What Are TEM Cells?

TEM cells, or transverse electromagnetic cells, are shielded test enclosures designed to generate and measure uniform transverse electromagnetic fields for electromagnetic compatibility (EMC) testing of small electronic devices and components. A TEM cell consists of a tapered coaxial transmission line with a flat conductor, called a septum, suspended midway between the top and bottom walls of a rectangular outer conductor. When a signal is applied at one port and the other port is terminated in a matched load, the cell propagates a wave whose electric and magnetic fields are oriented transversely to the direction of propagation, closely approximating the plane-wave conditions that a device would encounter in open-air operation.

Developed in the 1970s and described in foundational work at the National Bureau of Standards (now NIST), TEM cells became a standard tool for both emissions measurement and susceptibility testing well before reverberation chambers and fully anechoic facilities became widely available. Their compact, shielded structure eliminates the need for an antenna, and their behavior is governed by straightforward transmission-line theory up to the frequency at which higher-order modes begin to propagate.

Cell Construction and Operating Principles

The defining geometric element of a TEM cell is the septum: a conductive plate positioned at the midpoint of the outer enclosure, creating an upper and lower test region. A device under test placed in one of these regions is exposed to a controlled field while the outer walls prevent external electromagnetic fields from contaminating the measurement and prevent the cell's internal field from radiating outward. The characteristic impedance of the cell is typically designed to be 50 ohms, matching standard radio-frequency test equipment.

The usable frequency range of a TEM cell is bounded at the high end by the onset of the first higher-order resonant mode, which depends on the cross-sectional dimensions of the outer conductor. Standard rectangular TEM cells are practical up to several hundred megahertz for bench-top unit sizes, with research on expanding TEM cell bandwidth published in IEEE Transactions on Electromagnetic Compatibility demonstrating techniques for pushing the upper operating limit through tapered-septum and resistive-loading designs.

Electromagnetic Compatibility Testing

TEM cells are widely used for pre-compliance and full compliance testing under standards including IEC 61000-4-20, IEC 61967-2, ISO 11452-3, and IEEE 1309. Emissions testing involves placing the device under test in the cell, connecting measuring receivers or spectrum analyzers to the cell ports, and inferring the radiated field from the measured signal level. Immunity testing reverses the process: a source drives the cell to establish a calibrated field level, and the device is monitored for disruption or degradation. Guidance from NIST on using TEM cells for EMC measurements of electronic equipment established the measurement procedures that underlie many current test standards.

Because TEM cells are fully enclosed, they offer high repeatability and are immune to ambient electromagnetic noise, making them preferable to open-area test sites for diagnostic and pre-compliance work. A GTEM (gigahertz transverse electromagnetic) cell is a variant that extends operation into the gigahertz range using a tapered, asymmetric geometry and absorber-lined walls. IEEE Xplore publications on TEM-cell-based EMC test systems document current practices for characterizing emissions from integrated circuits and printed circuit board assemblies using these enclosures.

Applications

TEM cells have applications in a range of fields, including:

• Pre-compliance emissions and immunity screening of consumer electronics
• Integrated circuit and component-level EMC characterization per IEC 61967 and IEC 62132
• Automotive electronics testing under ISO 11452-3 and related standards
• Antenna calibration and field-probe calibration in RF metrology
• Bioelectromagnetics research requiring controlled, quantified field exposures