Conformity Magazine - December 2007 - (Page 43) T he antenna is an essential part of a test system. In some applications, selecting the best antenna for the job may be very important. Unfortunately, selecting the right antenna can be a difficult task, because of variations in the way manufacturers specify and describe them. This article explains the basic terminologies employed and their limitations. Antennas, by definition are devices that convert time-varying voltages or guided electromagnetic energy into a radiated electromagnetic field (transmitting) and vice-versa (receiving). The key word here is “radiated.” Many field generating devices, such as TEM (or Crawford) cells, GTEM, parallel plates, E/H field generators, striplines, parallel bar field generators and TEM wires are, strictly speaking, not antennas. The generated RF energies stay within the devices and are not radiated into space. In EMC applications, antennas are primarily used for radiated emissions measurements, radiated immunity testing, site qualification testing (normalized site attenuation), or other applications such as exciting a reverberation chamber. In a specific application, one set of parameters may be more important than another. For example, the gain of an antenna may not be of any concern if it is used to excite a reverberation chamber. Directivity and Gain Passive devices, such as most of the antennas used in EMC, cannot amplify the signals they receive, or radiate more energy than provided. Gain and directivity specify an antenna’s ability to concentrate a transmitted signal in a desired direction, or receive a signal from that direction. This differs from the definition of gain in such active devices as amplifier. Directivity describes how well an antenna concentrates radiation intensity in a certain direction, or receives a signal from this direction. This is in comparison to an isotropic antenna. Note that an isotropic antenna is simply a theoretical model, and is not possible to construct one physically. A theoretical isotropic antenna has a directivity 0 dBi (“dBi” means dB over an isotropic source) in all directions. A half wave dipole has a directivity of 2.14 dBi in the plane perpendicular to the antenna. This means a half wave dipole can concentrate 2.14 dB more energy in its maximum radiation direction than an isotropic source. Higher directivity is associated with narrower beamwidth. Gain, by IEEE definition, is the product of the directivity and the ohmic efficiency (sometimes called ohmic loss factor). Most EMC antennas are made of aluminum or other highly conductive metals. In these antennas, the ohmic loss is insignificant; therefore, gain is essentially the same as directivity. This is where confusion arises, since such a gain definition is rarely the one you encounter in an EMC application. Gain in EMC applications typically includes an additional mismatch factor. The IEEE definitions above are based on the net power delivered to the antenna. In practice, antennas are never perfectly matched to the source, and energies are reflected at the antenna port. The net power is given by the subtraction of the forward and reverse powers (in logarithmic, or dB terms). Although including mismatch in the gain figure differs from the IEEE definition, it is practical. This is sometimes referred to as the “apparent gain.” For example, a transmitting antenna can have a very directive pattern but, if its input impedance is not close to 50 ohms, very small electric field levels will be generated when the antenna is connected to a 50 ohm source (used for most instruments). In most EMC applications, gains published in the antenna catalogs include this mismatch factor. Figure 1 shows a hydraulic analogy to RF impedance mismatching: water flows through pipes with unequal diameters. Some of the water goes through and some is reflected. Figure 1 December 2007 conformity 43 http://www.thehowlandcompany.com http://www.thehowlandcompany.com
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