Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The published article describes the design of a matched load that provides good performance in a wide frequency band (up to several gigahertz). It is easy to repeat and assembled from commonly used parts.

When setting up and repairing VHF equipment, a matched load is often required, connected to the output of the transmitter (transceiver) and used in SWR measurements. Matched loads of industrial production, including those extracted from old measuring instruments, in most cases are designed for low power dissipation, which limits their scope.

Therefore, for radio amateurs, the design of a home-made matched load operating in a wide frequency band is of interest. For the manufacture of such a load, it is necessary to use only suitable resistors with a small intrinsic inductance and capacitance, otherwise the frequency range of the device is limited. Thick-film "chip" resistors for surface mounting PH1-12 and similar have very good frequency properties. They are widely used in a variety of electronic devices. These resistors are manufactured with a dissipation power of 0,125, 0,25 and 0,5 W, therefore, to manufacture a matched load with a dissipation power of 5 W (permitted power on VHF), it is necessary to use from 10 to 40 resistors. When such a number of resistors are connected in parallel or in series, it is almost impossible to obtain a low SWR value over a wide frequency range.

The way out of this situation can be a combination of parallel and series connection of resistors. To obtain a low SWR value, it is advisable to apply the circuitry of resistive attenuators. For them, there is also a simple method for calculating the resistance of resistors. The block diagram of such a load is shown in fig. 1. The device consists of several resistive attenuators connected in series, assembled according to a U-shaped circuit. If you use resistors RN1-12 with a power of 0,25 W, then as a resistor R4 with a total power dissipation of 1 W, you must use four resistors connected in parallel. In this case, you can limit yourself to four attenuators, each of which also dissipates 1 W. Therefore, each attenuator can be assembled using four resistors, for a total of 20 resistors.

If you correctly calculate the attenuation of the attenuators, then the dissipated power will be distributed between the resistors approximately equally. This can be achieved if the damping of the attenuators is distributed as follows: 1; 1,5; 2 and 2,5 dB. The circuit diagram of the load is shown in fig. 2. Resistor R9 is made up of four resistors connected in parallel, and all the others are made up of two. The calculated resistances and the nearest resistor values ​​(from the standard range) are shown in the table.

Structurally, the load is made as follows (Fig. 3). On a tinned metal plate 1 with a thickness of 1,5 ... 2 mm from a material with good thermal conductivity, a strip 2 of foil fiberglass is soldered or glued with a small amount of glue. Resistors R2, R4, R6 and R8 are installed on it, the remaining resistors are installed between the strip and the plate. As close as possible to the strip, cable 3 is soldered onto the plate with an outer braid, at the other end of which a coaxial connector (plug or socket) of the required type is mounted. It is desirable to use the cable with fluoroplastic insulation. The author used a semi-rigid cable PK50-2-25 and an SMA type connector (plug). The plate on the side where the resistors are installed must be closed with a metal cover, and on the other side it is desirable to install a radiator (Fig. 4).

The device uses resistors RN1 -12 of size 1206. They allow operation at temperatures up to 125 ° C, so the load can dissipate power up to 5 W for a long time, and several times more for a short time. If you use resistors with a power of 0,5 W, then the total power dissipation of the load will be 10 W. The attenuator resistor values ​​for input and output resistance of 50 ohms can be determined by the formulas: R1 = R3 = 50(K2-1)/(K2-2K+1); R2 \u50d 2 (K1-2) / 2K, where KXNUMX \uXNUMXd Pin / Pout.

The experimental characteristics of the layout were studied in the range from 1 to 5000 MHz in a 50-ohm path. In the frequency range of 1 ... 200 MHz, the SWR was no more than 1,05; 200 ... 1000 MHz - 1,05 ... 1,11; 1000...1500 MHz-1,11...1,15; 1500 ... 25 00 MHz - 1,15 ... 1,19; 2500...4000 MHz - 1,1 5...1,37; 4000.. .5500 MHz - no more than 1,5.

Author: I. Nechaev (UA3WIA), Kursk

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