Self-built 2m FM transmitter with tubes

Transmitters for the 2m amateur band, which were designed for portable operation, often worked with crystal control. In times when there were hardly commercially VHF amateur radio sets available, the use of 48 MHz overtone crystals was the most popular. With a crystal frequency of 48.333 MHz, for example, a transmitting frequency of 145.0 MHz was generated after tripling. Quite often, 72 MHz overtone crystals were also used. Here, when doubling the oscillator signal for a transmission signal to 145.0 MHz, a 72.5-MHz crystal was needed. The 48 MHz crystals were usually oscillating on the 3rd overtone. The 72-MHz crystals, however, worked mostly on the 5th overtone.

Such transmitter concepts made possible the simple structure required for use in portable radio systems. Transmitters operating in this way were designed for the AM mode, which first of all was the most commonly used in the 2m band for long time. With the advent of repeater stations, such transmitters were also built for FM. However, with 72 MHz crystals, it was difficult to get a sufficient frequency deviation. In that regard, control with 48 MHz crystals was clearly superior. Incidentally, you get such quartz crystals as a custom-made for any channel frequencies even today. Per crystal must be expected with a price of about 25.00 € plus shipping.

I made several experiments with such transmitters, which were constructed as shown in the circuit diagram. The achievable RF power was depending on the design of the coils at 2 to 3 watts. For the close range, a single-stage arrangement with an EF184 was already enough. It was then achieved a few hundred milliwatts, which was enough for the local 2m repeater operation. It could be achieved with the arrangement shown a completely sufficient result for the 25 kHz frequency raster, without causing any significant distortions. Transistor transmitters proved to be much more problematic in this respect. With high Q factors required for optaining maximum RF power, a fairly small bandwidth is achieved, as tubes only slightly damp the resonant circuits. As a result, it must be expected that in the case of a crystal change outside a range of approximately 500 kHz, the resonant circuits must be readjusted. It is therefore advantageous if the rotary capacitors can be adjusted on the front panel.

Since nowadays a higher cleanness of the transmission signal is required, a filter should be inserted at the transmitter output. Since tube transmitters already generate a fairly clean signal as a result of the lower damping of the resonant circuits, this does not require a great deal of effort. A tuned to 145 MHz series resonant circuit in the antenna line already causes quite a lot in this regard.