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Although the phenomenon of an antenna converting electric currents into electromagnetic (EM) waves is fascinating, in many analyses, we can simply model the antenna as an equivalent impedance (for example, a 50Ω resistor) connected to ground, without directly addressing the EM properties of the antenna. For off-the-shelf antennas, manufacturers specify the antenna impedance in the datasheet. However, for custom antennas, the impedance must be measured or simulated. The antenna impedance remains the same in both receive and transmit modes. It is also known as radiation resistance.
Antenna impedance is a measure of the resistance an antenna presents to the flow of electrical current at a given frequency. It is a complex quantity, consisting of both a resistive (real) component and a reactive (imaginary) component, and is typically expressed in ohms, Ω.
Resistive Component (R): This part of the impedance dissipates energy, usually in the form of heat or radiated electromagnetic waves. It reflects the actual power radiated by the antenna. A well-designed antenna might have a resistive component of 50Ω, meaning it is optimized to efficiently transfer power when connected to a transmission line with a characteristic impedance of 50Ω.
Reactive Component (X): This part of the impedance stores and releases energy without dissipating it. It is typically associated with inductance (positive reactance) or capacitance (negative reactance). The reactive component varies with frequency and affects how well the antenna is tuned to a particular frequency.
An antenna serves as an interface between circuits and space, facilitating the transfer of power between the communication medium and the transmitter or receiver. The antenna's impedance acts as either the load for the transmitter or the input impedance for the receiver. It consists of three components: 1) radiation resistance, 2) loss resistance, and 3) reactance.
The resistance observed by the transmitter or receiver at the antenna terminals will be equal to the sum of the radiation resistance and loss resistance and may also include a reactive component.
When there is no reactive component, the antenna is considered resonant. Maximum power transfer between the antenna and the transmitter or receiver occurs only when the impedance seen from the antenna terminals is the complex conjugate of the antenna impedance.
It is important to match the transmitter to the antenna not only to get maximum power transfer. Attenuation of harmonics relative to the fundamental frequency is maximized when the transmitter is matched to the antenna—an important point in meeting the spurious radiation requirements for license-free transmitters. The radiation resistance depends on the proximity of the antenna to conducting and insulating objects. In particular, it depends on the height of the antenna from the ground. Thus, the antenna matching circuit of a transmitter with integral antenna that is intended to be hand-held should be optimized for the antenna impedance in a typical operating situation.
Definition: Radiation resistance is a conceptual resistance that represents the resistance an antenna exhibits due to the radiation of electromagnetic waves. It indicates how effectively an antenna converts input electrical power into radiated electromagnetic power.
Explanation: When current flows through an antenna, part of the electrical energy is radiated as EM waves, while some may be lost as heat or through other mechanisms. The radiation resistance measures the portion of resistance responsible for the radiation of energy.
Definition: Loss resistance refers to the resistance caused by non-radiative losses in the antenna, including heat dissipation in its materials (conductors, dielectrics), resistance from imperfect conductors, and losses caused by nearby objects.
Explanation: This resistance results in inefficiencies in the antenna's operation by consuming power that is instead dissipated as heat rather than being radiated.
Definition: Reactance is a measure of an antenna's opposition to changes in current or voltage due to its inductance or capacitance. In other words, it describes the energy stored in the magnetic or electric fields around the antenna.
Explanation: Reactance can be either inductive or capacitive. It determines how the antenna's impedance affects its matching with the transmitter or receiver circuitry. For resonance, reactance ideally should be zero, meaning that the inductive and capacitive reactances cancel each other out.
Inductive Reactance: This occurs when the antenna behaves like an inductor, causing the current to lag behind the voltage.
Capacitive Reactance: This occurs when the antenna behaves like a capacitor, causing the current to lead the voltage.
