DRAFT Recommended Practice for Measurements and ...

1/29/98 59 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
termination at frequencies up to about 30 MHz. When the cell is open-circuited, it acts as
a parallel plate capacitor rather than a transmission line, and produces a field with a high
intrinsic impedance (high E/H ratio). Large E-fields can then be produced by connecting
a broadband step-up transformer at the cell input terminal. By contrast, when the cell is
short-circuited it acts as a single turn loop and produces a field having a very low intrinsic
impedance. This configuration is useful for producing large H-fields.
4.5.3.6 Wideband TEM Cells. If a TEM cell is ''loaded'' in strategic locations with
microwave absorbing material (such as the carbon-filled foam that is used in anechoic
chambers), higher-order modes can be suppressed. The resulting cell can therefore
generate a calculable, relatively pure TEM mode, at frequencies that are much higher
than would be possible using standard TEM cells [B35]. This condition degenerates
whenever reflecting (metallic or dielectric) objects are introduced in the cell. For
calibrating probes with very small sensors and nonperturbing leads (resistive or fiber
optics), specially designed absorber-loaded TEM cells may be useful over a very broad
range of frequencies (DC-18 GHz) [B83]. Careful design, testing, and error-analysis
should be performed for each specific combination of TEM cell and sensor under test, to
ensure reasonable performance at any frequency in the range where undesirable higherorder
modes may exist.
4.5.4 Magnetic Field Generators Using Coils.
At low frequencies, the axial magnetic field (in A/m) at the center of a circular loop of wire
is simply the current (in amperes) divided by the loop diameter (in meters). For a singleturn
coil in free space, a loop becomes self-resonant when the circumference
approaches the free space wavelength. For multiturn coils, the resonant frequency is
lower because of capacitance between the turns. Using a coil with a total wire length
less than λ/10, the input impedance is very low, but the field strength value is easily
calculable and given by
H
= NI
(Eq 4.13)
D
where:
H = Magnetic field strength at the center of a flat coil (A/m),
N = Number of turns in the coil
I = RMS Current (A)
D = Diameter of the coil (m).
The magnetic field at points on the axis of a flat coil is
2
NIr
H =
(Eq 4.14)
2 2 3 2
2( r + d ) /
where:
r = Radius of the coil (m)
d = Distance along the axis from the coil center to the field point where the H-field is to
be computed (m)
This type of coil is useful for probe calibration purposes up to about 30 MHz. Figure 4.7
is a sketch of a Helmholtz coil arrangement that consists of two flat coils on the same
axis, both carrying current in the same direction. This type of coil system generates a
more uniform H-field over a larger volume than the single coil. For the purpose of
generating a uniform H-field in which a typical hazard probe can be calibrated, Helmholtz
coils are useful up to about 10 MHz. This frequency limit is dictated by the dimensions of
the coil that should be small compared with a wavelength. The wave impedance (E/H
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.