Which transmission line configuration is effectively non-radiating in normal operation?
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ACoaxial line
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BOpen two-wire line
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CBoth
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DNone of the above
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ESingle-wire line over ground
Answer
Correct Answer: Coaxial line
Explanation
Introduction / Context:Transmission lines are intended to guide energy, not radiate it. Line geometry determines field confinement and susceptibility to radiation losses, interference, and EMC issues.
Given Data / Assumptions:
- Coaxial line: inner conductor, dielectric, outer shielding conductor (return), with TEM mode confined between conductors.
- Open two-wire line: parallel conductors exposed to free space, supporting external fields.
Concept / Approach:Coaxial cables are designed so that the electromagnetic field is almost entirely confined within the dielectric between inner and outer conductors; external radiation is negligible when the cable is properly terminated and shield integrity is maintained. Open two-wire lines, by contrast, have fringing fields in free space and can radiate, especially when unbalanced or near discontinuities.
Step-by-Step Solution:
Consider field lines: coax TEM fields reside between inner/outer conductors.The outer conductor serves as a shield, preventing radiation and pickup.Therefore, coaxial line is effectively non-radiating; open two-wire line is not.Verification / Alternative check:EMC measurements show far less radiated emission from coax than from open two-wire lines for the same guided power, assuming correct terminations and integrity.
Why Other Options Are Wrong:
Open two-wire: exposed fields → potential radiation, particularly at bends/mismatches.“Both” or “None” misstate basic field confinement differences.Common Pitfalls:
Damaged or poorly shielded coax can leak; the statement assumes proper construction and terminations.Final Answer:
Coaxial line