28 Sep 2013

Matching the supply line to the antenna output of transmitter

The figures below show matching circuit transmitter output stage and power supply line connected to the antenna. Some of these schemes are seldom used because they have a weak filtering harmonics . Figure a) shows a matching circuit in which the antenna is connected directly to the output circuit of the transmitter. Matching is done by changing the capacitor C1, or change the point of connection to the antenna circuit ( change in the number of turns ) . As an indicator that will show the output matching circuit and antenna can be applied ammeter or a neon bulb. But the maximum ammeter or glow lamps can not always talk about the good coordination of the system. The current can reach its peak in the event of a standing wave in the supply line and the devices may not be current at the antinodes . This situation can occur when a wire is broken antenna SWR and has an infinite value . The following diagrams b) and c) are diagrams in which approval is best filtering harmonics of the transmitter. But because of the presence of inductive coupling between the coils L1 and L2 in the antenna section of the inductive reactance , an additional character. Sometimes when a large inductive coupling between the coils L1 and L2 can occur a substantial redistribution of the currents in the power supply line and the antenna. In order to compensate for the inductive coupling between the coils can be applied capacitance C3 - Figure d). But in this case, the circuit is interrupted by the DC that prevents run-off from the antenna electrostatic discharges. In the figure e) shows the matching circuit and the power line transmitter output stage transistor . Then use the usual resonance system , increasing resistance and the degree of transformation is the selection of the capacitors C1 and C2. In the VHF band can apply an agreement in Figure f). In order to reduce the level of higher harmonics can apply a Faraday shield (all circuits except a)). As this screen, you can use the shell coaxial cable - figure h). This screen can be used when a particularly high level of harmonics . Loop cable is closer to the edge of the grounded coil output stage . Even more harmonic filtering can be applied if the agreement scheme on the image i). In this circuit, an antenna circuit is divided by filtering and matching circuit . As the coil connection can be used here as a coaxial cable as shown in the previous figures . Circuit performing the role of filtering and matching can be taken away from the transmitter output stage and place them in a more convenient location and the connection between these stages can be performed from any piece of cable with a characteristic impedance . Loss in this case is extremely small and can be neglected . The circuit in Figure i) is called a filter Collins.