Subject : Microwave Engineering
Unit : Microwave measurements
VSWR measurement
VSWR can be measured by detyecting V_{max} and V_{min }in the VSWR meter , as
There are two methods of VSWR measurement .
1. Low value VSWR measurement (S<20)
2. High Value VSWR measuremenmt (S>20)
Low value VSWR measurement (S<20) :-
Low values of VSWR can be measured directly from the VSWR meier using the experimental set-up shown in Fig.1 as follows
- The variable attenuator is adjusted to 10 dB. The microwave source is sc! to the required frequency. The 1 kHz modulation is adjusted for maximum reading on the VSWR meter in a 30 dB scale. The probe carriage stub is tuned for maximum delected signal in VSWR meter.
- The probe carriage is slided along the non-radiating slot from the load end until a peak reading is obtained in VSWR meter. The meter's gain control is adjusted to get the meter reading at 1.0 or 0 dB corresponding to the position of voltage maximum.
- The probe is moved towards the generator to an adjacent voltage minimum. The corresponding reading in VSWR meter directly gives the VSWR = V_{max}/V_{min} on the top of SWR NORMAL scale for 1 < S < 4 or on the EXPANDED scale for 1 < S < 1.33.
- The experiment is repeated for other frequencies as required to obtain a set of values of 5 vs f .
- For VSWR between 3.2 and 10, a 10 dB lower RANGE should be selected and reading corresponding to V'mm position should be taken from the second VSWR NORMAL scale from the lop.
- For VSWR between 10 and 40, a 20 dB RANGE sensitivity increase is required and reading is taken from the top of VSWR NORMAL scale < 1 to 4) at the voltage minimum and should be multiplied by 10 to obtain actual VSWR.
- For VSWR between 32 and 100. a 30 dB lower RANGE must be selected and reading is taken from the second VSWR NORMAL scale (3.2 to 10) from the top at the voltage minimum. The reading should be multiplied by 10 to obtain actual VSWR.
The possible sources of error in this measurements are
- Vmax and V_{min} may not be measured in the square-law region of Ihe crystal detector.
- The probe thickness and depth of penetration may produce reflections in the line and also distortion in the field to be measured. Depth of penetration should be kept as small as possible otherwise values of VSWR measured would be lower than actual.
- Mechanical slope between the slot geometry and probe movement may cause different values of VSWR for measurement at different locations along the slot.
- When VSWR < 1.05, the associated VSWR of connector produces significant error in VSWR measurement. Very good low VSWR (< 1.01) connectors should be used for very low VSWR measurements.
- If the modulating 1 kHz signal is not a perfect square-wave, the microwaves will be frequency modulated and at each frequency there will be a different set of standing waves. This causes reduction in the sharpness of voltage minima and there may be error in the reading of minimum position.
- Any harmonics and spurious signals from the source may betuned by the probe to cause measurement error.
- A residual VSWR of slotted line arises due to mismatch impedance between the slotted line and the main line as explained in Fig 1.
Figure 1 : Residual VSWR of a slotted line
Let :-
Then, the total reflected field at a point = | E_{s }± E_{L }|. The maximum and minimum VSWR and reflection coefficients on the main line are
...(1)