RF Propagation

Radio waves don’t travel in straight lines from transmitter to receiver and stop there. They reflect off walls, bend around obstacles, get absorbed by building materials, and arrive at the receiver by several paths at once — sometimes reinforcing each other, sometimes canceling each other out. The result is that signal strength in a real building is far less predictable than a coverage map suggests.

In this talk, Bastille Chief Engineer Dr. Bob Baxley builds an intuition for RF propagation: the mechanisms that carry a signal from one point to another, how much power survives the trip, and why moving a receiver a few inches can change everything.

These are not academic details. Propagation behavior determines where a wireless device can be detected from, how reliably a link holds, and why the same transmitter looks strong in one part of a facility and invisible in another — all of which shape how wireless monitoring has to be designed and deployed.

Key takeaways

• Four mechanisms move a signal: line of sight, reflection, shadowing, and scattering — and in a real building all four happen simultaneously.
• Shadowing depends on materials: a wall attenuates the signal directly behind it, while some energy wraps around it. How much passes through versus gets absorbed or reflected is a property of the wall material.
• Path loss follows an exponent: in free space, power falls off with the square of distance — a path loss exponent of two — because the same energy spreads over an ever-larger sphere.
• Real environments are worse than free space: path loss exponents of three or four are common, meaning power falls off with the cube or fourth power of distance.
• Multipath can help or hurt: signals arriving by different paths can cohere constructively — a receiver can see a signal 50% stronger than what was transmitted — or destructively, cutting it roughly in half.
• Small movements matter: because RF travels at the speed of light, the geometry that produces a dead spot changes over inches, which is why inching a car forward at a stoplight can restore radio reception.
• Moving objects modulate the channel: a hand waved near an antenna visibly changes the received spectrum — the same physical effect radar exploits to detect aircraft.
• Propagation shapes detection: signal quality and link reliability are functions of the environment, so understanding propagation is a prerequisite for locating and characterizing wireless devices in a facility.

The four propagation mechanisms

When a signal propagates from a transmitter to a receiver, several things happen at once:

• Line of sight. The signal travels directly from source to destination with nothing in the way.
• Reflection. The signal bounces off walls, floors, ceilings, and metal surfaces and still arrives at the destination — just later, and by a longer route.
• Shadowing. An obstacle sits between transmitter and receiver. Some energy permeates through it, but with reduced strength.
• Scattering. Diffused reflection off a cluster of small reflectors, spreading energy in many directions at once.

What ultimately matters is how much signal gets from the transmitter to the receiver, because that dictates signal quality and how likely the link is to be maintained.

Shadowing: what walls actually do

Simulate a transmitter in a room with two walls and plot signal intensity as a heat map, and the shadowing effect is immediately visible. As energy emanates from the transmitter and hits a wall, the region directly behind the wall shows up as a cold spot where the signal is attenuated.

Some signal wraps around the wall rather than passing through it — obstacles are not clean cutoffs. And the wall material dictates the balance: how much energy permeates through, how much is absorbed, and how much is reflected back. The intuition is the one you already have. If there’s a wall in the way — especially a dense or metal-lined one — it will adversely affect the wireless system.

Path loss: why distance costs power

Even with no obstacles at all, signals weaken with distance. The clearest way to picture it: energy leaves an isotropic transmitter as a sphere expanding outward from a point. The total power in that sphere is fixed, but as the sphere grows, its surface area grows — so the energy density at any point on the surface drops.

A receiver close to the transmitter intercepts a relatively large portion of the sphere. A receiver far away intercepts a tiny portion. Work through the math and you find that under this mechanism, power falls off with the square of distance. Double the distance, quarter the power.

The path loss exponent

That exponent — the two in “square of distance” — is called the path loss exponent. In free space, it is two. In other propagation environments it can be higher: three or four is common indoors and in cluttered environments. A path loss exponent of three means power falls off with the cube of distance, which is considerably worse than free space.

This is why the same device can be detectable across an open floor plan but nearly invisible three rooms away in a building full of interior walls, ductwork, and shelving.

Multipath: constructive and destructive interference

The most counterintuitive mechanism is multipath. A signal arrives at the receiver by several routes at once — the direct path plus every reflection — and those copies add together. Whether they reinforce or cancel depends entirely on the geometry of the space.

