Radio communication and antennas

Question

I'm trying to understand how an antenna is able to understand, but have a few quesions:

1. I understand that for an antenna to distinguish between 2 (or more) signals, they have to be in different frequencies. This means that, for example, every mobile phone transmits in a unique frequency? otherwise the signals would mix in the antenna..

2. So if I want to disrupt a signal that an antenna receives, I need to transmit some sort of noise in the same frequency (preferably the inverse of the received wave), and then the antenna couldn't understand that signal?

3. Also, a third question (not related to the last two): what is the tolerance of an antenna? How sensitive it can get? for example, how far away from a transmitter an antenna could receive the sent message (e.g. in communication between earth and a satellite).

Answer 1

1) Normally, the antenna isn't the only component that distinguishes between the various competing signals received. The antenna does have a bandwidth and will attenuate signals outside that band. A typical antenna on a cell phone mast for example, may receive in the range 1.8GHz - 2.4GHz (just an example, you would have to look up manufacturer data sheets for more accurate figures).

However the principal component that rejects competing signals in other frequencies is the filter. This is a tunable component that can be tuned to the frequency you wish to receive at any particular time and will strongly reject other signals.

There are other ways of transmitting signals to multiple cell phones (for example) than using different frequencies. Code Division Multiple Access is an example.

2 The most obvious way to disrupt signals is to transmit a high power signal on the same frequency band that the target signal is using.

3 An antenna may have a set of directions for which it is designed to achieve maximum gain. This directionality is encapsulated in the antennas gain pattern. The maximum achievable distance between an transmitter and receiver is a function of many things: The transmit and receive antenna gain patterns, the receiver sensitivity, the transmit signal power, the propagation loss... The calculation of these effects is called a link budget.

Answer 2

1. An antenna is just a receiving system... it doesn't actually decipher anything. It's the electronics behind the antenna that does the interpreting. Sort of like your eyes or ears. They don't do any of the interpretation themselves, it's your brain doing all that. Now that I've got that little nitpick out of the way... Mobile phones actually transmit at two different frequencies simultaneously. Each frequency carries one side of the conversation, so you can speak and hear at the same time. You can read about it in great detail here.
2. As @twistor59 mentioned, the easiest way to jam a cellphone signal is to send a high powered signal at the same frequency as the cellphone. This is a nice link to read.
3. The sensitivity of an antenna is basically defined as how small a signal it can detect in the presence of noise (to put it roughly). It can also be measured by the signal to noise ratio. This depends on two things - The receiving area of the antenna, and the electronics behind the antenna. For parabolic dish antennas, the signal to noise ratio increases linearly as the surface area of the dish increases. So if you want to detect signals from outer space (Radio/Microwave astronomy, signals from satellites etc.) you need a pretty massive dish. The size of the dish is determined by the longest wavelength that you want to receive on the antenna. These two (1, 2) links should explain the difference between a parabolic antenna and a dipole antenna, and how sensitivity changes for each etc.