Introduction to Wireless Data Transmission

When starting with wireless transmissions using radio waves, there is at first an overwhelming number of moving parts: frequencies, RF power, transmitter, receiver, modulations, legal regulations, and so much more.

In this section, I am explaining the basics of wireless data transmissions using radio waves in a fun and practical step-by-step series, including many sample projects that you can build or expand on.

Overview

Electromagnetic waves carry energy through space without the need for physical wires. They oscillate at certain frequencies and can travel long distances at the speed of light. The distance they can bridge depends on their frequency and power.

Which raises the question: what exactly are “radio waves”?

Spark-Gap Transmitters

Initially, it was discovered that sparks emit electromagnetic waves, and the first radio transmitters were spark-gap transmitters.

This is the Hertzian spark oscillator from 1902:

Coincidentally, the German physicist Heinrich Rudolf Hertz (1857-1894) influenced the research of radio waves so significantly that the unit for frequencies was named after him: Hertz (Hz) specifies the cycles per second, and today, radio frequencies are specified in Hertz, Mega-Hertz (MHz), and Giga-Hertz (GHz). For example, license-free radios in Europe often operate at 433MHz, and Wifi uses 2.4GHz and 5GHz.

Soon, the commercial value of radio waves was discovered: Guglielmo Marconi created radiotelegraphy communication on the basis of them which played a crucial role 1912 in the maritime rescue of the RMS Titanic disaster.

Oscillating Circuit

It is not the spark itself however that produces the radio waves. The spark just serves as a fast acting switch to excite oscillating electric currents.

The oscillating conductors - any oscillating conductor - radiate energy as radio waves. Or put differently, whenever you abruptly change the flow of currents, you produce radio waves - on purpose or on accident.

Modern Radio Wave Generators

Modern radio wave generators (and receivers) have much improved, mainly due to two factors:

• Transistor: there are much better fast-acting switches than sparks. For a while, vacuum tubes were used, and then largely replaced by semiconductors like transistors.
• Crystal: Uses a piezoelectric crystal (i.e. quartz) that oscillates at a highly stable frequency. Unlike classic LC circuits, the frequency is determined by the physical properties of the crystal, and does not depend on the values of the inductor and capacitor, which in turn are affected by temperature, humidity, and aging.

Spark gap transmitters are no longer used except for experimental reasons. Oscillarors today typically use transistors to keep them oscillating.

Crystals

The original spark gap oscillator oscillated at frequencies determined by its capacitance and other physical aspects of the components used. L-C circuits consisting of a coil and a capacitor are today the closest resemblence.

Simple L-C circuits share the fate that their oscillating frequency is not stable and can be influenced by the materials used, temperature, humidity, material aging, and many other factors.

Crystal Oscillators are much more precise because they use a quartz resonator that has a resonant frequency that is not affected by environmental variables such as temperature or production differences in the materials used.

Manufacturers have difficulty producing crystals thin enough to produce fundamental frequencies over 30 MHz, though. Modern electronics operate at much higher frequencies. Using frequency dividers, frequency multipliers and phase-locked loop (PLL) circuits, a much wider range of frequencies can be generated from one reference frequency.

Transmitters use almost always crystals as sending at a wrong frequency can cause severe interferences and harm to other services. Receivers benefit from crystals, too, however very cheap receivers, for example in toy remote controls, occasionally skip crystals for a less expensive design. Harm is limited on the receiver side: Should the receiver frequency be off, i.e. due to a sunny day with high temperatures, then the worst case scenario is a toy car that isn’t responding well anymore.

EMI (Electromagnetic Interference)

Since fast-acting switches are everywhere today - thanks to cheap transistors - the long abandoned “spark gap transmitter” is back in new shapes and forms. After all, physics hasn’t changed: any abruptly changing current generates radio waves.

Many modern electronics - switching power supplies as an example - switch currents in high frequency and produce unwanted radio waves which are then called EMI (Electromagnetic Interference).

