# Reflections on RF - How did it happen?

• Ivan Seeking
In summary: 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
Staff Emeritus
Gold Member
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.

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

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?

Last edited:
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).

Last edited by a moderator:
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!

Last edited:

## 1. What is RF and how did it come about?

RF stands for "Radio Frequency" and it refers to the range of electromagnetic waves used for communication and broadcasting. It was first discovered and studied by James Clerk Maxwell in the 1860s, but it wasn't until the early 20th century that scientists such as Guglielmo Marconi and Lee de Forest began to develop practical applications for RF technology, leading to its widespread use in the modern world.

## 2. How does RF technology work?

RF technology works by transmitting and receiving electromagnetic waves at specific frequencies. These waves, also known as radio waves, can travel through the air or through specialized cables and are used to carry information such as sound, images, and data. The receiver then converts the waves back into usable information.

## 3. What are the main uses of RF technology?

RF technology has a wide range of uses, including radio and television broadcasting, cellular and wireless communication, radar and sonar systems, GPS navigation, and many other applications. It is also a crucial component of modern technologies such as Wi-Fi, Bluetooth, and IoT devices.

## 4. How has RF technology evolved over time?

RF technology has continuously evolved and improved since its inception. In the early days, it was mainly used for long-distance communication through radio and television broadcasting. However, advancements in technology have led to the development of smaller, more efficient devices that can transmit and receive signals over shorter distances, such as smartphones and Bluetooth devices.

## 5. Are there any potential health risks associated with RF technology?

There have been concerns about the potential health risks of prolonged exposure to RF radiation, particularly from cell phones and other wireless devices. However, extensive research has been conducted, and so far, there is no conclusive evidence that RF radiation at levels within safety guidelines has any adverse health effects. It is always recommended to follow safety guidelines and limit exposure to RF radiation when possible.

• Engineering and Comp Sci Homework Help
Replies
1
Views
713
• Optics
Replies
1
Views
3K
• Other Physics Topics
Replies
1
Views
2K
• Electrical Engineering
Replies
1
Views
1K
• DIY Projects
Replies
5
Views
2K
• Electrical Engineering
Replies
2
Views
3K
• Electrical Engineering
Replies
20
Views
6K
• Art, Music, History, and Linguistics
Replies
1
Views
1K
• Electrical Engineering
Replies
4
Views
3K
• Optics
Replies
3
Views
2K