Antennas and Transmission Lines Flashcards
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Read the first 7 Antennas and Transmission Lines flashcards as text
At what frequencies do rectangular waveguides become practical transmission line alternatives to coaxial cable?
Answer: Above approximately 1 GHz (microwave)
Waveguides become practical above roughly 1 GHz where coaxial cable losses become prohibitively high, and waveguide physical dimensions become manageable relative to wavelength.
What is the cutoff frequency of a rectangular waveguide?
Answer: The minimum frequency at which the dominant propagation mode can travel through the guide
Below the cutoff frequency, the waveguide cannot support propagation of the dominant mode and instead attenuates signals exponentially.
Why is a Type N connector preferred over a BNC connector for RF applications above 1 GHz?
Answer: Type N connectors maintain consistent impedance and lower insertion loss at microwave frequencies
Type N connectors use precision construction that maintains 50-ohm impedance consistency and lower loss up to 18 GHz, well beyond the 4 GHz practical limit of BNC connectors.
What is the primary reason coaxial cable attenuation increases with frequency?
Answer: Skin effect confines current to a progressively thinner outer layer, increasing effective conductor resistance
Skin effect concentrates RF current in an increasingly thin surface layer as frequency rises, reducing the effective conductor cross-section and raising its ohmic resistance.
What type of transmission line is typically used for high-power AM broadcast station antenna feedlines?
Answer: Rigid coaxial transmission line with EIA flanged sections
High-power AM broadcast stations use rigid coaxial transmission line because of its very low loss, high power-handling capacity, and weatherproof construction for outdoor use.
Why must outdoor coaxial cable connectors be weatherproofed in commercial antenna installations?
Answer: Moisture ingress raises dielectric loss, promotes corrosion, and degrades SWR significantly
Water between a coaxial connector's conductors dramatically increases dielectric loss, corrodes the center pin, and can raise SWR high enough to trigger transmitter protection shutdowns.
What is the approximate usable upper frequency limit of a standard BNC coaxial connector?
Answer: 4 GHz
Standard BNC connectors maintain acceptable impedance match and insertion loss up to approximately 4 GHz, above which mechanical tolerances cause significant reflections.