Antennas The Aerial or the portion of the antenna that is in the air, captures all radio signals is usually metal and conducts the energy into the radio as an AC (alternating current).  The Antenna resonates or electromagnetically vibrates harmonically with a wave length that is a fractional multiple of the antenna length as the electromagnetic radiation propagates on to the antenna.   Resonant frequency.  'The natural frequency of a resonant circuit, given by 1/(2(Π)(LC)^(1/2)) where L is the inductance and C the capacitance of the circuit.  (Thewlis p.286)'  This is similar to how strings vibrate at specific pitches near their natural frequency, and in the antenna the electrons are being driven to the resonance frequency matching the antenna length to a harmonic sequence of the wave length.  (Serway p.  473, 558, 1043)  AM wavelength is constant because the frequency is constant, and the electromagnetic waves create resonance currents on antennas (conductors) of length equal to the wave length or some harmonic multiple of the wave length (i.e. If a wave length is 405 meters then an antenna which is 1/2, 1/3, 1/4, etc. the wave length will resonate properly). If we choose 740 kHz for a radio frequency the wave length is 405 meters or 1329 feet, c, the velocity of light, over f, the frequency, = λ, wave length. The optimal antenna length is a half wave or the distance between two nodes.  This is huge (202.5m), and the space required is not practical. Furthermore, we can only tune to one wavelength or station.  The Antenna coil is essentially a long antenna that is coiled up to save space, however a coil is an electromagnet and/or an inductor.  Most antennas are only a small  harmonic fraction length of the wave length, n.  i.e. 1/n ...Harmonic sequence.  Antennas are tuned by adjusting the capacitance or antenna length.  All materials have a capacitance, or ability to hold a charge.  By placing a capacitor parallel in circuit with the antenna the length is increased allowing a different resonance frequency i.e. different radio station.  A variable capacitor has a changing electric field size, which changes the circuit's capacitance. Tuning is capacitance adjustment.  In series the capacitor decreases the length of the antenna.  "I will discuss the magnetic field approach to antenna operation instead of the electric field approach, because I feel that it relates more directly to the pioneering experiments.... If a ring of wire (now called a loop antenna) is set up in a location where the alternating magnetic fields of a radio wave pass through the wires of the loop, an alternating current will be generated in the loop with the direction of the current being determined by the direction of the magnetic field as it passes through the loop. This is similar to the transformer, except that the input coils (the radio transmitter) and the output coils (the receiving antenna) are separated by a large distance and are coupled by electromagnetic waves not induction fields. The strength of the current in the coil will be determined by the strength of the magnetic field passing through it. After the radio energy leaves the transmitting antenna the wave expands in a spherical wave front filling space with electromagnetic energy. The strength of the magnetic field diminishes as the square of the distance from the transmitter, just as light from a flash light or a laser beam diminishes as the square of the distance from the source." (Adrian Popa)