Conformity Magazine - December 2007 - (Page 44) Reflection Coefficient/VSWR/Return Loss Reflection coefficient, VSWR, and return loss all describe the same physical phenomenon discussed above—impedance mismatch. If a mismatch occurs, there is a standing wave established in a transmission line. The voltage ratio of the maximum to the minimum of the standing wave is called the “voltage standing wave ratio” (VSWR). The closer the VSWR is to unity, the better the match is. Reflection coefficient is the ratio of the reflected voltage to the forward voltage. Reflection coefficient can be a complex number, as the reflected voltage does not always line up in phase with the forward voltage. The smaller the magnitude of the reflection coefficient, the better the match is. Since all these terms describe the same physical property, there is a one to one relationship among them. The simple equations are: the same field level, more sensitive receivers are needed for higher frequencies. AFs are normally provided by antenna manufactures or calibration labs. The accuracy of AFs directly affects radiated emissions measurement. It is recommended antennas be calibrated annually to minimize the measurement uncertainties. Antenna Pattern and Beamwidth Antenna pattern, in simple terms, is the response of an antenna as a function of viewing angle. In a strict sense, antenna pattern is a descriptor for the far field response. In practice, EMC measurements are often performed in the near field, and antenna pattern is taken quite liberally as well to include near field responses. Beamwidth is typically measured when power received has fallen by half (3 dB down) from the power measured along the boresight direction. This is called half-power beamwidth or 3 dB beamwidth. Beamwidth can be used to roughly estimate the size of a uniform field area in an immunity test. However, many other factors come into play for establishing uniform field, such as reflection from the ground or walls of a chamber. Beamwidth is not by itself a magic number that establishes the size of the uniform field plane. Phase Center A radiated wavefront has curvature close to the antenna. At far field distances, this curvature is so large that it can be regarded as a plane wave. The apparent center of the curvature is the phase center. For many EMC antennas, such as biconical antennas or dipoles, the phase centers are quite obvious. For log periodic antennas, the phase center moves from the back to the front as frequency goes up. The measurement distance from the antenna to the device under test is unclear (and varies with frequency). In practice, a compromise has to be made for the distance. Polarization The polarization of a radiated wave has to do with the radiated vector field traced out as a function of time. It is beyond the scope of this article to explain the full meaning of polarization in all its detail. Fortunately, many EMC antennas are linearly polarized, such as dipoles, biconical antennas, log periodic dipole arrays, and horns. Linearly polarized antennas radiate vector field in a single direction which is less complicated than other polarizations. Any imperfections are measured by the cross-polarization ratio (the ratio of the field level in the intended direction to that of its orthogonal direction). Some antennas are circularly polarized for special applications, such as conical log spiral antennas as required by MIL-STD 461. Balance Coaxial cables attached to the antennas are inherently imbalanced, because they are asymmetrical with respect to ground. In other words, the impedance from the cable shield where |G| is the magnitude of the reflection coefficient, and RL is the return loss in dB. In terms of power relationship: where Pnet is the net power, and Pfwd is the forward power. For example, if an antenna has a VSWR = 2:1, the magnitude of the reflection coefficient is 1/3, the return loss is 9.5 dB, and 11% of the power is reflected (or 89% net power is delivered to the antenna). Antenna Factor Antenna factor (AF) is another practical term commonly used by EMC engineers, although it is seldom used outside EMC applications. It provides a receiving antenna with the relationship between the incident electromagnetic field and the voltage on a 50 Ω load connected to the antenna. In equation form: where E is the incident electric field, and V is the voltage on the load (usually 50 ohms). AF has a unit of 1/m, or dB•m-1. Antennas with smaller AFs are more sensitive to the incident field. It is interesting to note that AFs generally increase with frequency. It comes as no surprise then that, to measure 44 Conformity DeCember 2007
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.