Side-by-side comparison
| Parameter | TEM | TE |
|---|---|---|
| Electric field (E_z) | Zero — no axial E component | Zero — TE means transverse electric |
| Magnetic field (H_z) | Zero — no axial H component | Non-zero axial H_z exists |
| Conductor requirement | Two conductors required (coaxial, twin-lead) | Single conductor (hollow waveguide) sufficient |
| Cutoff frequency | None — propagates from DC | f_c = (c/2)√((m/a)²+(n/b)²) for rectangular guide |
| Dominant mode example | Coaxial cable (any frequency < 18 GHz for RG-58) | TE₁₀ in WR-90 waveguide (f_c ≈ 6.56 GHz) |
| Phase velocity | v_p = c in lossless medium | v_p = c/√(1−(f_c/f)²) > c |
| Group velocity | v_g = c | v_g = c√(1−(f_c/f)²) < c |
| Power handling | Limited by coax dielectric breakdown | Higher power; air-filled waveguide handles kW |
| TM mode difference | Same as TEM — H_z = 0, E_z = 0 | TM: E_z ≠ 0, H_z = 0; TE: H_z ≠ 0, E_z = 0 |
Key differences
TEM requires two conductors and has zero cutoff — RG-58 coax supports TEM from DC to about 18 GHz. TE modes have H_z ≠ 0 and E_z = 0; the dominant TE₁₀ mode in a WR-90 waveguide cuts off at 6.56 GHz, meaning nothing below that propagates. TM modes flip this: E_z ≠ 0, H_z = 0, and the lowest TM mode in a rectangular guide is TM₁₁ — there is no TM₁₀ or TM₀₁ because the field would be zero everywhere. Hollow waveguides cannot support TEM at all.
When to use TEM
Use TEM (coaxial or microstrip) when your system spans a wide frequency range from near-DC through microwave, such as a 50 Ω SMA-connected circuit board operating at 1–6 GHz.
When to use TE
Use TE₁₀ in a rectangular waveguide when you need high power handling at a fixed microwave band, such as a 10 GHz radar transmitter using WR-90 guide rated at several kilowatts.
Recommendation
For most PCB and RF circuit work, choose coaxial or microstrip (TEM-like) lines — they are broadband and manufacturable. Choose waveguide TE modes only when power levels or losses demand it, as waveguides are bulky and narrowband.
Exam tip: Examiners will ask you to prove that TEM mode cannot exist in a hollow waveguide — be ready to show that ∇²_t E_z = 0 with E_z = 0 on boundaries forces E_z = 0 everywhere, leaving no axial field to drive propagation.
Interview tip: A microwave engineer interviewer will ask you the cutoff frequency of TE₁₀ in a WR-90 waveguide (answer: 6.557 GHz) and why TEM is impossible in that guide — have both answers ready with a one-line justification.