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21 | to a conjugate matched load. This measurement is given as a function of the direction to the source of the captured radiation for directional antennas with matched polarization. Antenna efficiency is how much of the energy captured from an electromagnetic wave is delivered to the antenna terminals or how much of the energy directed to the antenna terminals is converted to radiated energy. As all practical antennas have losses, antenna efficiency is always less than 100 percent, and efficiency impacts the antenna's effective area, gain, and directivity. Impedance Matching, VSWR, and Return Loss The impedance match of an antenna's terminals to the feed line affects the VSWR and return loss metrics. If the impedance match is perfect, the antenna carries 100 percent of the signal energy. However, practical systems cannot provide perfectly matched impedance, and some energy will reflect from the antenna terminals and form a standing wave. The standing wave is characterized by the standing wave ratio (SWR) or, if measured in voltage, VSWR. Antenna tuning circuits are often used to enhance the impedance match between the antenna and feed line over a desired frequency range. Reactance, which arises when inductors and capacitors oppose alternating current by storing and releasing energy in magnetic and electric fields, can affect this match. To compensate for this, tuning circuits use physical conductor or dielectric structures or a reactance network that cancels the reactance of the antenna at the feed point. Transformers at the antenna and feed node, such as a balun, may be used to match balanced/ unbalanced antenna and feed lines to prevent substantial reflections. Resonance and Bandwidth Optimization The physical structure of an antenna and tuning network (known as a transmatch) may be designed to allow for resonance to occur, or resonances may be the by-product of design and structure. Resonance is when an antenna's reactance is zero, presenting a purely resistive feed point impedance. Depending on the quality of the resonance (Q), the electrical length or effective area can Image Source: natrot/stock.adobe.com