DRAFT Recommended Practice for Measurements and ...

1/29/98 55 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
of leakage probes than calibration in a plane-wave field where the entire probe is more
uniformly illuminated. A careful error analysis of this problem has not been completed,
but it appears that if the maximum probe dimension is less than one-third the smallest
waveguide dimension, the total uncertainty will not exceed ±1 dB [B6, B126].
Fig 4.5 Rectangular Waveguide Calibration System
Figure 4.5 shows how a section of rectangular waveguide can be used for calibrations. A
reflectionless load is connected to the output end to prevent standing waves that would
cause serious errors in the calibration. The probe to be calibrated is usually inserted into
the waveguide through a hole in the side wall (as in Fig 4.5) and positioned in the center
of the guide where the field is most nearly uniform. (Entry through the top wall is not
recommended because spurious pickup by the leads that are then aligned with the E-
field is greater.) The access hole should be as small as possible to minimize
perturbation of the field distribution. Equations for calculating the field distribution from P n
(the net power delivered to the section) and the guide dimensions can be found in [B59,
B86]. The ''equivalent power density'' can be determined in terms of E 2
(not E ·H ) at the center of a rectangular guide in which the width, a, is twice the height,
b, from
2 −1/
2
Pn
W = ⎛ ⎝ ⎜ 4 ⎞⎡
⎟ − ⎛ a ⎠ ⎝ ⎜ λ ⎞ ⎤
2 ⎢1
⎟
⎣ a⎠
⎥
(Eq 4.8)
2 ⎦
A slide and slot in the side wall of the guide may be used to evaluate and reduce the
uncertainty produced by any standing wave within the guide. P n is determined in the
same way as P T in Eq 4.1. It is difficult to estimate the total uncertainty of this method
because the field strength at the probe sensor being calibrated will be modified by the
size and nature of the probe. The influence of the conductive walls on probe calibration
in a waveguide is analyzed and evaluated in [B69]. The error assessment of the probe
voltage for dipoles of lengths of 20 and 30 mm having terminations of 100 ohms in a
WR430 waveguide is 1% and 2.5%, respectively. (The 100 ohm termination impedance
is typical for thermocouple, but not for diode based probes used for microwave oven
leakage measurements). [B126] describes a system that operates from 400 MHz to 600
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.