Two reflections can be separated if the phase of one relative to the other can be continuously adjusted through at least one-half wavelength. By attaching a sliding load to the hybrid tee, it is possible to obtain the return loss of the load and the directivity of the hybrid tee separately.
Sliding loads in rectangular waveguide are constructed in such a way that the load element slides axially along the precision waveguide which forms its housing. The reflection from the load, therefore, remains constant in magnitude as it slides, but its phase varies relative to the internal reflections within the hybrid tee. By sliding the load over a distance of at least one-half wavelength in the waveguide, the combination of reflections from the hybrid tee and the load will pass through a cycle of addition and subtraction at any given frequency.
If the reflections are considered in terms of reflection coefficients (i.e. in terms of voltage), the maximum reflection coefficient reading will occur when the voltage reflection from the load and the voltage reflection from the hybrid add, while the minimum will occur when these two subtract. The sum and difference averages will identify the two components separately.
If
max is
the highest reflection coefficient reading and
min is
the lowest reflection coefficient
1,
and the smaller component
2
are given by: 1
= 1/2 ( max +
min)
2
= 1/2 ( max -
min)Consider, for example, readings of 33.5 dB (
max = 2.1%)
and 34.4 dB (
= 1.9%) at a given frequency:
1
= 1/2 (2.1 + 1.9) = 2.0% (34 dB)
2
= 1/2 (2.1 - 1.9) = 0.1% (60 dB)Using this principle, the directivity of the hybrid tee and the reflection coefficient of the loads can be obtained at any frequency with an accuracy based upon the accuracy of calibration of the calibrated mismatched load used as a reference in setting up the equipment.
It is important to recognize that the values of
1 and
2 are not assigned
specifically to the load and the hybrid tee, but represent
only the larger and smaller reflection coefficients,
respectively. To identify the hybrid component, it is
necessary to know that the sliding load has the higher
reflection coefficient over the entire frequency band.
The calibrated mismatched load satisfies this requirement.
The same principle can be applied to other combinations of reflections if their resultant at any frequency can be arranged to represent one fixed reflection and one reflection of variable phase, relative to the fixed reflection.
In the particular case of a matched load, there is a limit to the accuracy with which this can be calibrated because of the dynamic range available in detectors and sweep equipment. At very low levels of reflected signal, the noise level in the equipment