Impedance and reflection coefficient measurement
Since impedance is a complex quantity, both amplitude and phase of the test signal are required to be measured. The following techniques are commonly employed for such measurements.
Slotted Line Method :-
The complex impedance ZL of a load can be measured by measuring the phase angle ΦL of the complex reflection coefficient rL from the distance of first voltage standing wave minimum dmin and the magnitude of the same from the VSWR, S. The following relations are important for the computation of ZL.
Figure 1 : Determination of load impedance using a slotted line
The method of using slotted line to measure the impedance is described in the above figure 1. The steps for measurement are summarized as follows :-
- Measure the load VSWR to find ρL using equation (2) .
- Measure the distance d between two sucessive minima to find g=2d and β=2π/λg
- Measure the distance dmin of the first voltage minimum from the load plane twowards generator in the following manner : Since it may not be possible to reach the first dmin by the probe close to the load directly using slotted line, an equivalent load reference plane on the slotted line is established by means of a short circuit at the load reference plane where a voltage minimum now occurs. Since a series of minima are produced on the slotted line at intervals of λg/2, the load reference plane can be shifted to a conve- nient minimum position near the centre of the slotted line as shown in Fig. 2. The dmin can then be measured by observing the first minimum from this shifted reference plane when the load replaces the reference short.
- Phase angle ΦL of the ioad is calculated from Eq. 2 and hence is found.
- The unknown impedance ZL is then calculated from Eq. 1.
Figure 2: Determination of dmin
To ease the calculation , Smith chart can be used to determine ZL from the measurements of S and dmin .
Impedance measurement using Reflectometer :-
Impedance can be done also by using Reflectrometer with appropriate setup.
Reflection coefficient measurement :-
once the impedance is measured , the reflection coefficient can be measured from the following relation