How does radio jamming work?

A brief introduction to communications jamming and the factors that go into making it more or less effective.

This spun out of a question - "why aren't civilian drones being impacted by Russian electronic warfare assets in Ukraine?"

The question itself doesn't make sense, because they are, massively. I've included a source for that below.

Russia’s Electronic-Warfare Troops Knocked Out 90 Percent Of Ukraine’s Drones
The electronic suppression of Ukraine’s unmanned aerial vehicles blunted one of Kyiv’s biggest advantages in the early months of the war.

However, the question got me started explaining jamming (also referred to as electronic attack, or EA), so let's talk today about the concepts behind the question to build some sense of what factors are at play.

Jamming Fundamentals

The fundamental concept of jamming relies on the concept of noise. You can think of the most basic version of jamming as yelling over someone while they're trying to talk. The volume of the yelling has to be above the volume of the talking in order to prevent you understanding what's being said. If the yell-er is far away, while the talk-er is close, they'll have to yell even louder. The yell-er might cup their hands around their mouth to direct more of the sound toward you (or use an old-timey metal bullhorn for comedic effect), which takes the yelling they're already doing and makes sure more of it is directed toward you, the target.

In all these cases, the concept is the same: noise is being generated and directed at the listener (this part is a common but key mistake - you don't jam the transmitter, you jam the receiver) so that they can't pick the real thing they want to hear out of all the noise. Jamming a radio operates the same way. The situation I described above is presented in equation form below.

radar equation (courtesy Wikipedia)

In words, this equation says that the ratio of the jamming strength to the "true" signal strength (J/S) is equal to:

  • the power of the jammer divided by the power of the true signal
  • multiplied by a term that represents the distance the jammer is away from the listener
  • multiplied by the ratio of the bandwidth of the true signal to the bandwidth of the jam signal

The higher the J/S ratio, the harder it is to pick the true signal out from the noise. The J/S ratio required to "jam" a particular system varies, but generally anything higher than 1 will work. (For more detail, check out this article.)

This equation is simplified somewhat, because it's written for a radar system, where the transmitter and the receiver are the same system - for communications, you add in the complexities that come from having three locations involved (transmitter, jammer, receiver) and the various antenna configurations of each of them. Here's why I'm not going to delve too much deeper into those complexities:

my notes from David Adamy's book "EW 101"

Instead, we'll focus on the tradeoffs involved, rather than the exact math behind it. Because... yikes. Instead, let's use the diagram below (also from Adamy's EW 101) to set the scene and examine the factors involved.

conceptual diagram of a jammer, transmitter, and receiver from David Adamy's EW 101

There are three main factors when jamming a communications signal (I'm leaving out smaller ones so we don't end up like that equation above):

  • the relative power of the jammer and transmitter
  • the relative losses of the "JMR - RCVR" link and the "XMTR - RCVR" link
  • the relative bandwidth of the signal and the jammer

Relative Power

Jammer power and signal power are fairly straightforward: if you want to make it harder to hear someone talk, yell louder. If you want to make it easier to understand your signal over the yelling, speak louder. There are practical limitations, of course - you can only yell so loud, and eventually your voice will give out. For jammers (and transmitters) putting out more power means you need more electricity (draining batteries faster, or requiring more generators and fuel), larger and more complex circuitry, and all of the drawbacks that come with generally needing "more stuff." Trying to overcome the limitations on how much power a jammer or transmitter can put out is where the other factors come into play.

The relative losses of the two links is where a lot of little factors creep in and lead to the horrifying equation we looked at earlier. Losses in a communications link depend on a lot, with the main ones being distance, terrain, and antennas used by both sides of the link. Distance is a major factor because power drops off fast in radio communications - exponentially, in fact. Terrain acts the same way - trying to jam from the wrong side of a mountain is going to result in a significant decrease in the jamming power the receiver sees. The analogy here would be trying to do your yelling from far away, or from another room with a closed door in between. It's possible to yell loud enough to overcome that, but it's going to be a lot harder than moving closer, ideally to somewhere without obstacles between the yell-er and the listener. These factors apply to the transmitter as well - if the transmitter is close to the receiver, and the jammer is far away, it's going to take a lot more effort to jam the link than if the transmitter is also far away. Remember, all of these things are ratios - we're looking at the relative distance, relative power, relative terrain loss, and so on.

The last of the factors for link loss is the most technically complex - antennas. The quick explanation is that an antenna acts a lot like a bullhorn for the yelling - it focuses and directs it toward the target. It doesn't create new energy, just makes sure more of it goes where you want it. The tradeoff here, of course, is that you have to know where to point the antenna, which means you have to track the location of the listener - which can be hard! On the flip side, the listener might also be using a directional antenna pointed at the transmitter, which means that it's better at "ignoring" things that aren't in the direction the antenna is pointed. This makes things harder for the jammer if it's off in a different direction (at the extreme end, imagine something like a laser communications link - it's incredibly directional and trying to jam it from another direction would be just impossible, since the laser receiver could be set at the end of a tube).

highly directional receivers are hard to jam from a direction the antenna isn't pointed in - a laser communications link is an extreme example

Radio signals aren't as directional as lasers, so the effect isn't as extreme - but the same concept applies. Directional antennas help "focus" power when transmitting and suppress signals from directions they aren't pointed in. These can make jamming easier or harder depending on what antennas are used where.

Relative Bandwidth

The last of  the three big factors is the relative bandwidth of the jammer and the signal. I'm not going to go too deep into the concepts of frequency and bandwidth here, but the short version is that you can tune radios to different radio frequencies (these are the different channels on a radio - each is a different frequency) and use them to communicate. If you want to jam a radio, you need to be jamming the frequency it's using. If it changes frequencies, you need to start jamming the new one (and so on). There are several ways to approach this, shown below. The vertical axis is frequency, and the horizontal axis is time.

different types of jamming: barrage, spot, and swept

The characteristics of the communication signal can change the required jamming method. Military radios, for instance, "hop" frequencies extremely rapidly and in an unpredictable sequence, meaning that you can't jam just one frequency and instead have to simultaneously jam the entire set of frequencies the radios could be on. What this means is that the transmitter can put all its power into one small segment of the frequency spectrum at a time, while the jammer has to jam them all. This is a huge power imbalance caused by the difference in jamming bandwidth (large) vs transmission bandwidth (small). There are other methods, such as error correction codes, that can make a signal harder to jam - but that's  outside the scope of this post and can be very complex. Plus, it's entirely dependent on the technology in use by the radios you're trying to jam, so it's not something you have control of anyway.

In Short

Things that make jamming harder:

  1. Being far away from the receiver you're jamming
  2. The transmitter and/or receiver using directional antennas (assuming they're pointed correctly)
  3. The transmitter using more power
  4. The transmitter using less bandwidth than you're jamming

Things that make jamming easier:

  1. Being closer to the receiver you're jamming
  2. Using a more directional antenna pointed at the receiver you're jamming
  3. Using more power
  4. Being able to jam the exact bandwidth the signal is using (and no more)

All in - a lot less complex than you'd think (at least, I hope). Thanks for reading!