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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Two designs for the 430 MHz band. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications Antenna unit The maximum output power of small-sized portable transceivers is, as a rule, small, therefore, when operating in stationary conditions, and even with a long reduction cable that introduces large losses, this drawback can significantly reduce the range of stable radio communication. This happens due to a decrease in sensitivity during reception and a decrease in the already low output power of the transmitter signal supplied to the antenna. This problem can be solved by installing a special antenna unit (AB) near the antenna or (worse) next to the transceiver. It consists of a low noise amplifier (LNA), which operates during reception, and a power amplifier (PA), which operates during transmission. AB allows you to significantly increase the sensitivity of the antenna-transceiver system with large losses in the reduction cable and more efficiently use the allowed output power, since it goes directly to the antenna. It is advisable to use it with transceivers having an output power of up to 0,1...0,5 W . The battery supply voltage is 11...12 V, so it can be successfully used in a car. A similar device for the 2 m range has already been described in the magazine "Radio" (Nechaev I. Antenna unit for the 2 m range - Radio, 2001, No. 2, p. 64,65). A similar block for the 430 MHz band is described here. The AB diagram is shown in Fig. 1. It contains an input low-noise amplifier (LNA) based on a VT3 gallium arsenide field-effect transistor, which allows for high sensitivity and a large dynamic range of the receiver. An L6C29 circuit is installed at the LNA input, tuned to the center frequency of the range. Capacitor C3O matches the LNA input with the antenna connected to connector XW2. Diodes VD9 and VD10 protect the transistor from the transmitter signal or other powerful signals, for example, from neighboring transmitters, interference, lightning, etc. The DC mode of the transistor is set by the automatic bias resistor R9. The transistor is loaded onto a low-pass filter C10L3C11, from the output of which the signal goes through a section of cable W1 to the female coaxial connector XW1 and then to the reduction cable. Diodes VD7, VD8 protect the field-effect transistor on the output side. The supply voltage is stabilized by an integrated voltage stabilizer on the DA1 chip and is additionally filtered by elements C13, C16, L4.
The power amplifier (PA) is assembled on the DA3 amplifier module. It delivers an output power of 5 W with an input power of only 20...40 mW and a supply voltage of 9...11 V. A control device is assembled on diodes VD3, VD4 and transistors VT1, VT2 - high-frequency VOX, which turns the PA into active mode when receipt of a signal from the transceiver transmitter. The supply voltage is constantly supplied to the PA, but in receive mode (RX) it does not consume current, since there is no voltage at the output power control input (pin 2). In transmission mode (TX), this voltage is stabilized by an integrated stabilizer on the DA2 chip. The input matching circuit is assembled on elements C19, C20, L5, and the output low-pass filter with a cutoff frequency of about 7 MHz is assembled on elements L31, C9, L32, C10, L500. This low-pass filter additionally suppresses the second harmonic of the output signal by 35...40 dB. The supply voltage to the battery can be supplied either through the low-frequency connector XS1 and diode VD2 using a special cable, or via a reduction cable through the high-frequency connector X\L/1, low-pass filter L1C1 and diode VD1. Switching RX/TX modes can also be accomplished by applying a constant voltage of 5...12 V to socket XS1. The current consumed by the control circuit does not exceed 1 mA. Switching of the LNA and PA is carried out using pin diodes VD5, VD6, VD11, VD12 and two sections of cable W1, W2 with an electrical length of X/4. AB works as follows. When supply voltage is applied, it is in RX mode. The Pin diodes are de-energized, so the signal from the antenna socket XW2 through the cable section W2 goes to the input of the LNA. The amplified signal from its output goes through section W1 to socket XW1 and then to the reduction cable. The PA consumes practically no current, and the LNA consumes a current of 25...30 mA. When the transceiver is turned on in TX mode, its signal is rectified by diodes VD3, VD4 and transistors VT1 and VT2 open. The positive voltage through the DA2 microcircuit is supplied to the output power control input of the amplifier DA3 and through current-limiting resistors R4, R7, R8, R11, R12, R14 to the pin diode chains VD5, VD6, VD11, VD12. Current begins to flow through the pin diodes, and their resistance decreases to several ohms. The signal from the transceiver transmitter through the diode VD5 is supplied to the input of the PA DA3, at the same time a section of cable W1 with an electrical length of λ/4 is short-circuited at the end with a practically low resistance of the diode VD6. The resistance of this section at the connection point (C5, VD5) turns out to be large and does not have a significant effect on the transceiver signal. The output signal of the PA through the diode VD11 is supplied to the antenna connector XW2, and the cable section W2 is also short-circuited by the diode VD12 and does not have a significant effect on the output signal. Most AB parts are placed on a printed circuit board made of double-sided foil fiberglass, a sketch of which is shown in Fig. 2. The second side is left metallized and connected with foil around the perimeter with the metallization of the first side. In addition, both sides are connected to each other by short pieces of wire passed through the holes shown in circles in the figure.
