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Antenna Theory Crash Course Part 2: Key Antenna Characteristics - Gain, Polarisation, and Impedance Explained

In our previous post, we introduced the fundamental concept of an antenna as a transducer between the wired and wireless realms. Now, we'll delve into some of the most crucial characteristics that define an antenna's performance: gain, polarisation, and impedance. Understanding these properties is essential for anyone looking to grasp the basics of antenna theory.

Let's start with Gain.

When discussing antennas, the term gain refers to the power at the intended terminals and importantly, it includes antenna losses. As you delve deeper into antenna theory, you'll often encounter the term directivity. In fact, in electrical engineering curricula, the concept of directivity is often introduced before gain. While related, gain builds upon directivity by factoring in the inefficiencies of the antenna.

Antenna gain is typically expressed in dBi, which stands for decibels relative to an isotropic antenna. An isotropic antenna is a theoretical point source that radiates power equally in all directions. Therefore, a dBi value tells you how much more power an antenna radiates in its direction of maximum radiation compared to this hypothetical isotropic antenna. Some antennas, particularly those used with parabolic dishes, can exhibit significant amounts of gain, which plays a vital role in extending the range and enhancing the performance of an RF system. An ideal isotropic antenna has 0 dB of gain.

Mathematically, the gain of an antenna, denoted as G, can be related to its directivity (D) and its antenna radiation efficiency (ecd) by the formula: G = ecdD(𝜃,𝜙). This highlights that gain is essentially the directivity reduced by any losses within the antenna. Furthermore, the realised gain (Gre) takes into account the reflection efficiency (er) due to impedance mismatches: Gre = erG(𝜃,𝜙).

Next, let's explore Polarisation.

Polarisation is a fundamental property of electromagnetic waves that describes the time-varying direction and relative magnitude of the electric-field vector. More simply, it's a characterisation of the direction of the electric field vector of the radiated wave. Imagine tracing the tip of the electric field vector at a fixed point in space as time progresses; the shape it creates defines the polarisation.

Antenna polarisation is defined as the polarisation of the wave transmitted (radiated) by the antenna. It's important to note that the polarisation of the radiated energy can vary with the direction from the antenna. Generally, we talk about three main types of polarisation:

  • Linear Polarisation: The electric field vector always oscillates along the same straight line at every instant in time.
  • Circular Polarisation: The electric field vector has a constant magnitude and its direction rotates with time, tracing a circle. This rotation can be either clockwise (CW) or counter-clockwise (CCW).
  • Elliptical Polarisation: The electric field vector's magnitude and direction both vary with time, tracing an ellipse. Circular polarisation is a special case of elliptical polarisation where the major and minor axes of the ellipse are equal.

For efficient wireless communication, it is crucial to intentionally design for your antennas' polarisation to match wherever you're trying to transmit or receive from. A mismatch in polarisation between the transmitting and receiving antennas will result in a loss of signal strength, quantified by the polarisation efficiency (pe) or polarisation loss factor (PLF).

Finally, let's discuss Impedance.

In the context of antennas, impedance is a crucial electrical property that affects how efficiently power is transferred to or from the antenna. The antenna impedance (ZA) is generally a complex quantity consisting of a resistance (RA) and a reactance (XA): ZA = RA + jXA. The resistance component further comprises the radiation resistance (Rr), which represents the power radiated by the antenna, and the loss resistance (RL), which accounts for power dissipated as heat within the antenna.

Impedance matching is of paramount importance in antenna systems because it maximises the power transfer between the transmission line and the antenna, and it minimises the signal reflection back towards the source. When there is a mismatch, some of the power is reflected, leading to a reduction in efficiency. The degree of mismatch is often quantified by the voltage reflection coefficient (Γ), given by Γ = (Zin − Z0)∕(Zin + Z0), where Zin is the antenna input impedance and Z0 is the characteristic impedance of the transmission line. The magnitude of the reflection coefficient, |Γ|, is related to the reflection (mismatch) efficiency (er) by er = (1 − |Γ|2). Another related parameter is the Voltage Standing Wave Ratio (VSWR), which is defined as VSWR = (1 + |Γ|) ∕ (1 − |Γ|). A VSWR of 1:1 indicates a perfect match with no reflection.

Tools like Smith charts are invaluable for visualising and analysing impedance as a function of frequency, aiding in the design of matching networks.

In summary, gain tells us how well an antenna focuses power in a particular direction while accounting for internal losses, polarisation describes the orientation of the electromagnetic wave's electric field, and impedance dictates how effectively power is transferred between the antenna and the rest of the system. These three characteristics are fundamental to understanding and designing antennas for various applications, and we will continue to build upon these concepts in the upcoming parts of our crash course.

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