KI4CLZ - Antennas

Antennas

An antenna is a type of transducer. The word "transducer" derives from two Latin words meaning "to lead across" or "to transfer". Transducers transfer or convert one form of energy to another. Antennas convert radio-frequency electric current to electromagnetic waves. Antennas can either transmit or receive electromagnetic waves. The performance of an antenna is characterized by three important properties: Feed point impedance - Impedance can be considered an AC resistance. The impedance of the radio has to match the impedance of the antenna. Directivity, gain and efficiency - Directivity is the property of the antenna radiating more strongly in some directions than others. Gain is related to directivity. The gain is the power loss subtracted from the power supplied to the antenna; most antenna have a high efficiency therefore gain is basically equal to directivity. Polarization - The polarization of an antenna is defined to be that of it's electric field, in the direction where the field strength is maximum. Two common types of polarization are vertical and horizontal, perpendicular and parallel to the ground.

Current Transformers

In electronics, impedance matching is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source to maximize the power transfer or minimize signal reflection from the load.

Impedance matching to minimize reflections is achieved by making the load impedance equal to the source impedance. If the source impedance, load impedance and transmission line characteristic impedance are purely resistive, then reflection-less matching is the same as maximum power transfer matching.

A balun is an electrical device that converts between a balanced signal and an unbalanced signal. A balun can take many forms and may include devices that transform impedances. Transformer baluns can also be used to connect transmission lines of differing impedance.

As an electro-magnetic wave travels through the different parts of an antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance. At each interface, depending on the impedance match, some fraction of the wave's energy will reflect back to the source, forming a standing wave in the feed line.

The ratio of maximum power to minimum power in the wave can be measured and is called the Voltage Standing Wave Ratio (VSWR or SWR). An SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface (impedance matching) will reduce a high SWR and maximize power transfer through each part of the antenna system.