What is a feed through capacitor?

Answer 1

A feed through capacitor is a type of capacitor (if you don't know how a capacitor works, look this up before going on) for bypassing RF voltage frequencies to ground of a transmitters chassis. It is built by putting a dielectric around a conductor and encasing it in a metallic tube.. so that the conductor is one plate and the outer tube is the second plate. They are shown in a schematic diagram by a straight line and a half moon to the side much like standard capacitor with the exceptions there are three connections. Two for the conductor plate and usually one for the outer shield. The higher frequencies that travel down the conductor plate 'pass through' (no current flows through the dielectric of a capacitor, unless it is leakage current or breakdown current) the dielectric to the outer case (typically grounded), while the lower frequencies (and DC) continue on through the conductor. So, in effect, what a bypass capacitor does is pass the DC current through it while bypassing the AC to ground of the chassis. The value of the capacitor will determine the capacitive reactance and what frequencies will pass more freely to ground.