Transmitter power attenuator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Filters and matching devices

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Recently, an increasing amount of imported radio transmitting equipment for both CBS and KB has been supplied to the CIS countries, the output stages of which are assembled on transistors. A long experience of "communication" with tube technology, which is not afraid of random antennas, has taught radio amateurs not to worry too much about matching the antenna with the transmitter output. As a result of this, when random antennas are connected, even through a matching device, which for some time its tuning to a real antenna does not provide an acceptable SWR, quite expensive and scarce transistors of the transmitter output stage may fail.

To exclude such "accidents", it is necessary to make it a rule to turn on the transmitter for tuning with a matching device only through an attenuator. It can be either built into the matching device or made separately, as shown in the figure, unless, of course, the transmitter has smooth power control.

In order to be able to make a long-term tuning of a hundred-watt transmitter, the dissipation power of R1 and R2 must be at least 25 W, R3 -30 W. They are recruited from MLT-2 resistors up to the value indicated on the diagram (without brackets - for a 50-ohm path, in brackets - for a 75-ohm path). The SWR of the TX attenuator will be close to 1:1 for almost all real cases of mismatch between the attenuator and the antenna, and in idle and short circuit modes it will not exceed 2:1, which is quite acceptable for the operation of transistor transmitters.

This attenuator can be used in any case and as the simplest "matching" device, for example, in case of an emergency failure of the main antenna and the need to work on a surrogate antenna, resigned, of course, to a weakening of 10 dB -10 times in power with a matched antenna. In the case of using an unmatched antenna, the loss can be either more or less than 10 dB.

On the HF bands, where random antenna mismatch and transistor failure occur most often, even in QRP you can make the necessary QSO's and still be absolutely sure of the safety of the transmitter. The attenuation of signals by 10 dB in the receive mode, as experience shows, in many cases can be compensated by the "forces" of the transceiver, for example, its UHF.

In many cases, there is no need to "pump" all the power of the transceiver into the RA, and reducing the power by its driver can increase the noise during transmission. The attenuator will avoid this. It will also be useful when setting up antennas with devices that require low power, such as an RF bridge, and when measuring radiation patterns.

Author: Igor Grigorov, RK3ZK, Belgorod; Publication: N. Bolshakov, rf.atnn.ru

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