Reflections on RF - How did it happen?

[Ivan Seeking]

Here is a real brain teaser that has always baffled me. Years ago, back in the bad old pre-cable days of cheesy TV antennas, I was watching something of interest and, since I was working around the house, I had both of our TV sets on the same channel for optimum viewing from any location. I happened to be in a position to see both sets when some kind of interference caused a loss of the signal intermittently. The picture to turn to snow, then back to the picture, back to snow, and back to a picture. This repeated for a total of about four or five cycles. The on and off times appeared to be about the same - about 1 second.

Now here is the really perplexing part: The two TV sets were oscillating in unison but 180 degrees out of phase. They were on separate antennas and separated by about 30 feet. How in the heck can this happen? At the time I was immediately and completely baffled and have remained so since. Am I missing something obvious?

The only guess I have ever managed is that the polarity of the power to the two sets was opposite and that this anomoly was due to some kind of power fluctuation. These sets could have predated the polarized power 110vac plugs used today.

 

[Alexander]

Interference of direct wave and reflected from a passing plane. And your two sets were separated by about half wavelength path difference.

 

[Ivan Seeking]

Condisering the period of the oscillation this doesn't seem very likely. Also, the wavelength was much less than thirty feet. I considered reflection and interference scenarios but couldn't imagine one that yields the observed effect.

Am I missing something obvious?

 

[Alexander]

Yes. What you missing is that distance between interference FRINGES is NOT half wavelength. Path difference is. And the speed the fringes move over Earth surface is NOT c NOR v (plane speed). Fringes can move very slowly (say, few ft/sec) or even be stationary despite that one mirror (plane) is moving at high speed.

Say, imagine source and receiver separated by some distance L, say L=20 km. Now add one wavelength and ask yourself, say, at what elevation midway between receiver and transmitter shall plane be to reflect with 1 wavelength longer path (and let's say, wavelength is 2 m)? As you can easily calculate using Pithagorean theorem, it shall be at about 141 m elevation. For 2 wavelength - about 200 m, 3 wavelength - 244 m, 10 - 445 m, 11-468 m and so on. So, if a plane is at elevation ~450 m and ascending or descending at a rate ~10 m/sec, you'll see slowly (with period ~ 2-3 sec) changing signal on your TV from min to max and back. Horizontal motion of plane does not matter in this case (because surface of equal phase is ellipsoid with transmitter and receiver at focal points).

In reality it is rare that you get equal intencities of direct and reflected beams (although due to elevation and high reflectivity planes give quite strong signal) so usually picture does not disappear completely but goes up and down in quality. Also because planes are crossing ellipsoids of equal phase at arbitrary places, even going in and out of ellipsoid, then speed of change of picture may vary (like, say, from fast changing to slow and then to stop (when plane by chance moves tangent to ellipsoid at that moment) and then slow and fast again).

 

[Ivan Seeking]

Originally posted by Alexander
Yes. What you missing is that distance between interference FRINGES is NOT half wavelength. Path difference is. And the speed the fringes move over Earth surface is NOT c NOR v (plane speed). Fringes can move very slowly (say, few ft/sec) or even be stationary despite that one mirror (plane) is moving at high speed.

Say, imagine source and receiver separated by some distance L, say L=20 km. Now add one wavelength and ask yourself, say, at what elevation midway between receiver and transmitter shall plane be to reflect with 1 wavelength longer path (and let's say, wavelength is 2 m)? As you can easily calculate using Pithagorean theorem, it shall be at about 141 m elevation. For 2 wavelength - about 200 m, 3 wavelength - 244 m, 10 - 445 m, 11-468 m and so on. So, if a plane is at elevation ~450 m and ascending or descending at a rate ~10 m/sec, you'll see slowly (with period ~ 2-3 sec) changing signal on your TV from min to max and back. Horizontal motion of plane does not matter in this case (because surface of equal phase is ellipsoid with transmitter and receiver at focal points).

In reality it is rare that you get equal intencities of direct and reflected beams (although due to elevation and high reflectivity planes give quite strong signal) so usually picture does not disappear completely but goes up and down in quality. Also because planes are crossing ellipsoids of equal phase at arbitrary places, even going in and out of ellipsoid, then speed of change of picture may vary (like, say, from fast changing to slow and then to stop (when plane by chance moves tangent to ellipsoid at that moment) and then slow and fast again).

Yes you must be correct. I had not thought about a shifting fringe field but this does make sense. It seems highly unlikely to get such a clear effect, but considering nothing else has ever made sense, this seems to be the only reasonable explanation. Case closed. You get the golden peanut award for the week!