The six-port reflectometer is a measurement device that is mainly used in high-frequency electronics. It allows one to measure both the amplitude ratio and the phase difference of two electromagnetic waves. The most frequent use of this device is for measuring the so-called complex reflection coefficient of a device under test (DUT), which is the ratio of the wave reflected by the DUT to the wave incident to the DUT. The term "complex" here means that both amplitude and phase information are obtained. The reflection coefficient is directly related to the input impedance of the DUT.
What is special about the six-port reflectometer is that phase information is obtained by making only amplitude (or power) measurements of four different linear combinations of the two electromagnetic waves. This means that a six-port reflectometer is in principle simply a passive linear circuit with two input ports and four output ports (hence its name), which provides at its outputs four different linear combinations of the waves present at its inputs.
Of course, an indirect measurement technique like this requires both careful calibration of the measurement device (the six-port reflectometer) and a rather sophisticated mathematical procedure to obtain the quantity of interest (the complex reflection coefficient) from the raw measured data (the four amplitudes at the output ports). More details about the operation of the six-port reflectometer can be found in some basic articles on my six-port reflectometer publications page. A pretty comprehensive presentation is also available in a paper by Vladimir Bilik from S-TEAM Lab, which is associated with the Slovak University of Technology at Bratislava.
By using two six-port reflectometers in a dual six-port configuration, it is possible to perform the same measurements as with a traditional network analyzer, that is, to measure not only the reflection of waves in order to determine the input impedance, but also the transmission of waves through a two-port DUT in order to determine the gain or attenuation of the DUT.
The six-port reflectometer was developed by Glenn F. Engen and his colleagues of the National Bureau of Standards, NBS, (which today is called National Institute of Standards and Technology, NIST) in Boulder, Colorado, USA, starting in the early 70s.
Around this time, desktop computers became accessible to large laboratories like the NBS. It now made sense to develop measurement devices which no longer directly measured the physical quantities of interest, but where these quantities first had to be calculated from the measured data.
The biggest advantage of the six-port reflectometer is probably its simple structure. Whereas traditional network analyzers require high-quality components for frequency conversion and phase detection, a six-port reflectometer simply consists of a passive linear circuit in combination with some power detectors. This makes it in principle much less expensive than the traditional network analyzers.
Another advantage is important for metrology laboratories: As the measurement with a six-port reflectoemter contains some redundancy (three of the measured amplitudes determine the fourth up to a choice between two possible values), it is possible to give an estimate of the accuracy of every single measurement.
A six-port reflectometer is also very useful for measuring the behavior of a circuit under high-power signals. This is because in a six-port reflectometer, only the amplitude (or power) of signals needs to be measured. For high-power signals, power measurement devices are much easier to design than the circuits which are used in traditional network analyzers.
In spite of the advantages cited above, the commercial success of six-port reflectometers has been rather limited; they are rarely used outside dedicated laboratories, and if they are, it is generally for very special applications.
There are in my opinion several reasons for this:
Until very recently, it has been found difficult to develop a six-port reflectometer with a bandwidth comparable to that of traditional network analyzers.
The calibration procedure for a six-port reflectometer tends to be more lengthy than for a traditional network analyzer.
Like all homodyne non-frequency-selective systems, the dual six-port has problems when measuring insertion loss, for example of a notch filter. The fact that unlike for traditional heterodyne network analyzers, harmonics of the signal frequency are not eliminated here, results in a greatly reduced dynamic range for this kind of measurement.
People who want to measure the reflection coefficient of a DUT often are not interested in phase information but only in the amplitude. For this case, there exist simpler and less expensive solutions than the six-port reflectometer.
The reasons cited above make it interesting to look for new and different applications of the six-port reflectometer. These should be applications where phase information is important and where the simple structure of the six-port reflectometer can be an advantage over alternative solutions. Examples for such applications are digital demodulators, control of adaptive antennas, and security distance radar for automobiles.
For reasons of size, weight, and cost (when produced in large numbers), it is essential for the six-port reflectometer to be integrated in monolithic microwave integrated circuit (MMIC) technology if it is to be used for these applications, which may be part of mobile telephones or other portable devices.
I have designed a six-port reflectometer with integrated diode power detectors on gallium arsenide substrate. It has a very simple structure, occupies a surface of 2.2 square millimeters, and works in the range from 1.3 GHz to 3.0 GHz.
Due to problems in calibrating the six-port reflectometer with existing algorithms at some frequencies, I have also developed a new, very robust method for calibration. In the meantime, this method has been used successfully by other people and even for the calibration of an optical six-port reflectometer. An implementation of the method is available for download.
Finally, I have participated in the design of a new, very wideband six-port reflectometer covering the frequency range from 2 MHz to 2200 MHz, that is, more than three decades.
A good summary of this work can be found in three articles that have been published in the IEEE Transactions on Instrumentation and Measurement. My complete PhD thesis is also available for download (in French only).