Antenna switch for 144 MHz. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas
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It is known from the theory of long lines that a quarter-wave segment of a coaxial cable, short-circuited at the end, has an infinitely large resistance at the resonant frequency. Due to the lack of high-frequency coaxial relays, I have been using this property for a long time to switch the antenna amplifier, Fig. 1.
In the receive mode, the antenna amplifier and the relay windings are energized. Quarter-wave segments connected to the input and output of the amplifier are a continuation of the connecting cable and with Rin to Rout equal to the cable impedance, they do not affect the feeder parameters. Quarter-wave segments intended for "bypass", short-circuited segments from the side of the antenna and transceiver have a large resistance. Their amplitude-frequency characteristic (AFC) is shown in Fig.2.
Thus, in addition to purely insulating properties, they also have filtering properties, being a fairly effective filter with steep slopes due to the high quality factor at the resonant frequency.
In transmit mode, the power is removed from the amplifier, and its input and output are grounded, which provides an almost complete guarantee of protection against the penetration of the RF signal of the transmitter. In addition, the quarter-wave segments connected directly to the input and output of the antenna amplifier are notch filters for the transmitter, and the other two segments connected in series have a length of 0,5 lambda and have the property of a 1: 1 impedance transformer without affecting feeder parameters.
Any available relays can be used as switching relays. The main requirement for the relay is the smallest possible intercontact capacitance. Good results are obtained when using RES-34, RPV-2/7, etc. with output power up to 200 W.
The manufacturing option of the antenna switch is shown in Fig.3.
Four quarter-wave lengths of cable, 342 mm long for a frequency of 144,1 MHz, are placed in an dural tube 340 mm long and 30 mm in diameter. One end of the pipe is rolled into a small box with two high-frequency coaxial connectors (on the right in the figure), separated by a shield. The other end is rolled into a large box (on the left in the figure), in which there are two relays and a power amplifier board, Fig. 3. It is necessary to take into account the requirements for the installation of high-frequency devices and be sure to separate the input circuit from the output circuit with a screen.
On the same principle, based on the property of a quarter-wave segment, relayless switching of a 20-watt power amplifier installed in a car is carried out. Instead of high-frequency relays, diode switching is used - anti-parallel switching of KD503B silicon diodes, Fig. 4.
In the HF transmission mode, the transceiver signal opens the keys and the quarter-wave segment from the input side of the RA turns out to be short-circuited, and from the side of the antenna it has a large resistance. In the reception mode, the power of the signal coming from the antenna is not enough to open the keys and the signal through the quarter-wave segment passes unhindered to the input of the transceiver. With a very large signal, the diode switches perform a protective function.
The power amplifier is assembled on transistors KT925A and KT925V according to the scheme published in "R-D" No. 1-96.
100 mW is supplied to its input from a "manual" transceiver, and about 50 W is obtained at a load of 20 ohms. The amplifier works on a shortened quarter-wave magnetic antenna of the MW range with a partially cut off upper part - above the extension coil.
Author: S.Ksenofontov, ES4BW, Kohtla-Jarve; Publication: N. Bolshakov, rf.atnn.ru
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