Radio broadcast .. (1922-30)

Why You Should Have a Wavemeter 481 the wavemeter will give more sharply defined resonant points than would otherwise be the case. Condensers provided with a movable plate vernier or other means of fine adjustment utilizing an extra plate, are not recommended for this service as they require two readings for each wavelength setting. If a vernier adjustment is desired, and sometimes this is desirable, the vernier should take the form of a small vernier knob attached to the dial of the condenser. THE COILS THE coils used in the wavemeter should be strongly constructed, and should not change their size or shape with temperature or with use. Coils either of the form wound type (duo-lateral coils) or coils wound on hard wood or bakelite forms are suitable for wavemeter construction. Perhaps the most convenient type of construction for the amateur is that employing the form wound coils, such as duo-lateral, De Forest, Remler or Curkoid coils. This type of construction is particularly good since it permits the easy construction of a wavemeter covering a wide wavelength or frequency range. The manufacturers of these coils have constructed a number of coils which are designed to cover the entire wavelength band when used in combination with a .001 mfd. variable condenser. If the coils are mounted on plugs, it is a simple matter to cover any particular wavelength band with great convenience. MEASURING THE WAVE OF A TRANSMITTER IN FIG. i is shown the circuit of a wavemeter designed to measure the wavelength of transmitting or receiving sets. This wavemeter consists of one duo-lateral coil suitably fitted with a plug shunted directly by the .001 mfd. variable condenser. Connected as shown in the diagram is a vacuum tube. This vacuum tube serves as an indicator to show when the wavemeter is in resonance or tune with transmitting or receiving set. In the case of a transmitting set, resonance is indicated by a deflection of the meter from the position at which it would rest if the transmitter were not operating. The wavemeter should be placed a considerable distance from the transmitter when the measurement is taken in order to permit of accurate measurement. When a wavemeter of this character is connected as shown in Fig. 3, it can also be used to indicate the wavelength at which any regenerative or FIG. 2 Typical wavemeter circuit for the generation of oscillations. The use of a UV-2OI-A or WE-2i6-A tube is recommended although dry battery tubes will work radio-frequency receiver is operating. In the case of a regenerative receiver the receiving set must be placed in an oscillating condition before any indication will be received on the wavemeter. The same oscillating condition must be obtained with the radio frequency receiver. The condition of oscillation of a radio-frequency receiver can, of course, be easily obtained by the manipulation of the potentiometer. In Fig. 2 is shown the circuit connections for a wavemeter designed to create oscillations of a known frequency. This wavemeter utilizes the same coils and condensers used in the wavemeter of Fig. i. Two coils are added— one a feed-back coil — the other a pick-up coil for use in transferring the energy from the wavemeter over to the circuit where it is desired. Coil C, together with the variable condenser, determines the frequency at which the wavemeter will oscillate. Coil B, the feed-back coil, serves to create the oscillation, the frequency of which is determined by coil C and the variable condenser. Coil A, which can be of the size of coil B, must be varied for best results as the frequency is changed. The same size coil will cover quite a wavelength band; but as the wavelength increases it is necessary to increase the size of coil A if any appreciable amount of energy is required for the measurement. In constructing either of the wavemeters shown, dry battery or storage battery vacuum tubes can be used. The wavemeter in Fig. i will operate satisfactorily with any type vacuum tube. The wavemeter shown in Fig.