13 Jul 2026

Measurement of Microwave Wavelength by Standing-Wave Method

practical pg-i microwave standing-wave wavelength

Aim

To determine the wavelength of microwave radiation using a standing-wave measurement on a waveguide bench.

Apparatus

Microwave source, isolator, attenuator, slotted waveguide, crystal detector, standing-wave meter, movable probe, and short-circuit termination.

Experimental arrangement

The detector probe is moved along the slotted waveguide and the separation of successive minima is measured.

Theory

The microwave source launches an electromagnetic wave into the rectangular waveguide. At a short-circuit termination the wave is reflected, and the incident and reflected waves superpose. Their interference produces fixed nodes and antinodes, called a standing-wave pattern. The distance between two successive minima is half the wavelength measured inside the guide:

\[\lambda_g=2d.\]

The waveguide wavelength is not the free-space wavelength because the field must satisfy the boundary conditions at the conducting walls. For the operating mode, the free-space wavelength is related to the guide and cutoff wavelengths by

\[\frac{1}{\lambda_0^2}=\frac{1}{\lambda_g^2}+\frac{1}{\lambda_c^2}.\]

The probe position is therefore used to obtain $\lambda_g$, after which the known cutoff wavelength gives $\lambda_0$.

Observations

Minimum pair	Probe positions (cm)	Separation $d$ (cm)
1	12.4, 13.9	1.5
2	15.4, 16.9	1.5
3	18.4, 19.9	1.5

Thus $\lambda_g=3.0$ cm for the sample bench.

Result

The guide wavelength is approximately $3.0$ cm; the free-space wavelength is obtained after applying the cutoff correction.

Viva Questions

1. Why are minima separated by half a wavelength? Successive destructive-interference points are separated by $\lambda/2$.
2. What is a waveguide? A conducting structure that confines and guides electromagnetic waves.
3. Why is the detector probe movable? To map standing-wave intensity along the guide.