Finding the resonant frequency of the oscillator is not a problem - with a counter, auxiliary receiver, or wattmeter. Although a wattmeter does not achieve the same accuracy as other methods, it provides us with a significant advantage when setting up oscillators and transmitters. All this with maximum simplicity of the circuit.
The wattmeter consists of a tuned circuit and an amplifier with an indicator. If we bring the coil of the wattmeter close to the coil of the oscillator, part of the energy is induced into the tuned circuit Lx-Cx. The maximum voltage is induced when the resonant frequency of this circuit matches the resonant frequency of the oscillator.
The induced voltage is rectified by diode D and filtered by capacitor C1. We determine the sensitivity of the wattmeter with potentiometer P1. Transistor T is connected in a bridge with resistors R2, R3, and R4. We balance the bridge with potentiometer P2. A sensitive measuring instrument is connected diagonally across the bridge - I recommend a needle microammeter of about 200uA. A digital measuring instrument can also be used, but finding the maximum is a bit more difficult. The supply voltage is 5V (not mandatory).
The frequency range depends on the parameters of the tuned circuit Lx-Cx. For the KV range, I recommend using a tuning capacitor with a maximum capacity of about 100 - 200pF and several interchangeable coils. For VHF the maximum capacity is of course smaller, around 30pF.
The finished wavemeter still needs to be calibrated. For this, a precise RF generator will be useful, and it is advantageous to use an unlocked transceiver. Coils need to be wound precisely on quality skeletons without a core, and after winding, they should be fixed in a suitable way.
Parts list:
| R1 | M5 |
| R2 | 2.2k |
| R3 | 1.2k |
| R4 | 4.7k |
| P1 | 470k/N |
| P2 | 10k/N |
| C1 | 2.2nF |
| Cx | see text |
| Lx | see text |
| D | VF germanium/Schottky diode |
| T | NPN KC…, BC… |
| M | 200uA |