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1/29/98 56 C95.3-1991 Revision — 2 nd Draft 10/98 Draft MHz with an estimated uncertainty in power density of ±12% (0.5 dB). Later results at 2450 MHz have been reported by [B6], which claims an accuracy of ±5% (0.2 dB). In this case, the probe diameter was 1.6 cm compared with the narrowest guide dimension of 5.46 cm. 4.5.3 Calibrations Using TEM Cells. Another guided wave method suitable for calibrating EM-field probes below frequencies of about 500 MHz is the use of a transverse electromagnetic or TEM Cell. Like the rectangular waveguide, this device is fully shielded, and does not emit energy that may be hazardous or cause interference with nearby electronic equipment. Other advantages are excellent long-term stability of the calibration system and moderate cost (compared with an anechoic chamber). The basic TEM cell is a section of two-conductor transmission line operating in the transverse electromagnetic (TEM) mode, hence the name. As shown in Fig 4.5, the main body of the cell consists of a rectangular outer conductor and a flat center conductor located midway between the top and bottom walls. The dimensions of the main section and the tapered ends of the cell are chosen to provide a 50 Ω characteristic impedance along the entire length of the cell [B34]. When the cell is properly designed and terminated in a reflectionless load, the input voltagestanding-wave ratio (VSWR) is usually less than 1.05 for frequencies below the cutoff limit. In the center of the calibration zone, halfway between the center conductor and the top (or bottom) wall, the E-field will be vertically polarized and quite uniform. Also, the wave impedance (E/H ) will be close to the free space value of 377 ohms. Introduction of the probe into this region will alter the field distribution in the vicinity of the probe, but the total uncertainty in the field strength is less than 1 dB [B34, B35, B90] if the maximum probe dimension is less than b/3 where b is the distance from the top wall to the center plate. Cells can be made in various sizes to suit particular needs and to cover specific frequency ranges. However, since the width (surface parallel to the surface of the center plate) should be less than a half-wavelength to avoid higher order modes in the cell, the upper useful frequency of a TEM cell is approximately 500 MHz unless the cell walls are lined with RF absorbing material (see 4.5.3.6). Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft, subject to change.
1/29/98 57 C95.3-1991 Revision — 2 nd Draft 10/98 Draft Fig 4.6 Typical Transverse Electromagnetic (TEM) Cell For proper use of TEM cells, several factors should be considered, including the following: (1) the electrical characteristics of the cell (2) higher order modes (3) relative size of the probe being calibrated with respect to the plate separation, and (4) stability and calibration of the voltmeter, directional couplers and power meters used in conjunction with the cell to produce field strengths with an absolute, known value. Several of these issues have been considered in more detail by [B58] and [B90]. 4.5.3.1 Electrical Characteristics. The rectangular TEM cells available commercially are designed to have a characteristic impedance of approximately 50 ohms. This value can be calculated from the equation: 94. 2 Zo = w + ⎛ ⎜ + Coth g (Eq 4.9) 2 π ln 1 ⎞ ⎟ d π ⎝ 2b⎠ where the dimensions w, b and g are given in Fig 4.6 [B89]. The characteristic impedance can be measured with a time domain reflectometer (TDR). The TDR can also be used to check for and correct impedance mismatches, particularly at the transitions. The fields at the test point, i.e., the geometrical center of the center plate (septum) and midway between the center plate and the upper (or lower) wall of the cell, can be calculated from: Pn Zo E = V / b = (V/m) (Eq 4.10) b H = E/377 (A/m) where V is the voltage at the input or output port of the cell, Z 0 is the real part of the characteristic impedance of the cell and b is the distance from the upper wall to the center plate. P n (the net power delivered to the cell) is determined in the same way as P T , and the discussion in 4.5.1 applies. The equivalent plane wave power density W can be calculated from: W = E 2 /377 (W/m 2 ) or (Eq 4.11) W = 377 H 2 (W/m 2 ) These field values apply only at the test point for a well-matched cell and significant variation will be seen closer to or farther from the septum. Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft, subject to change.
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