HAM Radio Extra Class Test Antenna Theory and Design Questions and Answers Flashcards
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Read the first 6 HAM Radio Extra Class Test Antenna Theory and Design Questions and Answers flashcards as text
What is the far-field distance (Fraunhofer region) criterion for an antenna with aperture dimension D operating at wavelength λ?
Answer: 2D²/λ
The far-field region begins at approximately 2D²/λ, where measurements represent the true radiation pattern.
In a stacked Yagi array, what spacing between the upper and lower Yagi antennas maximizes gain?
Answer: Approximately one wavelength
Stacking Yagi antennas approximately one wavelength apart provides optimal gain by combining their patterns constructively at low elevation angles.
What is the polarization of the electric field radiated by a quadrifilar helix antenna?
Answer: Circular
A quadrifilar helix antenna produces circular polarization, making it ideal for satellite communications where signal orientation constantly changes.
What happens to the input impedance of a dipole antenna as it is shortened well below its resonant length?
Answer: It becomes capacitively reactive with lower radiation resistance
A shortened dipole presents capacitive reactance and reduced radiation resistance because it is electrically small compared to a half wavelength.
What is the approximate gain of a five-element Yagi-Uda antenna over a dipole?
Answer: About 10 dBd
A well-designed five-element Yagi typically provides roughly 10 dBd of gain, combining the effects of one reflector and three directors.
What is the trap in a trap dipole antenna used for?
Answer: To isolate sections of the antenna so it resonates on multiple bands
Traps are parallel LC circuits that electrically isolate outer sections of the dipole on higher frequencies, allowing multi-band resonance.