ISRO EC 2023 Q.37: solution and answer

Official answer: ASource: ISRO's revised and final answer key of 16 February 2024

1. A stripline is a thin conducting strip embedded in a homogeneous dielectric, sandwiched between two parallel ground planes.

2. It has more than one conductor (the strip and the ground planes), so a transverse electromagnetic (TEM) wave, with no electric or magnetic field along the direction of propagation, can exist. A TEM mode has zero cutoff frequency.

3. Because the dielectric is uniform, every field line sees the same permittivity. The phase velocity is $v_p=1/\sqrt{\mu ϵ}$ for the whole field, so the propagating mode is pure TEM.

4. Microstrip is different. Its fields lie partly in the substrate and partly in air, so the phase velocity is not the same for all field lines and the mode is only quasi-TEM. Option (d) describes microstrip, not stripline.

5. TE and TM modes are higher-order modes. They can appear only at high frequency, when the plate spacing becomes comparable to half a wavelength in the dielectric, and they are not the operating mode of a stripline. The mode a stripline supports in normal use is TEM.