G. Yassin et S. Withington, ANALYTICAL EXPRESSION FOR THE INPUT IMPEDANCE OF A MICROSTRIP PROBE IN WAVE-GUIDE, International journal of infrared and millimeter waves, 17(10), 1996, pp. 1685-1705
We present a closed-form expression for the the input impedance of a m
icrostrip probe in a rectangular waveguide. The probe extends only par
t way across the waveguide and is therefore compatible with RF compone
nts that require an open circuit at low frequencies. Our analysis is b
ased on the spectral-domain method and is able to take into account th
e orientation of the antenna with respect to the direction of propagat
ion. We have examined the validity of our model by carrying out extens
ive impedance measurements at 5GHz. In those cases where the probe did
not extend more than half way across the waveguide, excellent agreeme
nt was obtained. We show that the bandwidth of a probe that stretches
only part way cross the waveguide is very much greater than the bandwi
dth of a probe that stretches all of the way across the waveguide and
that is earthed at both ends. Moreover, the input resistance is lower
and more suited to submillimetre-wave detectors such as SIS tunnel jun
ctions. Our expression suggests that it should be possible to develop
low-impedance, wideband probes for nearly double-height waveguide, and
this implies that the upper frequency limit to which probes and waveg
uides can be manufactured can be extended well into the THz frequency
range. A related, and often neglected consideration, is that the ohmic
loss associated with an oversized waveguide is very much smaller than
the ohmic loss associated with a reduced-height waveguide.