Radio Basics for Hackers, Part 1: Electromagnetic Radiation, Frequency and Wavelength

Welcome back my aspiring cyberwarriors!

As you already know, radio hacking is the leading edge of cybersecurity! So many things in our digital lives operate from radio signals, such as;

Cellphones

Drones

Wi-Fi

Bluetooth

Near Field Payment Systems (NFC)

Automobile Key Fobs

Remote Control Systems such as home security

RFID

Satellite Signals

GPS and so much more.

To address this gaping knowledge gap of these technologies, Hackers-Arise began the SDR for Hackers training series. The goal of this series is to use inexpensive hardware and free software to hack a variety of radio signals to demonstrate the vulnerability of these devices and technologies.

Electromagnetic Radiation

At the risk of stating the obvious. all of us use radio signals in our everyday life. Radio communication is so ubiquitous and common in the 21st century that many of us forget what a magical thing it is. When I say radio, it often conjures up an image of the music-playing device in our cars and trucks, but radio signals are used for SO much more. I will attempt to start with the basics and build up from there so that no one misses the fundamentals that are critical when hacking these signals.

How does radio happen? You probably learned somewhere back in your primary or middle school science classes that when voltage is applied to two metal plates, an electrical field is generated. When current then runs through the wire, it generates a magnetic field (electric field is the force around a charged particle, while a magnetic field is the force a magnetic pole exerts on a magnetic material ). A changing electrical field can produce a magnetic field. Radio is the result of how those electrical and magnetic fields interact.

A change in the current in a wire produces a changing magnetic field. A wire in a changing field produces a voltage. A changing current in a wire will cause a changing magnetic field. This will cause a changing electric field perpendicular to the magnetic field. Note that in each case, it is the change that causes the development of these fields (hence, as you know, an electrical generator needs to always be moving to generate electricity). The movement causes the electrons to move through the wire. The movement of this field is called an electromagnetic wave.

These electromagnetic waves share the same properties as light. In fact, light is a visible electromagnetic wave! We already understand that light can be reflected, refracted, diffracted, absorbed, and filtered and so can our electromagnetic waves. This makes understanding electromagnetic waves a little easier.

As mentioned above, it is the changes over time that initiates these fields. A current can take many forms but the most common form looks like that below.

 

Frequency

This shape is referred to as sinusoidal (no, it has nothing to do with your nose or sinuses). This shape or wave form is used in most practical systems. This type of signal is said to have a frequency or the number of times it completes a cycle (think of it like a bicycle. A cycle is every time the pedal comes back to the same position. The frequency is how long it takes for your foot and pedal to return to the same position). Since electromagnetic waves travel the speed of light (186,000 miles per second) this cycle is measured in fractions of a second or how many cycles per second.

Frequencies are expressed in hertz. One cycle per second is one hertz. From there, the industry and science use the familiar Greek suffixes for thousand (kilo), million (mega), billion (giga) and trillion (tera). So, 1000 cycles per second is 1 kilohertz, 1 million cycles is 1 megahertz and 1 billion cycles is 1 gigahertz. For our purposes here, the range of frequencies between about 1 MHZ (1,000,000) to about 6GHZ (6,000,000,000) are where we will focus our attention.

 

Frequency is very important in understanding how signals transverse from one place to the next. The frequency of the wave will determine how it interacts with objects along the way in its path. As will see, lower frequencies are better able to bend and refract around objects than higher frequencies.

The frequency of a radio signal is important for a number of reasons.

First, it determines the range of the signal. Lower frequencies have longer wavelengths, which can travel farther through the atmosphere. This is why AM radio stations, which typically use frequencies in the 530-1700 kHz range, have a longer range than FM radio stations, which typically use frequencies in the 88-108 MHz range.

Second, frequency determines the amount of data that can be transmitted over a radio signal. Higher frequencies can carry more data than lower frequencies. This is why cellular networks, which use very high frequencies in the 1.7-2.2 GHz range, can transmit more data than traditional AM or FM radio stations.

 

Third, frequency determines the way that radio waves interact with the environment. Lower frequencies can penetrate through walls and other objects, while higher frequencies are more likely to be reflected or absorbed. This is why radio waves are used for applications such as weather forecasting and air traffic control, where it is important to be able to penetrate through objects.

In general, radio signals are described by frequency. Various frequencies are reserved and used for various purposes, so as we described above, Wi-Fi and Bluetooth operate at 2.5ghz (as does your microwave) while your cellphone operates at multiple frequencies between 700Mhz to 2.3 Ghz. Automobile lock dongles operate at 315Mhz (US and Japan) and 433.9Mhz (Europe). Aircraft ADS-B signals(communicating GPS coordinates, speed and altitude) operates at 1090 Mhz.

While we will examine intercepting, replaying, jamming and hacking signals across this range at this stage it is important to understand which SDR devices are capable of operating in these ranges. Here is table of the most common SDR devices and their specifications.

 

Not on this list is the Flipper Zero. It only operates at sub 1GHZ frequencies. This means that it can not operate (without modifications) at the frequencies of your Wi-Fi signal, Bluetooth, aircraft ADS-B or most cellular signals. The Ettus and HackRF One have the widest frequency bands and can receive and transmit at most of the frequencies we will be using here. The Lime SDR has two transmitters and two receiver channels, making it better suited for high-performance applications.

In summary, frequency is an important parameter for radio signals. It determines the range, data capacity, and interaction with the environment of a radio signal. The choice of frequency depends on the specific application for which the radio signal is being used.

Wavelength

Wavelength of a wave is the distance the wave travels during one cycle. Wavelength can be determined by dividing the speed of the signal (speed of light) by the frequency of the signal. So, if we have a signal at a frequency of 2.5Ghz (WiFi), the wavelength will be the speed of light (300,000,000 meters per second) divided by the frequency (2,500,000,000 cycles per second. This means that the wavelength of a Wi-Fi signal is .12 meter.

300,000,000/2,500,000,000 =.12m

That is how far the the wave travels during one cycle.

Since the speed of light is constant (remember that from your high school physics), each radio signal can be described by either its frequency or wavelength. In general, in this industry radio signals are described by their frequency (they just as easily could be described by wavelength). Various frequencies are reserved and used for various purposes. As we described above, Wi-Fi and Bluetooth operate at 2.5ghz (as does your microwave) while your cellphone operates at multiple frequencies from 700Mhz to 2.3Ghz. Automobile lock dongles operate at 315Mhz and 433.9Mhz. Aircraft ADS-B (communicating GPS coordinates, speed and altitude) operates at 1090 Mhz.

Summary

In order to become adept as a radio hacker, you must understand some basics concepts of radio waves. In this first of a series of radio basics, we attempted to understand the basics of electromagnetic radiation, frequency and wavelength.

For more on radio basics for hackers, keep coming back to this space or sign up for our Radio Basics for Hackers training June 29 or our SDR for Hackers training, July 11-13.