Q4Electromagnetics Waves

Two-wire (parallel) line: two parallel conductors separated by a fixed spacing, historically used for open-wire telephone and power lines and still used for low-frequency, low-cost applications (twin-lead TV antenna cable). Unshielded, so susceptible to interference and radiation loss, limiting its use at higher frequencies.

Coaxial cable: a centre conductor surrounded by a dielectric and an outer conductive shield, offering good shielding from external interference and low radiation loss, making it the standard choice for RF signal transport (cable TV, broadband internet, antenna feeders) from DC up to several GHz, though loss increases with frequency due to dielectric and skin-effect conductor loss.

Microstrip and stripline: planar transmission lines fabricated by etching a conducting strip on (microstrip) or embedded within (stripline) a dielectric substrate with a ground plane; central to modern RF/microwave integrated circuits (MMICs) and printed-circuit RF module design because they are compatible with standard PCB photolithographic fabrication and allow direct, low-parasitic integration with surface-mount and monolithic active devices.

Waveguide: a hollow single-conductor metallic pipe supporting only TE/TM modes above a cutoff frequency, offering the lowest loss and highest power-handling of any transmission-line type, used for high-power radar transmitters, satellite ground-station uplinks, and other applications in roughly the 1-30 GHz range where its size remains practical and its power/loss advantages outweigh its bulk (as discussed quantitatively in Part C, Q.1 of the companion 2024 Main/Back paper on this subject).

Each line type therefore occupies a distinct application niche determined by the trade-off between loss, power handling, integration convenience, shielding, cost, and physical size appropriate to the frequency and system context in question.

Frequency-driven selection pattern: viewed across the full frequency spectrum, these four line types form a natural progression — two-wire lines dominate at power/audio frequencies where wavelengths are enormous and radiation loss is not yet a concern, coaxial cable takes over through HF/VHF/UHF and into the low microwave region where shielding and moderate loss matter most, waveguide becomes preferable in the 1-30 GHz range where its power-handling and low-loss advantages outweigh its bulk, and microstrip/planar lines become preferable again at millimetre-wave frequencies (60 GHz and above) where fabrication precision and monolithic integration with active devices dominate the selection criteria (exactly the crossover argument developed quantitatively in Part C, Q.1 of the companion 2024 Main/Back paper). No single transmission-line technology is therefore universally optimal; a complete RF/microwave system typically uses several of these line types together, each in the frequency range and circuit location where its particular strengths are most valuable.