The audio analogy is exact. A voice reaches a microphone directly, but it also bounces off the walls and arrives slightly later. Audiophiles arrange their living rooms so that sound from the speakers coheres at the center of the couch; move a little to one side and you land in a dead spot. RF behaves identically. The only difference is that RF travels at the speed of light rather than the speed of sound, so the same effects play out over much smaller distances.

• Constructive multipath. In a simulation with a transmitter at the center of a room and a receiver off to one side, the reflected copies can arrive in phase. The received sine wave measures about 50% larger than what was transmitted — the receiver gets more than was sent.
• Destructive multipath. Move that same receiver a small distance and the geometry changes so the copies arrive out of phase. The signal nearly dies, dropping to roughly half of what was transmitted.

You have experienced this in a car. Stopped at a light in an urban canyon with buildings on all sides, radio reception is poor; inch the car forward and it changes drastically. Signals bouncing off the buildings are combining, and whether you are in a dead spot depends entirely on where you’re sitting.

Seeing multipath live

The demonstration in the video makes this visible. A software defined radio is tuned to the 500–600 MHz band, where TV stations transmit at fairly constant power, and the received spectrum is displayed as a spectrograph. The frequency response across the band shows a wavy, frequency-selective pattern — the signature of multipath.

Moving a hand near the antenna changes the physics of the environment, and the frequency-selective pattern visibly shifts in response. Move erratically and the spectrum churns; hold a flattened arm near the antenna and it is unmistakable in the spectrogram that someone is moving nearby.

This is the same phenomenon radar exploits. A radar system watching for aircraft is fundamentally looking for the multipath change a plane creates as it flies through the signal.

What propagation means for wireless security

Propagation physics sets the boundaries of what any wireless monitoring system can do. Because path loss exponents indoors are higher than in free space, and because walls shadow and materials absorb, a single sensor cannot see an entire facility. Coverage has to be engineered against the building, not assumed from a datasheet range.

Multipath cuts both ways. It makes raw signal strength an unreliable proxy for distance — a device can appear stronger from farther away if the geometry favors it — which is why accurate localization requires multiple sensors and processing that accounts for the environment. It also means the RF environment carries information about physical activity in the space, since motion changes the channel.

The practical takeaway is the one Baxley emphasizes throughout: build an intuition for how the physics of the environment affects the signal, because every question about wireless coverage, detection, and localization comes back to it.

Full Transcript

Hi, welcome to this talk on RF propagation. My name’s Bob Baxley, and I’m the Chief Engineer at Bastille, where I work on the radio and data science teams. The objective of this talk is to give you a feeling and an intuition for how radio waves propagate, which in turn gives you a feeling for how wireless devices communicate — and specifically, how well they communicate in various channel conditions, like walls and those sorts of things.

Propagation mechanisms

I’ve got a piece of clip art here that illustrates the various propagation mechanisms. When my signal propagates from here to there, various things happen. One thing that happens is I send a line of sight signal, which goes directly from here to there.

Another mechanism is that things bounce off the walls — these are reflections — and those signals end up at the destination. There’s also this mechanism of shadowing: if there’s a wall between us, some signal will permeate through the wall, but it will go through with reduced signal strength, so as we say, it’s shadowed a bit. And there’s scattering, which is like diffused reflection — there’s a bunch of reflectors gathered together and you have this scattering event.

There are all these propagation mechanisms. In the next few slides I want to give you a feeling for how they affect the signal, because what we’re ultimately interested in is how much signal gets from the transmitter to the receiver — that dictates the signal quality and how likely the link is to be maintained.

Shadowing

Let’s first start with shadowing. Here I’ve got a simulation where I’ve drawn two walls, and I’ve got a transmitter in two different spots — a star transmitter and a triangle transmitter. What the heat map shows is the signal intensity in space. As the signal emanates from that star and hits the wall, it’s shadowed by the wall. That’s why, directly on the other side of the wall, we have this blue region where the signal is attenuated.

As you can see, some signal ends up wrapping around the wall. Depending on the wall material, the material dictates how much permeates through the wall and how much gets absorbed or reflected. So that’s shadowing, and it’s pretty intuitive: if there’s a wall in the way, especially a lead wall, it’s going to adversely affect your wireless system.

