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?

The answer is no. Electromagnetic propagation is not dependent on air, unlike sound waves. It can travel in free space. That being said, electrically or magnetically charged air particles, and Earth's magnetosphere can alter the trajectory (and phase) of the EM wave depending on frequency (as will gravity). This frequency dependence in the atmosphere is the reason lower(HF and below) frequencies bounce back to Earth and higher frequencies pass through to space. I would be remiss not to mention the Earth magneto-sphere's influence on bounced polarization for HF frequencies, they tend to become elliptical upon bouncing depending on the incidence angle to the magnetosphere.

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?

This is a matter of height of the transmitting antenna, the power of the transmitting station and the frequency used.

In the US, AM stations are in the range of 550 to 1720 KHz. At these frequencies, the signal may bounce several times in the atmosphere, with power dissipating with each bounce. It is not uncommon for a 50 KW transmitter to be heard 1,200 to 1,700 KM away. More distance is possible, depending on solar activity.

The FM part of the question is answered in the response to the next question(3).

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?

This question is a matter of directed power versus atmospheric reflection. The FM acceptable bands, 29-~109MHz through the highest frequency current TV band, less than 700MHz in the US, are all reflected by the atmosphere (The higher the frequency the less atmospheric reflection occurs.)

The answer to wasting power by shooting signals into space, is to design the antennas to maximally radiate at the lowest take-off angles. This technique maximizes the reflection to receiving stations well beyond the radio horizon even at higher frequencies.

At US FM frequencies and maximum power levels(100 KW), the power on the third or fourth bounce is enough to receive a signal well beyond the radio horizon.

Answer 2

Radio waves (such as broadcast AM and FM) can travel through a vacuum (thus do not require air). They are electro-magnetic (or EM) waves, the same as light waves (from the sun and stars, etc.), that can travel through space, but at a vastly lower frequency.

The radius these EM waves can travel (unless blocked or absorbed) is the radius of the universe; but they fade with distance at r^3, so at some distance become smaller than background or thermal or quantum noise, and thus become undetectable. Plus you would need to wait a long time for a signal if you are a light-year or so away from the source.

EM Photons of different frequencies (or colors) do not interact or mix in free space, however a strong signal could jam your receiver from hearing a weaker signal that is nearby in frequency (because transistors and circuits, etc. turn out to be non-linear mixers.)

EM waves travel not just line-of-sight, but also refract and reflect off of various media, including buildings and atmospheric layers, and thus depending on the environment, can be received "around corners".

Answer 3

Question 2 & 3 not fully answered...

You probably actually are within line of sight of the FM transmitter. Broadcast transmitter antennas are typically placed on high towers specifically so that more people will be within line of sight of the tower. Of course, you may not actually be able to "see" the tower because of atmospheric attenuation of visible light, trees and buildings in the way, etc. Radio waves can easily pass through trees (with only some attenuation) and through or around buildings. The power of the transmitter helps with weak receivers and penetrating thick buildings, but doesn't really help with propagation distance much. The groundwave propagation distance is almost entirely reliant on the height of the antenna on the tower.

However, there are multiple ways for the radio waves to get to you:

Ground wave propagation is more or less line of sight, but also includes a slight refraction of the wave against the ground, which may extend propagation further depending on the wavelength. Longer wavelengths refract more; FM broadcast is roughly 3m wavelength, and the refraction distance is roughly 3-7 miles beyond the 50-200 miles you'd already get from the height of the tower. AM might get 100 miles groundwave propagation due to refraction.

When you are just slightly beyond the range of line of sight propagation, you can get edge propagation, where radio waves refract around the edges of obstructions (like tall buildings or mountains) and create fresnel zones, alternating bands of good and bad reception, caused by constructive and destructive interference of multipath propagation.

Finally, there's all kinds of strange propagation modes that are unreliable and a bit random. For instance, ducting, where weather events can cause sharp changes in air layers that radio waves can reflect off of -- this can allow a weak signals to propagate hundreds of miles, but this occurs maybe only a few weeks of the year. Meteor propagation, where the waves reflect off of the ionization trail from meteors (this might only last 10-30 seconds). Or reflecting off of airplanes even.

Radio waves can also reflect off of the ionosphere and travel 400 miles or more. Typically this can't happen with 3m, unless conditions are just right, like with a strong solar flare. However, this can easily happen under normal conditions with broadcast AM, which has a wavelength 150m - 500m, especially at night. Generally, wavelengths longer than 10m start reflecting off of the ionosphere, but the effects of this are variable depending on the time of day and the time within the sun's 11 year sunspot cycle (in other words, space weather).