The board is placed in a metal case with a conductive surface; it must be secured with screws around the perimeter in several places (the more, the better). The case simultaneously serves as a heat sink for the DA3 amplifier module. RF connectors are installed on the walls of the housing. In addition to those indicated, the device can use the following parts: DA3 amplifier module - M57714M-01, M57797MA-01, M67705M-01, M67749M-01, but they have a housing of a different design, and the topology of the printed circuit board conductors will have to be changed. Transistor VTI - KT315, KT312, KT3102 with any letter index, VT2 - KT814A...G, KT816A...G, KT836A, VT3 - ATF-10136. The latter has a noise figure of 0,4 dB at a frequency of 500 MHz, so the LNA assembled on it has very high sensitivity. You can replace this field-effect transistor with KP325, 2P602 and similar ones, but the results will be worse. Diodes VD1, VD2 can be replaced with KD212, KD257 with any letter indices, VD3, VD4 - with KD419, 2A120 with any letter indices. Trimmer capacitors - KT4-25, constant polar ones - tantalum for surface mounting (CHIP), the rest - K10-17v, K10-42 or similar imported ones, also for surface mounting. Fixed resistors - RN1-12, size 1206, tuning resistor - 3303W-3 from Bourns or similar, you can also use SPZ-19, SPZ-28. All coils are wound on a mandrel with a diameter of 3 mm, L1, L2, L6, L9 - with PEV-2 0,6 wire and contain 8, 1,5, 1,5 and 1,5 turns, respectively. L7, L9, L10 are wound with PEV-2 0,4 wire and contain 2,7, 3,7 and 2,7 turns, respectively. Chokes 12, L4, L6, each contain 10 turns of wire PEV-2 0,2. Cable sections W1 and W2 must have an electrical length of A/4. They are made of thin cable RK50-1-22, 12 mm long; during installation they must be rolled in the form of a spiral. You can use any suitable high-frequency connectors, but all connections must be made to a minimum length or using a coaxial cable. The low-frequency socket can be anything that allows current through the contacts up to 2 A. AB establishment begins in receive mode (RX). A supply voltage of 10...11 V is supplied to the battery and the functionality of the voltage stabilizer on the DA1 chip is checked; its output voltage should be about 3 V. By selecting resistor R9, the recommended drain current of the field-effect transistor is set, in this case 25 mA. Next, capacitors C10 and C11 adjust the LNA output circuit to the maximum transmission coefficient, and capacitors C29 and C30 adjust the input circuit to the maximum transmission coefficient with a minimum SWR at the center frequency of the range. Then the adjustment is carried out in transmission mode (TX). To do this, the resistor R13 slider is set to the lowest position according to the diagram, and an ammeter is connected to the power circuit. A matched load and an RF voltmeter are connected to socket XW2 to monitor the output voltage. The supply voltage (10... 12 V) is supplied to contacts 1 and 2 of the XS1 socket. In this mode, a current of 180...200 mA will flow through the pin diodes. The voltage at the DA2 output should be about 3 V. Using resistor R13, increase the current consumption by 30...50 mA - this will be the quiescent current of the DA3 amplifier module. Next, a signal with a frequency of 1 MHz and a power of 435...2 mW from a transceiver or RF generator is supplied to the “Tr” input (connector XW5). Capacitors C19, C20 achieve maximum output power. The input signal power is increased to 20...40 mW, and the setting is repeated. After this, you need to make sure that the input circuit is tuned to resonance. To do this, ferrite and brass cores are alternately brought to the L5 coil, and in both cases the output power should decrease. If this is not the case, then you will have to change the number of turns of this coil. Finally, the operation of the VOX system is checked. To do this, the supply voltage is turned off from pin 1 of XS1. When a signal with a power of 20 mW or more is applied to the input, the battery should automatically switch to TX mode. If you plan to operate the battery next to the transceiver, it is advisable to supply power through the XS1 female connector. Then from the circuit (see Fig. 1) you can exclude parts L1, C1, VD1, as well as LNA elements: DA1, VT3, VD7 - VD10, C9-C11, C13, C16, C18, C21, C22, C29, C30, L3, L4, L6, R9, R10. The right (according to the diagram) terminal of capacitor C7 is connected to VD12 with a piece of cable with an electrical length of X/2. The appearance of the AB is shown in the photo (Fig. 3).
