## 20150624

### Physics final exam problem: interference of two in-phase Wi-Fi antennae

Physics 205B Final Exam, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 25.31

"Wifi"
Jason Cole
youtu.be/6hcK9B4HHY8

A commercially available wireless router[*] broadcasts at a 0.12 m wavelength from two vertical antennae spaced 0.18 m apart. Assume that the two antennae are in phase. Determine how many destructive interference (minima) directions there will be (if any) in the 360° range of all possible directions. Show your work and explain your reasoning using the properties of source phases, path lengths, and interference.

Solution and grading rubric:
• p:
Correct. Approximates two in-phase antennae as a double slit, and equates path length difference approximation d⋅sinθ with destructive interference (minima) condition for in-phase sources to find θ = 19° and 90° as measured counterclockwise from the θ = 0° south direction, such that there are six unique directions of destructive interference in the 360° range of all possible directions. Okay if minima directions in the south-east quadrant are not correctly mapped via symmetry to find all minima directions, if at least the two unique θ = 19° and 90° directions in the southeast quadrant are found.
• r:
Nearly correct, but includes minor math errors. Only specifically searches over the cardinal directions, and finds of these that only east and west have destructive interference.
• t:
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
• v:
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner.
• x:
Implementation of ideas, but credit given for effort rather than merit. No clear attempt at applying path length differences and interference.
• y:
Irrelevant discussion/effectively blank.
• z:
Blank.