what is the amplitude of radiowaves ? and FM & AM bandwidth?

Question

hello,

I am in high school and I wanna study wireless communications in college. I have a very stupid question to ask.
I understand the electromagnetic spectrum and the frequency range that each type of wave occupies, but what is the amplitude of each type of these waves ? ( radio, microwave,xray,etc). Also, are these supposed to be sine waves, square waves, triangular waves ? According to fourier theorem, I understand that square waves and other nono-sinusoidal periodic waves can be written mathematically as a bunch of harmonically related sine waves, so when we talk about 95MHZ wave, are we talking about sine waves ?
Also, why r we running out of space since for example, in 95 mHZ range, one radio station can have 95.111 Mhz and 2nd?radio station can be at 95.112 and the third at 95.113 ?
please excuse me if my questions sound so stupid or if i m not asking the questions right.

Answer 1

A radio transmitter is shooting out countless photons/waves in all directions (most perpendicular to the antenna, but not many along the direction of the antenna). The further you are away from the antenna, the fewer the number of photons/waves that reach you (as 1/r^2, where r is your distance).

Since (ignoring reflected waves) all the photons/waves that reach you are all in phase with each other, so the amplitude of the signal is directly proportional to the RATE at which you are receiving photons/waves from the transmitter.

If you were 1 mile from a 100-watt transmitter, and 2 miles from a 400-watt transmitter, you would measure the same signal amplitude from each transmitter.

A individual photon does not have a meaningful amplitude. It is better to characterize a photon by its energy, which changes linearly with its frequency.

It is very difficult to make a radio receiver that is tuned finely enough to be sensitive to 95.111 MHz waves, while being completely insensitive to 95.112 MHz waves. There is a practical limit set by the quality of electronics hardware. This is why you mainly see radio stations set about 0.2 MHz apart, rather than 0.001 MHz apart.

Answer 2

There are no stupid questions, only stupid answers....there are no photons in radio waves.

Actually, radio waves can be any amplitude. In practice, when a radio station receives a license from the FCC to transmit, they are restricted to a certain level of ERP, or effective radiated power. This term is derived from both the transmitter power (which may be up to 50kW) and the antenna configuration. Antennas can provide gain and directivity, and usually what a broadcaster wants to do is to direct the radio waves to his listeners, who are on the ground and not in space. Often the tranmittting antenna will be outside of a city, in which case the antenna is designed to direct the radio energy towards the city. The effective radiated power is the antenna gain times the transmitter power. What you receive at a given location in terms of signal amplitude is ERP x 1/r^2 x gain of your receiving antenna, where r is the distance from the transmitting location.

In answer to your other question, for a pure sinusoid (i.e., a single frequency), you would be correct. You could divide the spectrum into infinitely smaller bands. However, a pure sinusoid can not carry any information, it must be modulated. When a single frequency (or carrier) is modulated, it spreads out in the frequency spectrum. Depending on the modulation technique, the required bandwidth could be very small, or very large. Cell phones and WiFi may cover several hundred MHz. FM radio stations are limited to +/- 200kHz (including the new HD radio standard), and I'm pretty sure AM stations are restricted to +/-2KHz. Generally, the FCC tries to keep stations farther apart than that in a geographical location, to prevent sideband interference, even though the sideband emissions are also limited by FCC standards.

Answer 3

Well first off, radio waves do not contain photons...when you look into the sky and radio waves are traveling I dont see a wave of light traveling through the sky. Photons are what are emitted from electrons to produce light. Anyhow, to answer your question as you want it to be, the typical amplitude that can be expected of radio waves coming from a radio transmitter does vary. It's typically on the order of a few mV. You can actually measure the strength of a signal coming from a particular station using a signal strength meter, as the name implies. The amplitude really is just dependent on the strength of the transmitter. If you know the output power of the transmitter and the distance between the transmitter and the receiver you can use a handy equation called the Frez Equation (may be spelled different). This equation relates the output power to the input power by the distance and the wavelength of the signal. Typical waves that are transmitted for analog systems are of sinusoidal nature, this is for simplification purposes, but digital signals may also be sent in which case a square wave may be used. As you stated, Fourier is a technique for turning any periodic signal into a series of sinusoidal waves in order to understand the signal better. The frequency 95MHz doesn't necessarily refer to a sine wave. It is referring to the period of the signal. It could be square wave or triangle wave or whatever. We're "running out of space" because the FCC only allocates so much "space" for different types of transmission (i.e. radio waves for radio stations, microwaves for govt communications and cellular systems). As you can see when we have millions of people trying to communicate over this spectrum we can "run out of space" pretty quick. Of course we could keep going up to the TeraHertz range and further, but when the frequency increases so does to energy and these waves can effect the human body in a bad way (i.e. cancer). They are being used in fiber optic communications though. To anser your question of why we cant just have a radio station at 95.111MHz and another at 95.112 MHz etc. is that at those frequencies...when you take communication systems in college you''ll understand this better, every station has two sidebands, which are +/- say...1kHz from the station frequency. If another station has its sidebands intersecting with those sidebands, then we create an effect called aliasing. Aliasing is very bad in communication systems. So for safety most stations are set with a reasonable distance apart.