Path loss

The next one down the level of intuition is path loss. As I travel out from my signal source, surely I lose some power, right? So if I want to transmit a long way away, I need more power. One way to visualize that is: if this is my transmitter and the signal leaves it and propagates in an isotropic pattern, it’s kind of like a sphere blowing up from this point.

At this point I have a certain amount of power, and as I make the sphere bigger, the surface area of the sphere contains all that power. So the further away I get, the more surface area there is, which means there’s less energy density. If I get a long way away, my receiver’s only seeing a tiny portion of the sphere. If the receiver is close, then it sees a much bigger portion of the sphere.

If you work out the math behind all of that, you end up showing that under that propagation mechanism, the power falls off with the square of distance. So if I take the distance from here to there and square it, that’s how much less power I have. That squaring, that two, is called the path loss exponent. In free space, we say the path loss exponent is two. In other propagation environments, the path loss exponent can be higher, like three or four. If it was three, that means the power falls off with the cube of distance, which is worse. So depending on the propagation setup, you might have more or less power loss.

Multipath

The last interesting mechanism is called multipath. Just like when I’m speaking now, my voice is going directly to the microphone, but it’s also bouncing off the walls and then coming back to the microphone. Depending on the geometry of the walls, that will dictate how these multiple signals cohere, or decohere, at the receiver.

You may be familiar with audiophiles who try to optimize their living rooms so that the speakers all cohere at the center of the couch, so they can hear the audio really well. If you move off from that a little bit, you might be in an audio dead spot. The exact same mechanisms apply in RF. The only difference is we’re talking about the speed of light instead of the speed of sound, so that’s much faster, on much smaller distance scales.

Here I’ve got an animation where I show a signal emanating from a black dot in the middle of the space, and you can see these wave patterns. On the right there’s a pink dot, and I’m plotting the black and pink signals up above. This is constructive multipath. As the signal bounces off those walls and then arrives at the pink dot, you can see the pink sine wave is actually larger — I’ll mark it out here — it’s 50% larger than the black signal that was transmitted. So this is coherent interference. I’m getting more than I sent.

There’s also destructive interference. If I move that pink receiver just a little bit spatially, the geometry might be such that the signal is attenuated a lot. That’s what we’re seeing here. As all these multipaths decohere, you can see the signal almost dies, and is about half of what it was when I transmitted it.

You can see a similar phenomenon when you’re in a car in an urban canyon. There are buildings all around you, and you’re at a stop light listening to the radio station, and the reception is not great. If you inch up your car just a little bit, the reception might change drastically. It’s the same mechanism — the signals are bouncing off of the buildings, and whether you’re in a dead spot or not depends on the geometry of all this.

Multipath demo

Now I’m going to show you a demo where I’m going to plug in this radio and move my hand around, and you’ll be able to see in the signal how the multipath changes.

What I’ve done is I’ve taken my software defined radio, and what you can see on the screen is a spectrograph. I’ve tuned the radio to the 500 to 600 megahertz band, and in this band what we’re seeing are TV stations. TV stations are fairly power constant, and you can see this frequency pattern is kind of wavy.

What I’m going to demonstrate is that when I start moving, you’ll see the effect of multipath. Remember how I said slight changes in the physics of the environment can change the characteristics of the signal? Here the characteristics are changing with the multipath changing, and it’s going to be manifested by this frequency-selective pattern changing.

You can already kind of see, as I move my hands, the pattern changes. I’m going to move them closer to the antenna. When I move them erratically, you’ll see the spectrum changing as my hands change. When I do that, it’s very obvious that you can see my hands change.

This is a very similar phenomenon to the phenomenon that radar uses. Radar’s looking for planes, and the effect that it’s really looking for is the multipath change the plane is creating with the signal as it flies by. Again, you can see, as I flatten my arm like this, it’s very clear in the spectrogram that somebody’s moving nearby.

So it’s a super cool phenomenon, and to the higher-level point, it’s just about getting your own intuition for how the physics of the environment can affect your signal. Again, I’m Bob Baxley with Bastille. Thanks for listening.