Car radio transmitters. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Car radio transmitters. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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When using radio transmitters in automobiles, the most difficult task is the implementation of the antenna. This article discusses the use of a car body as a hidden and coordinated receiving-transmitting antenna in relation to the ranges of personal (27 MHz) and service (150 ... 180 MHz) radio communications. Overview of Existing Types of Automotive Antennas. The minimum dimensions of the effective radiator must be at least L/4, i.e. 2,7 m for 27 MHz. With a decrease in size, its efficiency drops very sharply - in proportion to the square of the shortening. When the antenna is transmitting, its low efficiency irrevocably reduces the radiated power and, accordingly, reduces the radio range.

It is desirable that the antenna be hidden and inaccessible to influence on it from the outside (the hijacker will first break the external antenna, and the radio security device becomes useless). To solve this problem, as a rule, a short helical antenna is used, located inside the passenger compartment near the glass. It is actually a folded L / 4 pin, but since it is placed inside a metal body, its effectiveness is greatly reduced due to the shielding effect. In addition, the helical antenna has a very narrow bandwidth (tens of kHz), which requires careful tuning of the antenna and makes it impossible to use it in the entire frequency band.

In the range of 150 ... 180 MHz, L / 4 pins with a length of about 0,5 m are almost exclusively used. They are most suitable for almost all cases, except for the case when it is necessary to classify the presence of a radio channel on cars, for example, as part of a theft prevention and search for stolen vehicles and / or for covert radio communications. To solve the problem of stealth, a whip antenna is usually used, located inside the cabin near the glass. The main disadvantage of this solution is again the reduction in its effectiveness due to the shielding effect. In addition, the presence of people inside the cabin leads to a noticeable mismatch between the antenna and the cable and, accordingly, a decrease in the radiated power, and there is a significant electromagnetic field in the cabin, which is harmful to health.

Let's formulate our requirements for a car antenna:

- high value of efficiency, exclusion of the shielding effect of the body, linear dimensions comparable to L / 4;

- secrecy and inaccessibility for damage from the outside;

- wide bandwidth;

- minimal influence on the antenna tuning of people and objects in the cabin and around the car;

- powered by a matched coaxial cable, which allows you to connect any transmitters and radio stations.

Theoretical background

Since the body of the car is metal, an antenna is needed that does not "overcome" the body, but uses it in its composition. And such a class of antennas exists. These are almost unknown in civilian equipment (and widely used in military) slot antennas (SHA), which are a narrow (0,001 ... 0,02 L) and long (0,2 ... 1,5 L) slot in a conductive screen of infinite dimensions, excited in the center. To understand the operation of an SC, the principle of duality is important, which states that the radiation of a slot in an infinite conducting screen is completely equivalent to the radiation of a metal tape of the same length and width as the slot, and placed in free space. The difference is only in the value of the input resistance and in the rotation of the radiation polarization plane - a horizontal slot radiates a vertically polarized wave.

In reality, the surfaces on which the SA are located are limited and irregular in shape, however, the resulting changes are not too large, and in most cases it is possible to find engineering solutions suitable for practice. So, for example, SHAs are successfully placed on the hulls of aircraft, missiles, metal towers, etc.

Directional pattern (DN) SChA depends on the following factors: the length and width of the slot, its shape, screen size. For a qualitative assessment, we give the following remarks: with a flat slot, its RP is usually bidirectional, similar to an equivalent dipole, and if the slot has curved edges or an annular shape, the antenna turns out to be weakly directional without pronounced minima and maxima in the radiation pattern.

Using the car body as a scooter

Our task is as follows - to find a constructive gap in the car with a total length of at least 1,5 ... 2 m and ensure its excitation. You can find several such places on any car, but it is desirable that the slot configuration be approximately the same for different types of cars so that the antenna itself, its parameters and the cable matching device are unified.

It is also necessary to minimize interference with the vehicle structure when making the slot excitation device.

In view of the foregoing, a gap was selected between the trunk lid (VA32101-2107 and similar) or the back door (VA32108, 2109, AZLK2141 and similar) and the car body. To ensure excitation, it is necessary to insulate the trunk lock latch as shown in Fig. 1, while the input power points of the slot are standard bolts for fastening the trunk lock latch, screwed into the body (body terminal), and the lock latch isolated with additional gaskets from the fastening bolts, having mechanical and electrical contact with the trunk lock and trunk lid (signal output). The specific method of insulation may be different - depending on the design features of the car.

Car radio transmitters

Figures 2 and 3 show experimental dependences of SWR on frequency for some vehicles. One can see the presence of several natural resonant frequencies of the gap. If the operating frequency coincides with one of the resonances, then it is enough to connect the cable to the power points of the slot, and the SC is ready! But such a coincidence is rare. As a rule, the input impedance of the slot at the operating frequency is complex, which excludes the direct connection of the supply cable. Since the dimensions of the gap are determined by the design features of the car and we, of course, cannot change them, to ensure the SWR in the cable is close to 1, a matching device (CD) is used, which ensures the transformation of the complex resistance of the SC into active, equal to the wave impedance of the cable used. SU is placed directly near the feed points of the slot.

Car radio transmitters

In the range of 27 MHz, the gap reactance is inductive, and the active resistance lies within 10 ... 15 Ohms. The control system shown in Fig.4 is used. Broadband transformer TV1 1:4 (on ferrite, providing operation at a given power and frequency) matches the cable resistance of 50 ohms with the active part of the radiation resistance of the ShchA, and the capacitors C1, C2 provide compensation for the inductive component. Exact matching is achieved by a single adjustment - capacitor C2. With a 50 ohm cable at a resonant frequency, SWR is not more than 1,3. SHA band in terms of SWR <2 - 500..800 kHz. Eto-SU provides a good match with most types of vehicles, including foreign cars.

Car radio transmitters

In the range of 150...180 MHz, another CU is used (three trimmer capacitors and two frameless coils), the parameters of which are chosen to ensure coordination with any type of car. Unlike the 27 MHz SU, in this case, careful individual adjustment of all three capacitors to a specific type of car and operating frequency is required. The antenna is matched with a 50 Ohm cable (at the resonant frequency SWR <1,3) in the range of 150 ... 180 MHz (center frequency tuning limits). SHA band (at the center frequency) in terms of SWR<2 is 5...8 MHz.

Results

In both described ranges, the following were noted:

- almost complete absence of influence on the parameters of the SC of objects in the car (people, cargo) and around (precipitation, other cars, etc.);

- low level of electromagnetic field strength in the cabin;

- vertical polarization of the radiation, which goes well with most car antennas, which also have vertical polarization.

In the range of 27 MHz, in the horizontal plane it is close to an ellipse with a small (about 3 dB) emission maximum, backward along the vehicle axis. The communication range with the SC is the same as with a spiral antenna 20 cm high or with a whip antenna 0,6 m high with an inductance coil, which were placed on the roof of the car.

In the range of 150 ... 180 MHz, the DN in the horizontal plane depends on the type of vehicle and the operating frequency and is a complex multi-leaf structure without deep dips with a small maximum of radiation back. The communication range with the SC is 40%...50% of the range when using the L/4 pin on the car roof. The worst parameters are explained by the location of the antenna feed points closer to the ground and the poor conductivity of the body iron in this range.

Using the described principles, with an appropriate choice of connection points to the body and the control system circuit, it is possible to ensure the operation of the car body of any brand as a resonant antenna at any point in the 10 ... 200 MHz range.

Authors: A. Boglov, I. Goncharenko (EU1TT), Minsk; Publication: N. Bolshakov, rf.atnn.ru

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