The Polarization Loss Factor in Antenna Communication

Key Takeaways

• Whenever there is a polarization mismatch in antennas, electromagnetic (EM) power is lost.

• Lost EM power can result in faulty transmission and poor signal reception.

• The loss of EM power due to a polarization mismatch is defined by the polarization loss factor (PLF).

A polarization mismatch in antennas leads to EM power loss

Electromagnetic (EM) waves are characterized by electric and magnetic fields; in a plane EM wave, electric and magnetic fields are perpendicular to each other and the direction of the wave propagation. The polarization of an EM wave is a term that describes the orientation of its electric field vector.

Polarization is an essential concept in antenna-to-antenna communication. According to the shape traced by the electric field vector, polarization can be classified in three ways: linear, circular, or elliptical. Signal reception is damaged if the polarization of the antennas does not match; any polarization mismatch is characterized using the polarization loss factor.

Let’s take a closer look at the issues posed by a polarization mismatch and the importance of the polarization loss factor.

Polarization Mismatch

The polarization of an antenna refers to the direction of the radiated electric fields from the antenna. Consider transmitting and receiving antennas of the same polarization, such that the angle between their radiated electric fields is zero. These two antennas are polarization matched and no EM power is lost.

When transmitting and receiving antennas are of different polarizations, for example, with horizontal and vertical polarization, respectively, then the angle between the radiated fields is 90° and no power is transferred between the antennas.

Now, let's consider a case of transmitting and receiving antennas of the same linear polarization. Two antennas are rotated from each other by an angle ‘ϕ’. The angle between the radiated fields becomes non-zero even though the polarization of the antennas is the same. This would be a case of polarization mismatch.

Whenever there is a polarization mismatch, EM power is lost in antenna communication and this loss is defined by the polarization loss factor (PLF), also known as polarization efficiency.

The Polarization Loss Factor

PLF is often mistaken for the term power dissipation. Power lost in the antenna after reception or in reradiation is related to power dissipation. Instead, PLF is the measure of power that is not captured from the incident wave by the antenna. It can also be described as a multiplier that defines what fraction of the incident power is captured into the receiving antenna.

 Percentage of Polarization Mismatch PLF value (dB) 100% -∞ 0% 0

The desired PLF for no loss of communication is unity or 0 dB

PLF Values

When an antenna receives the maximum possible power from an incident wave, the polarization mismatch is nil. The value of PLF at no polarization mismatch is equal to 1, 100%, or 0dB. When the PLF is equal to 0 or -∞ dB, there is a 100% polarization mismatch and no power is received from the incident wave by the antenna.

The possible values of PLF are:

Calculating PLF

To calculate PLF, let Pr be the power received by an antenna and Pi be the incident power. If there is any polarization mismatch, the power received by the antenna is less than the incident power. A reduction in power received by the antenna is given by the PLF. The actual power received by the antenna is denoted by the equations:

Let the polarization vector of the incident wave be Wand the polarization vector of the receiving antenna be A, in its transmitting mode.  In this case, the PLF is defined as:

In the case of linearly polarized antennas with an angle between radiated electric fields equal to ϕ, PLF can be determined by the following equation:

Antennas are sensitive to polarization—without polarization matching, proper transmission and signal reception are hindered. You can use the PLF equations above to calculate any EM power loss caused by polarization mismatch. Quality antenna communication will require engineers to orient antennas so that the angle between the radiated electric fields is nearly zero, providing a PLF value close to unity.

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