Antenna Measurements | Communication Systems

Antenna Measurements

The antenna is an important element of radio communication, remote sensing and radio localisation systems. The measurement of the antenna radiation pattern characteristics allows one to verify the conformity of the antenna. The simplest measurement method consists of the direct far-field measurement. The near-field techniques also make it possible to carry out the diagnosis of the antennas, i.e. to find defects on the antenna.

Antenna must undergo various measurements before installing on the system. Most common desired measurements: radiation, gain, antenna efficiency, directivity, impedance, VSWR (Voltage Standing Wave Raio), beam width, polarization. Note that most wireless applications use the far-field wave. Any antenna radiation patterns are valid only if measurements are taken on the far-field.

The required equipment for antenna measurements includes:

• A source antenna and transmitter: This antenna of known radiation pattern that can be used to illuminate the test antenna. Source antennas are often horn antenna or dipole with a parabolic reflector.
• A receiver system: Determines how much power is received by the test antenna
• A positioning system: Used to rotate the test antenna relative to the source antenna, to measure the radiation pattern as a function of angle. To measure radiation pattern of a test antenna as a function of angle (in spherical co-ordinate system), we need to rotate the test antenna. Source antenna illuminates the test antenna from different angles.

Anechoic chambers

Measurements are usually conducted either in open filed or closed rooms called “ANECHOIC CHAMBERS”. Anechoic chamber testing configuration simulates conditions like weather operation, temperature control and freedom from interference.

Anechoic indoor measurements are called indoor antenna ranges. The walls, ceilings and floor are lined with special EM wave absorbing materials. Indoor ranges are desirable because the test conditions can be tightly controlled than that of outdoor ranges.

Solved problem

Q. An engineer is designing a communication link at 3 GHz where the receiver sensitivity is 1 µWatt of power needed to overcome receiver noise. The receiving antenna gain is 8 dB, the transmitting antenna gain is 10 dB, the transmitting power level is 25 Watt and the distance between the two elements is 1 Km. Will the communication link will work?

Sol. Friis transmission equation used to calculate the signal power arriving at the receiver, which is given as:

Where:

P_r = Received power level =???
P_t = Transmit power level = 25 W
l = Transmit wave length = 1/f = 3 GHz
G_t = Gain of the transmit antenna = 10 dB
G_r = Gain of the transmit antenna = 8 dB
R = Separation distance between antennas = 1 Km

Note that dB values must be converted to numerical before substituting in the formula. So after substituting values, we will get, Received power Pr = 99.7 nWatt.

Communication link will not work because minimum received power should be greater than 1 µWatt. Communication link will not work unless the G_t, G_r or P_t levels are dynamically increased.