What are Radio Frequencies (RF) and how are they used for indoor location

Most indoor positioning systems work by sending a signal between the object you're tracking and a known (fixed) point. In radio frequency (RF) systems, that signal travels as an electromagnetic wave, and the nature of that wave determines how it interacts with walls, machinery, people, and furniture, and therefore determines why different indoor location technologies perform so differently in their range, accuracy, cost, and reliability.

What Is A Radio Wave and Radio Frequency?

A radio wave consists of electric and magnetic fields that move outward from a transmitting antenna. A receiving antenna converts some of that energy back into an electrical signal. When visualised, the radio wave looks very much like a wave you would see in the ocean, with peaks and troughs. The wavelength is the distance between the peaks of the waves. The frequency is how many times the peak of the wave (or wave cycles) occurs every second and is expressed in hertz:

1 kHz = 1,000 cycles per second
1 MHz = 1 million cycles per second
1 GHz = 1 billion cycles per second

As you would imagine, frequency and wavelength are directly connected. The higher the frequency, the shorter the wavelength. A 1 GHz signal has a wavelength around 30 cm. At 2.4 GHz, it drops to about 12.5 cm. At 6 GHz, it's roughly 5 cm. That's because the signal is cycling through more waves every second and therefore the 'peaks' are closer together.

The wavelength matters because its size affects how signals interact with objects in the environment. More on that below.

The radio frequency (RF) expressed in herts, refers to the part of the electromagnetic spectrum that is used for transmitting signals through space.

Radio Waves Lose Energy Over Distance

A signal weakens as it travels. Even in open space, energy spreads across a growing area, so a receiver captures less of it the farther away it sits. Obstacles in the way of the signal, such as structures or water, add significant losses to the signal. As signals pass through plasterboard, glass, brick, concrete, timber, furniture, machinery, and people, some energy gets absorbed, some bounces back, some scatters in different directions.

These effects are called attenuation, and it varies widely depending on material type, thickness, and even moisture content. That's why frequency alone doesn't determine range. Transmit power, antenna design, receiver sensitivity, bandwidth, environmental conditions, and regulatory power limits all play major roles.

What Changes as Frequency Rises?

A common belief holds that lower frequencies travel farther and higher frequencies travel shorter distances. There's some truth in that, but it's not a hard rule.

Imagine two radio transmitters sending signals across an empty room, one at a lower frequency and one at a higher frequency.

If both systems use receiving antennas with the same gain (the amount of focus on the signal) the receiver will generally receive less power from the higher-frequency signal at the same distance. This does not mean that higher-frequency systems must always have shorter range (engineers can compensate with antenna design, receiver sensitivity, transmit power, and other techniques). It simply describes what happens to the radio signal itself when the other factors are held constant.

Higher-frequency signals also tend to suffer more from obstruction and penetration loss through building materials. Lower frequencies, with longer wavelengths, bend around obstacles more easily.

But higher frequencies bring advantages. The shorter wavelengths let you pack directional antenna arrays into smaller spaces and support narrow beams (and therefore allow for more precise angle measurements in systems that determine the location of objects based on triangulation of signals). More on this in our location measurement article.

What Happens When RF Hits a Material?

When a radio wave meets an object, several things can occur:

Reflection – Some energy bounces off surfaces. Metal reflects strongly. Large metal objects like machinery, shelving, structural steel, and ductwork create multiple copies of the same signal bouncing around a building.

Absorption – Some energy enters the material and converts into other forms. Water-rich materials—people, liquids, food products—cause significant attenuation at certain frequencies, changing the indoor RF environment.

Transmission – Some passes through. How much depends on the material, thickness, frequency, and angle of incidence. Real-world building measurements show wide variation between different wall types, glass, doors, and structural elements.

Diffraction – Waves bend around obstacles. Longer wavelengths generally diffract more easily around objects of a given size, allowing receivers to pick up signals even without a direct line of sight.

Scattering – Irregular surfaces or objects sized similarly to the wavelength scatter RF energy in multiple directions. Industrial spaces with pipes, equipment, racks, and vehicles create complex scattering conditions.

Common frequency bands used for indoor location

A few frequency ranges come up repeatedly in indoor positioning. You will notice the different characteristics as the frequencies rise.

Low frequency (LF), around 125–134 kHz: long wavelength, short range in practice because so little power can be radiated legally at this frequency, but good penetration. Used in some proximity and access-control RFID systems.
High frequency (HF), 13.56 MHz: used by NFC and many access cards, again a short-range, near-field technology rather than a true positioning one.
High frequency (HF), 433 MHz: good penetration and useful in environments with lots of liquid or blocking materials, lessa accuracy and difficulty confining known locations to within a specific room.
Ultra-high frequency (UHF), 860–960 MHz: the band used by most passive RFID inventory systems, offering read ranges of several metres.
2.4 GHz: shared by Wi-Fi, Bluetooth Low Energy and Zigbee. A practical middle ground of range and penetration, widely used for coarse-grained indoor location.
3.1–10.6 GHz: the band used by ultra-wideband systems, chosen for its wide available bandwidth rather than for any particular propagation advantage, which allows for the very short signals to be distinguished more easily from multi-path, reflected signals to improve accuracy.
Millimetre wave, 24 GHz and above: short range, poor penetration, but very fine angular and distance resolution, increasingly used in radar-style sensing.

Location Systems Face Harder Problems Than Communication Systems

Radio waves carrying information are also used in communication systems. For a communication system, the main question may be whether enough signal reaches the receiver to recover the data.

A location system has a harder problem. It must extract information about where the signal came from. That information may come from determining how strong the signal is and therefore how far it has travelled (received signal strength indication or RSSI), the time the signal has taken to travel between two points (Time of Flight, or ToF), the difference in time when signals are received at different fixed points, the direction or angle the signal arrives across an attena array, or a combination of measurements. More on this in our location measurement article.

The problem with radio waves is that a signal that is reflected off a surface, or partially absorbed by some material, may carry data perfectly while giving misleading location information because it travelled farther than the direct route or has a weaker signal than would otherwise occur in open space.

This understanding is central to indoor location or real-time location systems (RTLS) that use radio frequencies.

That is why selecting a frequency for indoor location involves more than asking how far the signal travels. The important question is how predictably the signal behaves in the intended environment and whether the system can extract the measurements needed to calculate location.