8MHz. Calculations show that the directivity of the whip antenna is about 4.80 and the absolute gain can reach 6dB. Through the analysis of the electromagnetic wave field strength, this gain can basically meet the signal reception of the remote wireless remote control system.

1.2 Magnetic induction antenna

The magnetic induction antenna is also called an electric small loop antenna. There are two types of loop antennas: circular loop and square loop. The electric small loop antenna in this design is a circular loop antenna, and its size is much smaller than the wavelength. Therefore, square loops or circular loops of the same area have the same far-field lobe pattern. The calculation formula of the field component of the magnetic induction antenna is as follows:

Compared with the dipole antenna, the dipole antenna contains an imaginary factor, while the loop antenna does not. This shows that the fields radiated by the dipole antenna and the loop antenna are orthogonal in time under the same current feeding, which is also the biggest difference between the two. Therefore, the loop antenna is suitable for horizontal arrangement and orientation, while the dipole antenna is generally oriented parallel to the z-axis. This also meets the requirement that the antenna length is much smaller than the wavelength, that is, the antenna size approaches zero with respect to the wavelength. Table 1 lists the far-field comparison of the loop antenna and the dipole antenna.

1.3 Combined Antenna

Since the receiving antenna is located in the buoy, the buoy is prone to change its direction and position under the influence of waves and wind direction, and may even capsize. Therefore, the use of a single antenna mode cannot ensure reliable signal reception. Therefore, a composite antenna of a whip antenna and a magnetic induction antenna can be used to enhance the reliable reception of signals. In this way, no matter which antenna fails (such as the whip antenna is broken or capsized), the other antenna can be used to complete the signal reception. The whip antenna is a vertically polarized antenna that can receive the magnetic field of the vertical polarization component; the magnetic induction antenna is a horizontally polarized antenna that can receive the magnetic field of the horizontal polarization component. In this way, through mutual complementation, the two antennas can well receive the omnidirectional waves from the transmitting antenna. The structural design of the combined antenna is that the magnetic induction antenna is arranged on the inner wall of the buoy in a horizontal manner; the whip antenna is arranged in the center of the buoy, perpendicular to the magnetic induction antenna. The two antennas are received using the same feeding method. Figure 2 is the design model of the combined antenna.

In order to ensure the independent reception of the two antennas, the whip antenna and the magnetic induction antenna need to be isolated by adding insulating materials, thereby eliminating the parasitic factors of the antenna itself and preventing signal crosstalk. The actual layout of the combined antenna is shown in Figure 3. The conductors in the figure refer to the whip antenna and the magnetic induction antenna, and there is insulating material between the two antennas.

2 Signal reception

The signal received by the combined antenna passes through the two frequency discrimination inductors L1 and L2, and then is filtered and output by the field effect transistor V1 to be sent to the high-frequency amplifier circuit. The frequency selection circuit is shown in Figure 4.

The high-frequency amplification receiving circuit is composed of two transistors of the same model. The receiving signal from the frequency selection network is amplified by two transistors and filtered by capacitors before entering the demodulation circuit for demodulation. Figure 5 shows its high-frequency amplification circuit.

3 Experimental and simulation results analysis

After the integrated system is realized, HFSS11 software can be selected to simulate the combined antenna. The simulation results show that the combined antenna has a strong ability to receive radio signals, and can still achieve reliable signal reception when the antenna rotates or flips over. The lobe diagram of the antenna when the cross section of the buoy barrel is 20cm is shown in Figure 6. It can be seen from the figure that the combined antenna can basically meet the system requirements under the field conditions, and the antenna directivity coefficient reaches 3.17 and the gain reaches 30dB. It can fully meet the system requirements.

Through CAD simulation of the frequency selection circuit and high-frequency amplifier circuit, it can be seen that the transmission system adopts 2FSK modulation with a carrier frequency of 1.8MHz. First, the crystal oscillator generates a center frequency f0 of 1.8MHz, and then the logic circuit generates two carriers with frequencies f1=f0+2kHz and f2=f0-2kHz, with a frequency deviation △f=4kHz. The command data sequence can be modulated by directly acting on one of the carriers to obtain a 2FSK signal. The signal is received by a combined antenna within a range of 100 kilometers on land and 300 kilometers at sea. The simulated waveform output after the signal passes through the frequency selection circuit and high-frequency amplifier circuit is shown in Figure 7.

4 Conclusion

This article presents a method for designing a receiving system that receives radio signals through a combined antenna. Combining the advantages of the combined antenna, the antenna receiving system can work in an all-weather state, thus avoiding the defect that the receiving system cannot work properly in bad weather conditions. The article also presents the signal receiving circuit (including the frequency selection network and high-frequency amplification circuit, etc.).