Two radio antennas are 120 m apart on a north-south line.?

Question

The two antennas radiate in phase at a frequency of 5.6 MHz. All radio measurements are made far from the antennas. The smallest angle, reckoned north of east from the antennas, at which destructive interference of the two radio waves occurs, is closest to:

a) 13 degrees
b) 9.7 degrees
c) 19 degrees
d) 6.4 degrees
e) 16 degrees

Answer 1

λ = 300,000,000/5,600,000 = 3000/56 m
λ/2 = 1500/56
200/λ = 56/15 cycles
Now the question becomes, "measured North of East from where?".
Destructive interference occurs when 1/(nλ)^2 = 1/(λ(m + 1/2)^2, or 1/(n)^2 = 1/(m + 1/2)^2

Answer 2

I reckon C @ 19 degrees.