Once the AM signal is obtained at the final stage, it is necessary to “couple” this signal into the antenna. The circuit which is used to couple the final stage with the antenna is called the antenna coupler. The antenna coupler network for this system is comprised of inductors (L3 and L4), and capacitors (250pF and 150 pF) as shown in Figure 1. This filter configuration is termed a double-pi network. To obtain maximum power transfer to the antenna, it is necessary that the transmitter’s output impedance be properly matched to the antenna’s input impedance.
This means equality in the case of a resistive antenna or the complex conjugate in the case of a reactive antenna input. If the transmitter was required to operate at a number of different carrier frequencies, the antenna coupler circuit is usually made variable to obtain maximum transmitted power at each frequency. Coupling circuits are also required to perform some filtering action (to eliminate unwanted frequency components), in addition to their efficient energy transfer function. Conversely, a filter invariably performs a coupling function, and hence the two terms (filter and coupler) are really interchangeable, with what they are called generally governed by the function considered of major importance.
The double-pi network used in the citizen’s band transmitter is very effective in suppressing (i.e., filtering out) the second and third harmonics, which would otherwise interfere with communications at 2 x 27 MHz and 3 x 27 MHz. It typically offers 37-dB second harmonic suppression and 55-dB third harmonic suppression. The capacitors and inductors in the double-pi network are resonant so as to allow frequencies in the 27-MHz region (the carrier and sidebands) to pass, but all other frequencies are severely attenuated. The ratios of the values of the two capacitors determine what part of the total impedance across L4 is coupled to the antenna, and the value of the 150-pF capacitor has a direct effect on the output impedance.
After assembly, it is necessary to go through a “tune-up” procedure to get the transmitter on the air. Initially, L1’s variable core must be adjusted to get the oscillator to oscillate. This is necessary to get its inductance precisely adjusted so that, in association with its shunt capacitance, it will resonate at the precise 27-MHz resonant frequency of the crystal. The tune-up procedure starts by adjusting the cores of all four coils and one-half turn out of the windings.
Then tune L1 clockwise until the oscillator starts and continue for one additional turn. Ensure that the oscillator starts every time by turning the dc on and off (a process termed keying) a number of times. If it does not reliably start every time, turn L1 clockwise one-quarter turn at a time until it does. Then tune the other coils in order, with the antenna connected, for maximum power output. Apply nearly 100% sine-wave intelligence modulation and retune L2, L3, and L4 once again for maximum power output while observing the output on an oscilloscope to ensure that overmodulation and/or distortion do not occur.