Radio Waves propagation (Intuitive Understanding of Wave propagation)

Question

I have few similar questions regarding Radio waves communication particularly FM and AM type of Radio wave communication. Please bear with the naivety of the questions.

Questions:

Radio waves in FM and AM communications are transmitted through Air and we pick those signals with the help of our receivers.

Assume few FM Radio stations such as 91.1MHz, 93.5MHz and 94.3MHz. (Area size radius - 10-15km)

All these stations are operating simultaneously and depending on what frequency I tune receiver, that radio station is played. But the idea is that all the above 3 radio stations are working and operating simultaneously.

Since the radio waves are EM waves, they propagate in air in the form of waves. So, technically, do these EM Waves propagate through the atmosphere by disturbing the air particles, I mean like, compression and rarefaction? I just want to understand how does these waves move from the transmitter to the receiver.

Question 1 : Do the EM waves travel by creating disturbances in the air particles such as compression and rarefaction?

If they travel like that, won't the compression and rarefaction pattern of the 91.1MHz radio station affect the compression and rarefaction pattern of the 93.5MHz radio station and cause any disturbance/interference/cross-talk between each other before they even reach my receiver? How won't those 2 radio stations interfere between each other ?

Question 2 : What would be typical distance range in terms of radius (in km) of an FM/AM radio station? I tried to find. But unable to find it. Somewhere, I have read that the distance coverage is dependent upon the power of the FM Radio Transmitter? Could someone please throw some more information like a certain table which would provided the distance covered and the radius of coverage of the FM Radio?

Question 3 : If the FM Radio communication are a type of line of sight communication, how am I able to receive the FM Signal even if my receiver is not in direct line of sight of the FM Transmitter?

P.S. The reason why I didn't make it as 3 separate questions because all these questions are very closely related so that I didn't want to burden the answer-er to the question or a similar fellow person who is having the same queries to find the answer under a single question itself.

Answer 1

Question 1 : Do the EM waves travel by creating disturbances in the air particles such as compression and rarefaction?

No. They are an electromagnetic field. They are the same type of thing as light (and travel at the same speed). For AM/FM radio stations the only difference is that the wavelengths are a lot longer than visible light.

• They can travel in space where there is no air (that's how all space based communications work)
• They can also pass through solid objects like wood for short distances (that's how you can pick up the radio from inside your house).
• They can pass through water (for short distances), for example to control a small unmanned submarine.

If they travel like that, won't the compression and rarefaction pattern of the 91.1MHz radio station affect the compression and rarefaction pattern of the 93.5MHz radio station and cause any disturbance/interference/cross-talk between each other before they even reach my receiver? How won't those 2 radio stations interfere between each other ?

The radio waves essentially pass right through each-other the same way that two flashlight beams pass right through each-other. While they are crossing, the field values just add together, and emerge out the other side at their original values.

At the receiver, the waves for multiple stations are all present and being received by the antenna. For an AM or FM radio there is a band-pass filter that selects only the waves falling within a specific frequency range (for example only 93.4MHz to 93.6MHz). Typically such filters can be made using transistors, op-amps, inductors capacitors, and resistors.

The operation of such filters depends on mathematical tricks that exist for linear systems, which make it easy to separate the signals. You would need a background in linear circuit theory, and calculus to understand the math. The basic principle is that even if you have a signal composed of multiple frequencies added together the filter can take just the part that is of the desired frequency and mostly reject the other part (though nothing is perfect).

Question 2 : What would be typical distance range in terms of radius (in km) of an FM/AM radio station? I tried to find. But unable to find it. Somewhere, I have read that the distance coverage is dependent upon the power of the FM Radio Transmitter? Could someone please throw some more information like a certain table which would provided the distance covered and the radius of coverage of the FM Radio?

Question 3 : If the FM Radio communication are a type of line of sight communication, how am I able to receive the FM Signal even if my receiver is not in direct line of sight of the FM Transmitter?

The range of the signal does depend on the transmitter power, and also on the sensitivity of the receiver. As the signals spreads out from the source antenna it covers a larger and larger area. As a consequence the amount of power in any given spot "thins out" the further you go.

The signal never really disappears, but at some point there is not enough power at the receiver for you to use the signal. That distance is determined by how much RF background noise there is as well as the sensitivity of the receiver. Receivers made to communicate with spacecraft (for example the mars rovers) use gigantic antennas in order to pick up really weak signals.

Radio towers are usually pretty tall, so their view of the horizon extends many miles. Additionally signals from AM/FM radio stations can travel through some amount of trees and buildings. Lastly, AM/FM radio waves at the frequencies used by music radio stations can reflect off of parts of the earth's atmosphere (like a mirror) to extend their range beyond the horizon by bouncing back and forth between the atmosphere and ground.

Answer 2

Do the EM waves travel by creating disturbances in the air particles such as compression and rarefaction?

No.

Radio waves are electromagnetic waves. They propagate in the electromagnetic field itself.

They do not require any physical medium to travel through and can travel through vacuum just as well (better, actually) than through air. If EM waves couldn't travel through vacuum, we wouldn't be able to see the sun.

What would be typical distance range in terms of radius (in km) of an FM/AM radio station? I tried to find. But unable to find it. Somewhere, I have read that the distance coverage is dependent upon the power of the FM Radio Transmitter? Could someone please throw some more information like a certain table which would provided the distance covered and the radius of coverage of the FM Radio?

What you read is correct: It depends a great deal on the transmitter power. Other factors such as antenna configuration, terrain, atmospheric conditions, etc., can also strongly affect broadcast range.

Normally, transmitter power is limited by regulations to allow for transmission over a few 100 km or less.

AM radio often has greater range than FM due to the lower frequencies used. The most powerful stations in the US (for example, KGO can be heard over nearly half the country, a range of well over 1000 km.

For US stations, sites like radio-locator.com provide coverage maps showing the areas stations are licensed to broadcast to.

Here's an example for one of my local stations, broadcasting with 8.9 kW from an altitude of 412 m (but in fairly hilly terrain):

If the FM Radio communication are a type of line of sight communication, how am I able to receive the FM Signal even if my receiver is not in direct line of sight of the FM Transmitter?

FM radio at broadcast frequencies is not truly a line-of-sight transmission. Due to diffraction, the signal can often be received even if a building or hill intervenes.