Antenna for radio security alarm. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / VHF antennas

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Automotive radio security devices operate at a frequency of 26,945 MHz. The radio watch contains two blocks - transmitting and receiving. The structure of the transmitting unit includes the guard itself with the necessary set of sensors, an encoder and a transmitter with an antenna mounted on a car. The receiving unit includes an antenna, a receiver, a decoder and an alarm sound generator.

When the auto-guard sensor is triggered, the transmitter starts emitting a radio signal modulated by a pulse code generated by the encoder. The receiver with a decoder extracts "its" signal from the mass of signals and turns on the alarm signal generator.

The use of protruding transmitting, for example, whip, antennas in car radio security alarm systems is practically excluded due to the possibility of a malfunction of the system in case of mechanical damage to a clearly visible and easily accessible protruding external antenna.

The range and stability of the operation of radio security systems to a certain extent depends on the correct choice of the type of antenna-feeder system and the quality of its implementation.

The requirements for the radiation pattern and, consequently, for the antenna gain are determined by the fact that the location of the receiver in the horizontal plane is arbitrary, and in the vertical plane it is limited by a small angle between the earth's surface and relatively low-lying antennas. That is, the antenna pattern should be close to circular in the horizon plane and narrow vertically.

(click to enlarge)

Since at a low location of the antennas relative to the ground, the field at the receiving point has an interference character, i.e. is the sum of the fields of the direct and the rays reflected from the earth, the choice of radiation polarization turns out to be very important. With horizontal polarization, the depth of the interference dips is greater than with vertical polarization. Therefore, it is expedient to use vertical polarization in low-level radio communication [1].

Because the car alarm system antenna is placed inside the vehicle's conductive body (usually near windows), its radiation pattern is significantly different from that of the same antenna in free space. This is due to the fact that the radiating antenna will create currents on the conductive surface of the car body, and the radiation of these currents, together with the field of the antenna itself, forms the resulting diagram.

The interaction of these two fields leads to the fact that in part of the space the field is weakened, and in part it increases.

In the long-wave section of the VHF range, various options for slot antennas are effective. Typically, a slot antenna is a narrow slot of length L / 2 cut in a plate that is flowed around with current. Coordination of the gap with the space is made by adjusting the width of the gap. When the slit is located horizontally, the radiation polarization is vertical.

However, for this section of the range, slot antennas in the classical version are bulky and structurally inconvenient. Therefore, it is advisable to use wire analogues of slot-type antennas [2, 4]. The excitation of such antennas is carried out by a symmetrical cable through a loop, and therefore such antennas are called loop antennas. In order to simplify the power circuit, it is more practical to use an unbalanced connection of the transmitter using a coaxial cable [1]. The electrical extension of the radiating part of the antenna is achieved by additional loops W2 and W1, as well as by the capacitor C1 (Fig. 1). Capacitor C50 is included in the points l of the antenna, corresponding to the maximum antiphase voltage, which is equivalent to an increase in the terminal capacitance of the elements. It should be noted that such an antenna has a higher efficiency than a whip antenna with extension inductors. In this case, it is possible to obtain acceptable sizes of loops and input resistance, with a resonance close to XNUMX ohms.

An antenna similar to [4] is made of MGTF 0,3 wire. Loops W1, W2 are made from the same wire. The conductors of the loops are parallel and close to each other. The antenna wire is located under the rubber seal of the rear passenger compartment window. Loops W1, W2 are folded three times in length and, together with the capacitor C 1, are placed in a dielectric tube attached to a spacer made of fiberglass with a thickness of 1 mm. The spacer is located vertically in the middle of the rear window and is fixed in a rubber seal. Since the shortening capacitor is included in the voltage antinodes, with a transmitter power of 1 ... 2 W, the voltage amplitude across the capacitor C1 reaches several tens of volts. This must be taken into account when choosing the type of capacitor. Two stub antennas were manufactured and tested for VAZ 2107 and ZAZ 968M vehicles (dimensions in Fig. 1 are shown in brackets). The adjustment is made according to the maximum radiation (minimum SWR) by rotating the rotor of the capacitor C1.

The results of testing antennas in the form of a radiation pattern according to the level of stable operation of the guard in an open area are shown in fig. 2. The diagram of the manufactured stub antennas is indicated by the number 1, and the number 2 is the diagram of the ring loop antenna of the "Signal-RK" kit. The polarization of the radiation of the stub antenna is vertical.

From fig. 2 shows that the stub antenna, located along the inner perimeter of the rear window inside the cabin, is not only hidden, but also provides a greater range. Structurally, stub antennas are simple, reliable, cheap and easy to manufacture. When the stub antenna is not being used for its intended purpose, it can be used as an efficient receiving antenna for a VHF radio or radio, eliminating the need for an external whip antenna.

Literature

1. Black F.B. Propagation of radio waves. - M.: Sov. radio, 1962.
2. Eisenberg G.3. and other VHF antennas. T.1. - M.: Communication, 1977.
3. Stakhov E.A. Antenna: Auth. witness. No. 369853 dated 23.09.71/XNUMX/XNUMX.
4. Boglov A., Goncharenko I.//Radio amateur. - 1991. -N8.-C.43.

Author: E. Stakhov, Grodno; Publication: cxem.net

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