The adjusted block has the following parameters. With a supply voltage of 12 V and an input signal of 20 mW, the output power was 3,8 W (current consumption 1 A), with an input power of 80 mW, the output power was 7,5 W (current 1,4 A). LNA gain - 21 dB, SWR at the center frequency - 1,1, in the range 431...438 MHz - no more than 1,5, in the range 429...440 MHz - no more than 2. The output voltage of the LNA with a decrease in the transmission coefficient by 1 dB was 290 mV. The bandwidth at the -3 dB level is 18...20 MHz, the sensitivity together with the FM transceiver at a signal-to-noise ratio of 12 dB turned out to be 0,08 µV. Adder-divider VHF range When constructing VHF antenna arrays, a necessary element is a power combiner-divider, or splitter, which ensures matching with the transceiver, addition of signals received by array elements, or uniform division of signal power between them during transmission. Readers are presented with a simple design of such a VHF power adder-divider for the 430 MHz range. The described device is designed to connect four antennas with their own feeders, each with a resistance of 50 Ohms, to one coaxial transmission line with a characteristic impedance of also 50 Ohms. In the VHF range, such devices are often made on the basis of quarter-wave transformers. Moreover, if the antenna feeders are connected in parallel, then their total resistance (Za) will be 12,5 Ohms. Then, to match the antenna feeders with a transmission line having a characteristic impedance Zl = 50 Ohm, it is necessary to use a quarter-wave segment with a characteristic impedance Ztr \u1d (Za Zl) 2/12,5 \u50d (1 2) 25/XNUMX \uXNUMXd XNUMX Ohm. It is possible to produce a line with such a characteristic impedance by connecting in parallel two sections of coaxial cable with a characteristic impedance of 50 Ohms. The adder-divider circuit is shown in Fig. 4. It contains a coaxial socket XW1, to which the reduction cable going to the transceiver is connected, two pieces of coaxial cable W1 and W2 with an electrical length of λ/4 and four pieces of coaxial cable W3-W6 of arbitrary length, at the ends of which female coaxial connectors XW2 are installed -XW5. Antennas - array elements - are connected to these connectors through pieces of 50-ohm cable of the same length. Although the device is made from pieces of coaxial cable and RF connectors, it has a rigid and durable design. This was achieved using cable RK50-2-25. A copper tube with a diameter of 3 mm is used as its external conductor. The internal cable insulator is made of fluoroplastic (shortening factor - 1,42). This cable has no external insulation and can be bent (carefully) and soldered (without overheating) anywhere without fear of the insulation melting. The design of the device is shown in Fig. 5. When manufacturing it, you first need to prepare two sections 2 of cable with an electrical length of λ/4 (for the 430 MHz range, the length of the sections will be 122 mm along the outer conductor). The central conductor should protrude 7...10 mm on each side. These segments are mounted (by soldering) into connector 1 and soldered to each other along the entire length. Then prepare four identical sections 6 of cable 40...70 mm long with connectors 3 at one end and with a central conductor protruding a few millimeters from the other end. All six segments are folded close to each other, bandages 4 made of tinned wire are applied and soldered together. Then the central conductors are soldered. The length of all central conductors at the soldering point should be minimal.
To remove the outer copper conductor of the cable, it must be sharpened in a circle with a file, carefully bent, broken and removed from the internal insulator. The place where the center conductors are soldered should be sealed with epoxy glue. It is advisable to solder a metal cap 5 on top for protection and shielding. The device uses the following parts: coaxial connector XW1 - SR-50-163FV, connectors XW2-XW5 - SR-50-725FV. These connectors are suitable when using cable RK50-2-22. But you can also use other 50-ohm connectors that allow you to install the PK50-2-25 cable, while the XW1 connector must allow the installation of two cable sections at the same time. A similar design can be made for the frequency ranges of 144 and 1300 MHz. The parameters of the manufactured prototype (see Fig. 6) when connecting loads with an SWR of no more than 2 to the XW5-XW1,1 sockets were as follows: the minimum SWR was 1,12 at a frequency of 430 MHz, in the frequency range 405...447 MHz the SWR was not exceeded 1,2, and in the frequency range 368...485 MHz -1,5. Author: I. Nechaev (UA3WIA), Kursk
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