How to calculate the effective aperture of an antenna

 The concept of antenna aperture is easy to explain if you look at a horn antenna, though it applies to all antennas including a dipole antenna which has no physical 'aperture'. In the figure shown here, the area marked with pink dotted line corresponds to the physical aperture of the horn antenna, where the incident electromagnetic wave strikes in case of a receiving antenna. But the effective area will be the total power received by the antenna divided by the power density of the incident wave. The effective area of the antenna may be less than the physical area of the aperture of the horn. In case of other types of antennas which do not appear to have a physical aperture like, the wire antenna, the effective aperture of the antenna can be found using the same method.


While calculating the total power received, the resistive loss in the antenna material will also have to be considered. Power from the antenna is transferred to the receiver as its input power. If this power received by the terminating load of the antenna is calculated and the resistive loss in the antenna added to it, we get the total power received by the antenna. Power density of the incident electromagnetic wave has to be measured. The voltage generated by the power received at the receiver end can be measured. If the impedance of the receiver and antenna are known, the current in the circuit can be calculated. From these values, power received by the antenna can be found out. After that, effective antenna area can be calculated by dividing the total power received by the antenna with the power density of the incident wave.

This means that if p is the power density of the incident wave in Watts/m2 and Pt the power available at the antenna terminals to the receiver, then, Pt = p x Ae, where Ae is the effective antenna aperture. The effective antenna aperture is proportional to the square of the wavelength and the gain of the antenna. The formula for calculating the effective antenna aperture if the gain of the antenna is known is: Ae = (λ2/4π) x G, where λ is the wavelength and G the gain of the antenna as its linear value and not dB.

Antennas with large effective apertures are considered as high gain antennas or beam antennas. They are very sensitive to radio waves coming from the preferred direction compared to radio waves coming from other directions, which are considered as interference. On the transmitting side, most of their power is radiated in the same direction, while radiation in other directions are minimized.

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