Jamming is part of Electronic Warfare. One of the original parts, actually. Specifically, it is an Electronic Counter-Measure, and thus its purpose is to deny the enemy the ability to make use of the electromagnetic spectrum.
Jamming works by filling the receiver of the enemy's equipment with "noise." Generally the target of a jammer is the enemy's radar systems, although it is possible to jam other systems, such as communications and GPS navigation.
Now radars, and communications, and anything else that uses the electromagnetic spectrum to relay or divine information, relies upon the ability to distinguish the intended signal from the background noise. And yes, there is always background noise. This can be caused by the existence of actual background EM radiation hitting the receiver's antenna, or simply by the random vibration of electrons in the receiver, causing false fluctuations in voltage levels.
In any case, to be able to trust the information, be it 1s and 0s being sent over a digital radio comms channel, or the return of a radar off an incoming missile, the signal that we are looking for must be stronger than the noise. How much stronger depends on a number of factors, such as the sensitivity of the receiver, and upon some processing techniques that can be performed to help get rid of noise. However, for it to be working correctly, you need the power in the signal to be above the power of the noise, by a certain ratio. This is called the Minimum Signal to Noise Ratio, and is generally denoted as SNR min.
So, while some ECM techniques work by attempting to reduce the signal available to the enemy, jamming works by attempting to increase the noise, thus reducing the SNR. To use an analogy, it would be like attempting to find the location of a hidden speaker, with someone yelling in your ear. A somewhat difficult task is all of a sudden made much more difficult.
There are two main factors when you consider jamming. How much power you are sending out, and what kind of bandwidth you are spreading it out over. The power is going to be one of your primary limitations, as no one has an unlimited amount. What kind of platform you're mounting the jammer on is going to be very important here. A jammer aboard a jet fighter is going to have less power available to it than a jammer aboard a warship with 4 electric generators.
The bandwidth is more a choice than a limiting factor. While it is true that you can't transmit on every frequency with one antenna, you can certainly get ones with a very large frequency range, and you are usually able to cover most of that with one antenna. Instead, the bandwidth is a matter of deciding how to spread, in order to make sure that you're actually jamming the enemy's receivers. It doesn't help you much if you decide to transmit a whole lot of EM radiation at 10 GHz if the enemy's radars are using 15 GHz.
There are two primary choices here, with some variation. You can do what is called "spot jamming", which is concentrating all of the power that you are transmitting into the same (or within a very narrow bandwidth around) frequency that your enemy is transmitting upon. When you do this, it floods the enemy's receivers with a great deal of noise, and renders their radar ineffective.
Now, in order to do this, you also need to know what frequency the enemy is transmitting on. It is rather unlikely that you'll know this ahead of time, but most platforms with jamming capability will also come with some type of Electronic Support Measure that detects incoming EM radiation. You can use this info to tune the jammer.
This is, of course, assuming that the frequency stays the same. Many types of radar out there have the capability of using different frequencies, and especially do so when they detect that they are being jammed.
The other main option of jamming is called "barrage jamming", and it is when you spread out your energy being transmitted over a wide frequency range. If you do so, you will be sure that you won't be getting as much noise into the enemy's receiver, but you can be much more certain that at least some is getting there. This helps counter frequency agile radars, and indeed may be possible to jam several radars at the same time.
A hybrid method is known as "sweep jamming", and is basically taking a spot jammer, and continually changing the frequency, back and forth over a wider bandwidth, for example, the same bandwidth that a barrage jammer would use. Using this method, you'll hit all of the radars some of the time, hopefully enough to confuse them.
If you are jamming a radar, as you get closer to the radar, the strength of your jamming noise in the receiver increases. This is rather intuitive, as you know that standing next to a speaker at a concert is going to pump a lot more noise directly at your eardrums than being in a seat on the other side of the arena. This is because EM radiation, like acoustic noise, spreads out spherically. The amount of energy that is going to hit the antenna is inversely proportional to the surface area of the imaginary "bubble" of radiation that is spreading out from the transmitter. Thus, jamming signal drops off at 1 / R^2, as range increases, or more important, increases thusly as range decreases.
The signal from the radar itself, on the other hand, has a two way trip. It loses energy both as it travels out towards the target, and again on the way back after it has bounced off something. The radar signal, therefore, drops off at 1 / R^4, as range increases, or, again, increases as range decreases.
From this you can see that as a jammer moves towards something, the strength of the actual radar signal shall increase faster than the strength of the jamming signal. Eventually, as the range drops, the radar signal may get strong enough that it can pick up the target again. The SNR is going to exceed the SNR min, and the radar is going to start tracking the target.
The range at which this happens is called the burnthrough range. How to calculate the burnthrough range is dependant upon the relation between the jammer, and whatever the jammer is trying to screen.
The first example is "self screening," when the jammer is mounted upon whatever it is that you want to screen. This would be, for example, an attacking fighter jet, using its jammer to try to cover itself as it flies in to fire upon a ship. The jammer will take up space that could otherwise be used for more fuel, sensors, or weapons. As well, self screening leaves you vulnerable to weapon systems with home on jam capability.
Somewhat similar to self screening is escort screening, which is when you try to determine if the prostitute that you hired is an undercover cop... Just kidding! It's when the unit doing the jamming accompanies the unit that requires the screening. This helps alleviate some of the problems with self screening, as the weight and volume of the jamming equipment isn't taking up space on the plane that is going to fire all the extra missiles that it has room for. As well, if home on jam is used to destroy the jammer, the other plane can still fire its weapons and, hopefully for them, get the heck out of dodge. The disadvantage of escort jamming is that you obviously need two planes to do it. In either escort of self screening, the burnthrough range is calculated by the following formula, based upon the radar range equation:
RBurnthrough = ( Ppk Gr σ )1/2
( _______________________________ )
( 4 π (PJammer / Hz) Gj BW SNRmin )
The next scenario is called stand-off jamming, and it is when you send the target being screened ahead of the unit doing the jamming. An example of this would be when a ship fires a missile at another ship, and uses its jammers to attempt to prevent the other ship from detecting the missile until it is too late. Ideally, the platform doing the jamming would stay out of the range of the enemy's weapons, thus eliminating the threat of home on jam weapons. The disadvantage in this scenario is that the attacking unit will be more vulnerable, since less jamming energy would be hitting the receiver than if we were self-screening, resulting in a higher burnthrough range.
As well, there is stand-forward jamming, which is when the jammer unit is sent ahead of the attacking unit. This is rather rare, as it leaves the jammer rather vulnerable to being destroyed. In either stand-off or stand-forward, the burnthrough range is calculated thusly:
RBurnthrough = ( Ppk Gr σ Rj2 )1/4
( 4 π (PJammer / Hz) Gj BW SNRmin )
As well as the classic jamming method of flooding the enemy receiver with noise, there are also more advanced techniques, designed to convince them that you're somewhere that you're not. Because, you know, if they fire at you, they'll miss. Meanwhile, you won't miss. These types of deception jamming will be covered later, under Range Deception, Angle Deception, and Velocity Deception. I promise!