The First Trapdoor

The picture gives a very rough indication of the first stage of 'filtration' in the receiver. The very small current entering the antenna passes through the left-hand coil and continues on to ground. The second coil picks up this incredibly small current and a current starts to circulate around it and the capacitor.

Since the second coil and the capacitor form a resonant circuit at one particular frequency, current at that frequency will begin to increase. As a result, the voltage between point 'A' and ground will increase also and this voltage - higher at one frequency than all the others - is fed on to the rest of the receiver.

In practice, the capacitor is generally a variable capacitor because a useful amount of filtering can be achieved here. Never-the-less, a comparatively wide range of frequencies still manages to squeeze through but at least it is now a manageable range rather than the entire radio spectrum!