Transistorized AM transmitters for the 10/11 m band

For small homebrew radios, this small transmitter is suitable for the 10m band. In principle, the use in CB radios or as a telemetry or remote control transmitter is conceivable, but this is not readily permitted. In amateur radio, the use as a telegraphy or beacon transmitter would be possible.

The transmitter is crystal controlled. It is suitable overtone crystals whose third overtone is at the desired transmission frequency. The oscillator works with a PNP transistor. This minimizes the expense of matching to the directly following PA stage, which is constructed with an NPN transistor. The transmitter output has a bipartite Pi low-pass filter, which significantly reduces the emission of harmonics.

The value of the achievable RF carrier output power of this small transmitter is on the order of about half a watt. The modulation takes place by coupling the audio signal to the emitter of the PA transistor. This variant, often referred to as emitter modulation, causes current flow angle control in the case of the RF output stage operating in C mode. As a result, it is possible to achieve a very useable and nicely intelligible amplitude modulation with little effort. As a modulation amplifier is a circuit suitable, that is actually intended to operate a small speaker. The required audio power is much smaller in this type of modulation, as the RF power of the transmitter. If the power of the audio amplifier is too large, a resistor should be inserted in the modulation line to avoid overmodulation. Otherwise the output of the used modulation amplifier is connected directly from the output electrolytic capacitor to the modulation input of the transmitter. With a suitable switching arrangement, which can be done for example by means of a multi-pole switch or a relay, the modulation amplifier can be used in a transceiver in the receive mode to drive the speaker.

With the measure instrument emitter current and modulation can be easily monitored. In a transceiver, the same instrument can be used as an S meter in receive mode. As the photo shows, such a transmitter can be built easily and without special components in a small space. The board dimensions are only 55 x 45 mm. A shielding tin between oscillator and power amplifier also serves as a heat sink for the RF power amplifier. It consists of copper sheet. The transistor is mounted on it by means of a plastic screw and with a mica plate isolated.

The coils are designed as 8 mm diameter air coils. They were made by winding over the shaft of a matching twist drill. The oscillator coil has 12 turns, the PA coil 16 turns, the PA side coil from the output filter 10 turns, and the antenna side 8 turns. The tapping of the oscillator coil is calculated at 3 turns from the ground-side end. In order to avoid mutual interference, the coils are each arranged at an angle of 90 ° to each other.

Three-stage transmitter for the 10m / 11m band

Similar two-stage transmitters, but without the NPN-PNP transistor combination and using coils with adjustable core, were used in my homebrew walkie talkies, which are presented here elsewhere. The three-stage transmitter shown here is essentially a replica of the circuit from an old CB radio device. It turned out to be particularly reliable. The required coils can be taken from a device with a corresponding circuit. With the specified values, a power of approx. 1 watt could be achieved with the transmitter.

When connecting the modulation transformer, it is important that the DC currents of the transmitter PA and the modulator output stage flows reversed through the transformer. The windings of the push-pull output transformer used, which came from a transistorized audio power amplifier had therefore to be connected in opposite directions. With another dimensioning can be achieved with the RF circuit of the transmitter, a carrier power of about 4 watts. In order to obtain a sufficiently powerful modulation, a push-pull modulator should then be used.

The photo shows the driver and output stage of such a transmitter. It is a self-made assembly from a DIY transceiver using etching technology. The relay on the far right is for switching the antenna from the receiver to the transmitter.

The output stage with its high-frequency grounded collector is very stable. It could also be connected following the two-stage transmitter indicated at the beginning.