Can a radio station's wavelength be measured by driving through its interference? The other night, while driving I was listening to KGO radio. KGO is a San Francisco AM station that broadcasts at 810khz. I was driving north, away from the station, at about 70mph. I notice that the signal would occasionally fade for a duration of about three seconds. 
You have a lot of time to think while driving. That's why I noticed the three second fade. The fade was consistent and happen several times. Then I thought I must be driving through interference patterns. 
So I started to do some rough mental calculations trying to see if my three seconds at 70mph was somehow related to the wavelength of the station. Later, while not driving, I worked out some numbers. KGO's wavelength is 1,247 feet. My three seconds at 70mph is 309 feet. I was disappointed that it wasn't a match. Trying to make it work, I remembered hearing something about radios and 1/4 wave and found a match. KGO's 1/4 wave is 311 feet very close to the 309 feet I traveled in the three seconds. 
My questions are these: Is there likely any truth to this? Is it possible to calculate a stations wavelength by driving through its interference?
Thanks for giving your thoughts to this just for fun and curiosity question.
 A: Yes, in principle it is possible to calculate the wavelength, but you would need much more information than the distance between successive peaks in signal strength. Although your basic idea is correct, your calculation fails to consider the geometry of the interference pattern (which could be very complex) and your motion through it. Your assumption that it is appropriate to use 1/4 wavelength is just wrong. (Quarter wave refers to the length of the antenna, not the wavelength of the broadcast.) It is a pure coincidence that your answer is close to the true value.     

The interference pattern between two point sources is illustrated below. In your case the two sources would be images of the broadcasting antenna reflected from large objects. Or the interference pattern could result from diffraction around the sides of "sharp" objects. The interference results from multi-path propation.

Source : The Physics Classroom
The red and blue lines join points at which there is maximum constructive and destructive interference respectively.   
You can see from the diagram that the distance between places at which you intersect the red lines (at which the radio signal is strongest) depends on where you are and what direction you are driving in. If you were driving along any of the coloured lines you would hear no interference at all. If you were driving at a small angle to these lines (almost parallel) you would have a long distance to go between points of maximum signal strength. If you were driving along the line joining the two beacons you have to go the shortest distance (half a wavelength).
