Transistor transmitters for the 2m band
In order to achieve a power of about 1 watt with a crystal-controlled 2m transistor transmitter, a three-stage arrangement is sufficient. The prerequisite for this is that a crystal oscillator in the 48 MHz or 72 MHz range is used. So a single frequency multiplier stage is sufficient to be able to drive the PA on the transmission frequency in the 144 MHz range. So the oscillator is followed by a doubler or tripler and behind that directly follows the output stage. While the circuitry is much more complex, as for a tube transmitter of this power class, the transistor transmitter simplifies the effort in the power supply. A DC voltage of 12 volts is sufficient, so that such a transmitter is ideal for mobile and portable use.
Such transmitters were offered around 1970 by various manufacturers such as Semcoset / Lausen KG, CTR (Conrad) und HAEL (Hanschke Elektronik) as ready-made and pre-adjusted assemblies for self-build of VHF amateur radio devices. Initially, these were only designed for the amplitude modulation that was predominantly used in the 2m band at the time. With the increasing spread of frequency modulation and the advent of relay radio operation, models came onto the market that were designed for both AM and FM. In terms of circuitry, these transmitters differ only slightly from one another. The circuit of a typical HAEL transmitter shows an overtone crystal oscillator (48MHz), a frequency tripler in a basic circuit, an RF output stage, a two-stage microphone amplifier and a collector current modulator for AM.
For FM, the modulation is done by a capacitance diode with which the crystal frequency is pulled back and forth by the microphone signal. With AM, the collector current modulation has the consequence that the carrier power with AM is reduced to around 300mW, while with FM it is around 1 watt. The method with collector voltage modulation by means of a modulation transformer used in many transmitter assemblies from Semco, however, enabled the full carrier power to be available with AM as well. But the transistor 2N4427 is unsuitable for this because of its lower voltage strength, since more than twice the supply voltage can occur at the collector in the modulation peaks. With a view to that, for example the 2N3866 or even better the 2N3553 could be used for this. Taking into account the increase in power when modulating, a more powerful version must be selected for 1 watt carrier power, since the power increases to up to 4 watts at 100% modulation in the peaks.
As a result of today's higher demands on the cleanness of the transmission signal, a combined bandpass and lowpass should be inserted at the transmitter output of such transistor transmitters. Otherwise, relatively strong harmonics and remnants of the 48 MHz signal respectively the 96 MHz harmonics will reach the antenna. Also it is advisable to provide reverse polarity protection. If plus and minus are interchanged, this can lead to the destruction of the as frequency tripler working driver stage. The base grounded circuit design in this case leads to a not inconsiderable current flows through the collector-base diode path of the transistor.
As can be seen, the transmitter is designed for three switchable crystal controlled channels. As with many models from other manufacturers, there is also a VFO connection, so that independent, frequency-variable operation is possible on any frequencies in the range from 144 to 146 MHz. A VFO suitable for this was again manufactured by the Semcoset company under the name Varios 48 as a ready-made module for self built devices. Instead of the otherwise used crystal frequency in the 48 MHz range, this circuit generates a tunable frequency in this range and feeds it to the oscillator transistor, which then only works as an amplifier. The amplitude modulation takes place in the same way as with crystal operation. For FM, on the other hand, the modulation should take place directly on the VFO. As the circuit of the VFO shows, the amplified modulation signal is fed to the here existing FM demodulator with the capacitance diode BA149. The modulation quality is much better with FM in VFO mode than with crystal mode.
In the interests of greater frequency stability, the actual VFO, which is equipped with a BF115 transistor, operates at 24 MHz. This frequency is doubled in the subsequent stage with the BF167 and then selectively amplified in the subsequent amplifier stage with the BF173. In this way, a 48 MHz signal with low spurious waves and low harmonics is ensured.
When, towards the end of the 1970s, FM operation had generally prevailed over AM on the 2m band, there was still a 6-channel transmitter module from HAEL, the SB-6/1, which was intended exclusively for frequency modulation. Apart from the larger number of quartz slots - now in the smaller size HC-25/U instead of the HC-6/U - it hardly offered anything new in terms of circuitry. By the way, suitable crystals for such transmitters can still be ordered today (e.g. from quartslab.com UK). These are custom-made products that are manufactured according to your own frequency requirements. You can get the crystals for any transmission frequency in the 2m amateur band.
Based on experiments with such a module, which was only designed for FM, I rebuilt the circuit several times with slight changes and in different constructive ways. Regardless of the construction, the resulting circuit shown below showed a surprisingly high level of replica security, provided that attention was paid to RF-compatible construction (f.e. short cable routing, sufficient spacing between the resonant circuits, arrangement of adjacent coils at 90 degrees). The transmitter worked best in a kind of chamber construction made of epoxy resin plates soldered together and coated on both sides with copper, such as are available as a raw material for the production of printed circuit boards. In one case I added an additional PA to the circuit so that about 4 watts of RF power could be achieved. This stage in principle was constructed the same as the last stage in the circuit shown. Only the resonant circuits had to be dimensioned a bit differently. In addition, to adapt the extremely low-resistance input, an air coil with one turn had to be inserted to the base connection of the output stage transistor. Type 2SC1971, for example, is suitable as a transistor for such an additional PA in the by that four-stage transmitter.
The modulation of all such transmitters with 48 or 72 MHz crystals was mostly somewhat distorted, since the frequency of the overtone oscillator can only be pulled very little. It should be noted, however, that a larger frequency deviation were used for FM in the 2m band at that time. At first they worked with a 50 kHz grid and then, very soon, for a long time with a 25 kHz grid. With today's 12.5 kHz grid and the frequency deviation adapted to it, significantly better results can be achieved with such transmitters. The modulation quality can be significantly improved if an limiter amplifier for the audio signal is inserted instead of the second stage of the microphone amplifier. It must be set in such a way that the limitation starts before the varactor modulator causes distortion.
In general, with FM transmitters it should be ensured that the LF modulation signal is limited, since the maximum intended frequency deviation should not be exceeded regardless of the properties of the FM modulator. It must match to the characteristics of the receivers used by the other stations. The IF bandwidth at the receiver end is decisive. In order to limit the maximum modulation deviation to a suitable, fixed value, a control circuit like an ALC (Automatic Level Control) would have to work with practically no delay. Otherwise, the FM signal can leave the range of the IF bandwidth during the control time. As a result strong non-linear distortion occurs, which affects the intelligibility. For these reasons, you will almost always find a modulation limiter in more modern FM transmitters. Such circuits were often referred to as speech clipper in the amateur radio literature of the tube era. While they are suitable for increasing the signal density for SSB and are therefore only suitable for achieving DX or contest modulation, they are also ideal for the close range with FM with the overall better sound properties there.
The circuit shows such a modulation limiter built up with separately transistors. With the two-stage, DC coupled preamplifier, the operating point is adjusted so that the signal is symmetrically limited for both half-waves in the event of overdrive. If the input sensitivity is not sufficient for the microphone used, there is no limit at all. In this case, an audio preamplifier should be inserted at the input. The limiter amplifier is followed by an active low-pass filter, which suppresses the harmonics of the audio signal caused by the limitation. This keeps the bandwidth of the modulated RF signal small. The microphone gain (Mike Gain) is set with the trimmer on the input side and the frequency deviation on the output side. Microphone sensitivity, frequency deviation and limiting symmetry can thus be set independently of one another.
I took over the circuit mainly from a Lorenz VHF radio system of the type SEM-57 and used it with good success in other homemade transmitters or transceivers. A hand-wired assembly of such a circuit is shown here.