79 Radio Wave Basics for IoT

79.1 Learning Objectives

By the end of this chapter, you will be able to:

Explain the relationship between frequency, wavelength, and the speed of light
Identify the key frequency bands used in IoT applications
Compare the trade-offs between different frequency bands (sub-GHz, 2.4 GHz, 5 GHz)
Understand why radio waves propagate differently through various environments

79.2 Prerequisites

Before diving into this chapter, you should be comfortable with:

Basic algebra and logarithms (dB math): Mathematical Foundations
Core networking vocabulary (links, range, reliability): Networking Basics

For Beginners: What is Wireless Propagation?

Think of wireless communication like shouting across a field. The farther away your friend is, the harder it is to hear you. Several things affect how well your message gets through:

Distance - Farther = weaker signal
Obstacles - Walls, buildings, and trees block or absorb some energy
Interference - Other people shouting (other radio signals) make it harder to hear
Frequency - Like pitch in sound; higher frequencies travel differently than lower ones

Radio waves follow similar principles, but we can measure and predict their behavior mathematically. This chapter gives you the tools to: - Estimate how far your wireless devices can communicate - Understand why some technologies work better indoors vs outdoors - Choose the right wireless technology for your application

Key Takeaway

In one sentence: Radio signal strength decreases with distance and frequency, and a link budget calculation tells you whether your wireless system will work before you deploy it.

Remember this rule: Every 3 dB of antenna gain doubles your effective range (or lets you halve transmit power), and lower frequencies (sub-GHz) travel farther and penetrate walls better than higher frequencies (2.4/5 GHz) at the cost of data rate.

79.3 Why Propagation Matters for IoT

Every wireless IoT system must answer a fundamental question: Will my devices be able to communicate reliably?

The answer depends on understanding:

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flowchart TD
    subgraph Questions["Key Propagation Questions"]
        Q1["How much power<br/>do I need to transmit?"]
        Q2["How far can my<br/>signal reach?"]
        Q3["What data rate<br/>can I achieve?"]
        Q4["How reliable will<br/>the connection be?"]
    end

    subgraph Factors["Propagation Factors"]
        F1["Frequency Band"]
        F2["Path Loss"]
        F3["Fading & Multipath"]
        F4["Interference"]
    end

    Q1 --> F2
    Q2 --> F1
    Q2 --> F2
    Q3 --> F1
    Q3 --> F4
    Q4 --> F3
    Q4 --> F4

    style Questions fill:#2C3E50,stroke:#1A252F
    style Factors fill:#16A085,stroke:#0D6655

Figure 79.1: Mapping Wireless Propagation Questions to Physical Factors

Alternative View:

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flowchart TB
    subgraph DESIGN["Design Stage"]
        D1["Select Frequency Band"]
        D2["Choose Transmit Power"]
        D3["Select Antenna Type"]
    end

    subgraph PHYSICS["Physical Phenomena"]
        P1["Path Loss<br/>(distance attenuation)"]
        P2["Multipath Fading<br/>(reflections)"]
        P3["Interference<br/>(other signals)"]
    end

    subgraph OUTCOMES["System Outcomes"]
        O1["Coverage Range"]
        O2["Link Reliability"]
        O3["Data Rate Achieved"]
    end

    D1 --> P1
    D2 --> P1
    D3 --> P1
    D1 --> P2
    D3 --> P2
    D1 --> P3

    P1 --> O1
    P2 --> O2
    P3 --> O2
    P1 --> O3
    P2 --> O3
    P3 --> O3

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    style D2 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style D3 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style P1 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style P2 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style P3 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style O1 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style O2 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style O3 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff

Figure 79.2: Process Flow: From Design Decisions to System Outcomes - This layered view shows how wireless propagation works as a three-stage process. Engineers make design decisions (teal): selecting frequency band, transmit power, and antenna type. These choices interact with physical phenomena (orange): path loss attenuates signals over distance, multipath causes fading from reflections, and interference comes from other signals. The physics then determine system outcomes (navy): coverage range, link reliability, and achievable data rate. Understanding this flow helps engineers trace how each design choice impacts final performance. {fig-alt=“Three-layer process flow diagram showing wireless propagation from design to outcomes. Top layer Design Stage (teal): Select Frequency Band, Choose Transmit Power, Select Antenna Type. Middle layer Physical Phenomena (orange): Path Loss with distance attenuation, Multipath Fading with reflections, Interference with other signals. Bottom layer System Outcomes (navy): Coverage Range, Link Reliability, Data Rate Achieved. Arrows flow from design choices through physics to outcomes, showing how each decision affects multiple physical factors and ultimately determines system performance.”}

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graph TB
    subgraph RADIO["Radio Communication"]
        R1["How loud<br/>to shout?"]
        R2["How far<br/>will it carry?"]
        R3["Can they<br/>understand me?"]
        R4["Is it<br/>reliable?"]
    end

    subgraph SHOUTING["Analogy: Shouting Across a Field"]
        S1["Volume = <br/>Transmit Power"]
        S2["Distance depends on<br/>obstacles, terrain"]
        S3["Speaking rate =<br/>Data rate"]
        S4["Background noise =<br/>Interference"]
    end

