DRAFT Recommended Practice for Measurements and ...

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.