Question:

Published on: 16 September, 2024

**Explain how high value of VSWR can be measured by the double minimum method.**

Answer:

Measurement of VSWR gives the degree of mismatch between the load and the transmission line. Complex load shifts the face and hence minimum position, so load impedance can be calculated from the VSWR and the position of the minimum point on the line with respect to the load. VSWR can be directly measured by a VSWR meter. The set up is shown in Fig.18

When the line is determinate with an impedance different from that of the characteristics impedance of the line, the incident and reflected wave sets a standing waves on the line. The probe is first move to voltage maxima in the slotted line and the detector is tuned to give the maximum output. The input power is so adjusted that the meter gives the full scall reading then the probe is moved to the next minima. The meter then given the VSWR which can be directly read on the VSWR scale of the meter. This method is quite suitable for they measurement of moderate values of VSWR≤10.

Fig. 18 VSWR measurement set up

For high value of >10 , when the reflection is very high then the probe insertion at the minimum should be high and which may cause disturbance to the field. The result may be errorless. Hence, another method called double minima method is used. In this method the probe dept does no introduced an error, the probe is inserted to dept where the minimum can be read without difficulty and then move to a point where the power is twice the minimum. This position is denoted by d_{1}. The probe is moved to the twice power pointy on the other side of the minimum the position designated by d_{0}. The VSWR may be found by using the relation

\(VSWR=\frac{\lambda_g}{\pi(d_1-d_0)}\)

Where λ_{g} is the guide wavelength in the slotted line.

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