ANTENNA IMPEDANCE

Let's say an antenna has an impedance of 50 ohms. This means that if a sinusoidal voltage is applied at the antenna terminals with an amplitude of 1 Volt,

then the current will have an amplitude of 1/50 = 0.02 Amps. Since the impedance is a real number, the voltage is in-phase with the current.

Alternatively, suppose the impedance is given by a complex number, say Z=50 + j*50 ohms.

Note that "j" is the square root of -1. Imaginary numbers are there to give phase information.

If the impedance is entirely real [Z=50 + j*0], then the voltage and current are exactly in time-phase. If the impedance is entirely imaginary [Z=0 + j*50],

then the voltage leads the current by 90 degrees in phase.

If Z=50 + j*50, then the impedance has a magnitude equal to:

The phase will be equal to:

This means the phase of the current will lag the voltage by 45 degrees. That is, the current waveform is delayed relative to the voltage waveform.

To spell it out, if the voltage (with frequency f) at the antenna terminals is given by

The electric current will then be equal to:

Hence, antenna impedance is a simple concept. Impedance relates the voltage and current at the input to the antenna.

The real part of the antenna impedance represents power that is either radiated away or absorbed within the antenna.

The imaginary part of the impedance represents power that is stored in the near field of the antenna. This is non-radiated power.

An antenna with a real input impedance (zero imaginary part) is said to be resonant. Note that the impedance of an antenna will vary with frequency.