    R1 -.-> S1
    R2 -.-> S2
    R3 -.-> S3
    R4 -.-> S4

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    style R2 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style R3 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style R4 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style S1 fill:#16A085,stroke:#2C3E50,stroke-width:1px,color:#fff
    style S2 fill:#16A085,stroke:#2C3E50,stroke-width:1px,color:#fff
    style S3 fill:#16A085,stroke:#2C3E50,stroke-width:1px,color:#fff
    style S4 fill:#16A085,stroke:#2C3E50,stroke-width:1px,color:#fff

Figure 79.3: Alternative view: Shouting Across a Field Analogy - Radio propagation works like shouting to a friend across a field. How loud you shout is your transmit power. How far your voice carries depends on obstacles and terrain (path loss). How fast you can speak understandably is your data rate. Background noise from wind or crowds represents interference. This everyday analogy helps beginners understand why wireless systems need careful engineering. {fig-alt=“Two-row analogy diagram comparing radio communication to shouting across a field. Top row (navy, Radio Communication): How loud to shout, How far will it carry, Can they understand me, Is it reliable. Bottom row (teal, Shouting Analogy): Volume equals Transmit Power, Distance depends on obstacles and terrain, Speaking rate equals Data rate, Background noise equals Interference. Dotted lines connect each radio concept to its physical analogy.”}

79.4 Radio Wave Basics

79.4.1 The Electromagnetic Spectrum

Radio waves are part of the electromagnetic spectrum, characterized by frequency (cycles per second, measured in Hz) or wavelength (distance between wave peaks).

Key relationship: \[c = f \times \lambda\]

Where: - \(c\) = speed of light (3 x 10^8 m/s) - \(f\) = frequency (Hz) - \(\lambda\) = wavelength (meters)

Show code

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      question: "You're deploying IoT sensors in a warehouse with thick concrete walls. Which frequency band will provide BETTER indoor penetration?",
      options: [
        {text: "2.4 GHz (Wi-Fi, Zigbee) - shorter wavelength", correct: false, feedback: "Higher frequencies are absorbed more by obstacles like walls."},
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        {text: "5 GHz (Wi-Fi) - fastest data rate", correct: false, feedback: "5 GHz has the worst wall penetration of these options."},
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79.4.2 IoT Frequency Bands

Band	Frequency	Wavelength	IoT Technologies
Sub-GHz	433 MHz, 868/915 MHz	~0.3-0.7 m	LoRa, Sigfox, Z-Wave
ISM 2.4 GHz	2.4-2.485 GHz	~12 cm	Wi-Fi, Bluetooth, Zigbee, Thread
ISM 5 GHz	5.15-5.85 GHz	~6 cm	Wi-Fi 5/6
mmWave	24-86 GHz	~3-12 mm	5G, Radar sensing

The Frequency Trade-off

Lower frequencies (Sub-GHz): - Travel farther (less path loss) - Penetrate walls and obstacles better - BUT: Lower data rates, larger antennas

Higher frequencies (2.4 GHz, 5 GHz): - Higher data rates possible - Smaller antennas - BUT: Shorter range, blocked by obstacles

79.4.3 Band Comparison Table

Factor	Sub-GHz (868/915)	2.4 GHz	5 GHz
Range (open)	5-15 km	50-200m	30-100m
Range (indoor)	500m-2km	20-50m	10-30m
Wall penetration	Excellent	Good	Poor
Data rate	0.3-50 kbps	Up to 2 Mbps	Up to 1 Gbps
Interference	Low	High	Medium
Antenna size	8-17 cm	3 cm	1.5 cm
Power efficiency	Excellent	Good	Moderate
Regulations	Region-specific	Global (ISM)	Global (mostly)

79.5 Visual Reference Gallery

Static Diagram: Key Propagation Questions and Factors

Flowchart mapping four key wireless propagation questions (transmit power needed, signal range, achievable data rate, connection reliability) to four propagation factors (frequency band, path loss, fading and multipath, interference). Navy blue boxes show the questions while teal boxes show the factors, with arrows indicating which factors affect each question.

Key Propagation Questions and Factors

This diagram illustrates how fundamental IoT wireless questions map to propagation factors that must be considered during system design.

Static Diagram: Wireless Protocol Evolution Timeline

Wireless Protocol Evolution Timeline

Historical timeline showing the evolution of wireless protocols for IoT, from high-bandwidth technologies through low-power mesh networks to modern LPWAN solutions.

79.6 Summary

Concept	Key Points
Frequency-Wavelength Relationship	\(c = f \times \lambda\); higher frequency = shorter wavelength
Sub-GHz Bands	Best range and wall penetration, lower data rates
2.4 GHz Band	Balanced range/speed, crowded spectrum
5 GHz Band	Highest data rates, shortest range
The Trade-off	Lower frequency = more range, less speed; Higher frequency = less range, more speed

79.7 What’s Next

With radio wave basics understood, continue to:

Path Loss and Link Budgets - Calculate whether your wireless link will work
Fading, Multipath, and Interference - Understand real-world signal variations
Practical Wireless Lab - Hands-on experiments with RSSI and packet loss

Related Chapters

LPWAN Introduction - Long-range IoT technologies
Signal Processing Essentials - Deeper dive into signal handling
Mathematical Foundations - Logarithms and dB calculations explained