Total posts : 45366
In a reply to one of my previous posts, Neil asked me how circuit capacitance can cause power loss in a Part 15 transmitter. After all, it is understood by everybody that resistances dissipate power, but capacitances do not. I will give an example:
A loading coil used for antenna tuning has an inductance, of course, but it also has an equivalent parallel capacitance. The loading coil capacitance increases losses. Here’s why:
Because of the equivalent parallel capacitance of the loading coil, the inductance of the loading coil is less than needed to resonate with the antenna capacitance. The capacitance causes the inductive reactance of the coil to increase. The loading coil capacitance causes the bandwidth of the system to decrease below what it would be if the impossible condition of there being no loading coil capacitance existed. This lower bandwidth suppresses the higher audio frequencies to be transmitted. To restore the audio bandwidth, the Q of the system would have to be decreased by adding losses to the system. For similar reasons, it is undesirable to tune a loading coil using a variable capacitor. It is better to use a variable inductor.
Capacitances at the output of the final stage, and in the matching network that couples the final stage to the antenna, all cause losses by reducing bandwidth. The bandwidth is restored by decreasing Q by increasing losses.
