Tuned Circuits
One thing you need to understand is Tuned Circuits. Tuned circuits are used extensively in the electronics we use for making Amateur Radio equipment. To understand Tuned circuits you need to understand Inductors.
When an inductor and a capacitor are used together they form a tuned circuit. If we take it as read that the Capacitor is charged, the energy from the capacitor will flow into the inductor creating a magnetic field, discharging the capacitor in the process. The energy in the Inductor coil is then transferred back to the capacitor re-charging it to start the process of again.
Whilst we are working on Tuned circuits, we know a air wound inductor can be worked out by using a formula, and although this formula is not required until you do your Full licence its worth aquainting yourself with it now.
The Frequency at which a capacitor and an inductor transfer energy backwards and forwards. Known as the Resonant Frequency. If you use really good quality components the Tuned curcuits can be very very accurate, and tuned to exact frequencies. Whilst lower quality components will offer less accurate tuning. Other non-resonent frequencies will not be affected by the components which act independently.
The Resonant Frequency of a tuned circuit depends on the values of the Capacitor and inductor, changing the value of either will change the Resonant Frequency of the circuit.
A Practical Use of a Tuned circuit
Below is a schematic of a Trap Dipole. as you can see it utilises two tuned circuits working as Traps. the first part of the antenna works on a higher frequency, as the Trap is tuned to the lower frequency only and rejects all others, if the signal is the same as the tuned circuit the whole length of the dipole is used.
You will also need to understand the basics of Filters, these being Low Pass, High Pass and Band Pass. Rather than replicate the details here, have a look at the Filters section in the Advanced study area here.