الفهرس 
Six-Port Reflectometers, a ReviewOnly 14 pages are availabe for public view
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Abstract
The six-port reflectometer is an alternative to the network analyzer, it
is simple device that does not need high precision components and requires
only power meters in order to measure complex impedances. The six-port
reflectometer is a device which allows the phase and magnitude of an
unknown complex impedance to be measured directly at the operating
frequency and in terms of power measurement alone using a standard
) impedances and a calibrating procedure. The calibrating procedure is carried
out in order to take into consideration the imperfections of the reflectometer
hardware and imperfect matching and isolation between different ports. There
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are different configurations of six-port reflectometers such as three lines
coupler, four directional couplers, quadrature hybrids, five-port junction plus
a coupler, ...etc.
In this thesis, the analysis and design of the three-section backward
couplers is carried out by taking the advantages of the four fold symmetry of
the structures and the chosen excitations. The three-section coupler has two
symmetry planes corresponding to an electric wall or a magnetic wall, and
consequently one has to determine two even-mode and two odd-mode
impedances. Then its S-parameters and physical dimensions can be
determined.
from the analysis it was found that, the S-parameters are not uniform
around the center frequency. Changes of the electrical lengths for the center
and outer sections lead to uniform performance keeping the overall coupling
lengths equal to 3/ 4A. at the center frequency. Accordingly, some shift in the
center frequency occurs. A curve fitting process was carried out to determine
this shift with an error not exceeding 2%.
Two couplers (single-section coupler and three-section coupler) with
coupling coefficient of -15 dB were designed and fabricated using thin film
technology and photolithography technique on Teflon substrate (sr =2.2,
HO.062”) at center frequency 4 GHz. It is found from the measurements of the S-parameters that, the three-section backward coupler gave broader
bandwidth (2-6 GHz), better isolation, reflection and transmission as
compared with the single section coupler.
A six port reflectometer is analyzed and designed by using four of the
prescribed three-section backward coupler. After some mathematical
analysis, three equations for the power ratios are obtained. To make these
equations similar to that of the six-port reflectometer traditional equations,
there are three solutions which depend on the variations of inter-connection
sections between couplers and terminating impedances at couplers ports. The
system is arranged so that it has two sliding short circuits adjusted at certain
positions, matched load and a fixed short circuit at four ports of the system,
while the other ports are connected to the IIP, DUT and four power meters.
The system was first simulated by IE3D software package and then
fabricated and tested. The measured S-parameters gave good agreement with
the theoretical ones. The six-port was calibrated with a matched load and
three positions of a sliding short circuit in order to obtain the calibration
constants.
The measurements of an unknown using the six-port reflectometer
leads to three circles equations. The intersection point of these circles give the
required reflection coefficient (magnitude and phase angle). Due to the errors
in the measurements, these circles do not intersect in one point but they
intersect in six points. The three points (which have reflection coefficient less
than unity), represent the vertices of a triangle. from this triangle, four
solutions can be obtained which are barycenter, circumcenter, incenter and
orthocenter which represent the intersection point of each of, the medians,
the perpendicular bisectors of the triangle sides, the bisectors of the triangle
interior angles and the three altitude of the triangle, respectively. In fact the
choice of one of the above four solutions depend on the triangle type
(equilateral, bilateral, acute and obtuse). The measurements of some unknown
impedances using the realized and calibrated six-port reflectometer give good
agreement with the measurements of the same unknowns using the vector
network analyzer (HP851OC),with an error not exceeding 5% in magnitude
and 7· in phase in the frequency range (3-5 GHz).