20160705

Physics final exam question: reception from dipole antenna

Physics 205B Final Exam, spring semester 2016
Cuesta College, San Luis Obispo, CA

A radio transmitter broadcasts using two horizontal electric dipole antennae, with one mounted along the north-south direction, and the other mounted along the east-west direction. A Physics 205B student holding a horizontal receiver is located to the west of the transmitter, and walks to the south of the transmitter, but the receiver is aligned such that is always pointed directly towards the center of the transmitting antennae. Discuss whether the signal (if any) received by the Physics 205B student will increase, decrease, or not change as he walks from west to south. Explain your reasoning using the properties of light and polarization.

Solution and grading rubric:
  • p:
    Correct. Discusses/demonstrates that a constant zero signal will be received by the Physics 205B student's receiving antenna as he walks from west of to the south of the horizontal north-south and east-west transmitting antenna array because:
    1. the transmitting antenna array effectively transmits horizontally polarized radio waves in all horizontal directions;
    2. the receiving antenna axis always points longitudinally in towards the transmitting antenna array, thus is unable to receive any transversely polarized signal at all points along the horizon.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. At least understands that no signal is received when west or south of the transmitting antenna array, but claims some signal is received when southwest of of the transmitting antenna array.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. At least understands that a receiving antenna must be held in the same transverse orientation as a transmitting antenna in order to receive a signal.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying the properties of light (radio waves) and polarization.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying the properties of light (radio waves) and polarization.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 30882, 30883
Exam code: finalm4u1
p: 21 students
r: 6 students
t: 5 students
v: 6 students
x: 1 student
y: 0 students
z: 0 student

A sample "p" response (from student):

Another sample "p" response (from student):

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