That’s why it is important to enclose such devices in RF-tight housings (Faraday Cage) and/or smoothen the switching process in an effort to not create abrupt current changes.

Modern RF Receivers

Radio waves serve as carrier for the information, so at the receiving end, there needs to be a device that re-converts the radio waves back into electrical impulses.

Today, there are three widely used designs for receivers:

• Superregenerative: uses positive feedback to amplify weak RF signals. This is simple and cheap, but it also doesn’t work reliably. Superregenerative receivers can be found in toys, and occasionally they are also bundled with decent RF sender boards (i.e. MX-RM-5V). Try and avoid superregenerative receivers as they often don’t work, and they are just a few cents cheaper than decent superheterodyne receivers anyway.
• Superheterodyne: Converts RF signals to an intermediate frequency (IF) for better filtering and amplification. This is how most professional transceivers and walky-talkys work, too, and there are plenty of superheterodyne receiver modules available (i.e. AK-RXB59R, RX470-4, SYN480R, RXBxxx).
• SDR: Software Defined Radio uses digital signal processing (DSP) to handle RF signals, replacing many analog components. SDR does not target data transmission in IoT devices but complements it for analyzing RF signals. SDR can monitor an entire frequency range, visualize signals in waterfall diagrams, and examine modulations and bandwidths.

It is important to know about the major differences so you can pick the best receiver circuit for your use case.

Super-Regenerative vs. Superheterodyne Receivers

Conclusions

In a nutshell, superregenerative receivers are dirt cheap but can cause a lot of issues. They are suitable for very simple scenarios like short-range remote controls at best.

Superheterodyne receivers cost only little more but provide a much better performance. Always opt for a superheterodyne receiver if you can.

Antenna

For an electromagnetic wave to be radiated into space (i.e., to transmit), or picked up again and converted to an electric signal (i.e., to receive), the resonant circuit should be connected to an antenna.

The antenna optimizes the radiation of electromagnetic waves, and a good antenna can radiate the majority of the electrical energy. Without an antenna - or when using a mismatched antenna - most of the electrical energy stays within the circuit and is converted to heat which can eventually damage the circuit, especially when using high power (which is why RF power amplifiers are so susceptible to permanent damage when the antenna is bad or missing).

Omnidirectional and Unidirectional

In its most basic form, an antenna is simply a piece of wire. In this case, electromagnetic waves are emitted equally in all directions (omnidirectional antenna).

More sophisticated (uni)directional antennas can focus electromagnetic waves in a particular direction, strengthening the signal considerably in this direction (and weakening it in all other directions).

Antenna Matching

Whichever antenna type you prefer, the antenna must match the desired frequency. In the case of a simple omnidirectional wire, it must have exactly the right length so that the oscillating currents can resonate. Here is how this works:

Wavelength (λ)

Any radio signal has a frequency (cycles per second). Any frequency has a wavelength (distance a single cycle travels). This is not as complicated as it initially sounds:

Radio waves travel with the speed of light. When you divide the speed of light by the frequency, you get the wavelength in meters: the distance that a single cycle travels:

λ (wavelength) = speed of light / frequency

Calculating Wavelengths

Lets put the theory at work and calculate antenna lengths.

In Europe, many license-free transmitters work at 433.92 MHz. How long needs a good antenna wire be to resonate at this frequency?

Put the values into the formula:

λ (wavelength) = 29979.2458 / 433.92 
               = 69.0893385877581

The wavelength for radio waves at a frequency of 433.92 MHz turns out to be 69.1 cm. If you need the wavelength for other frequencies, just change the frequency in Mhz.

When you use 29979.2458 as the speed of light, you can divide it directly with a frequency specified in MHz and get back the wavelength in centimeters. Using the scientific default base units (meter, Hertz), the formula would be 299792458/433920000 and return the wavelength in meters instead of centimeters.

Antenna Length

For a 433.92 MHz transmitter, a full-λ antenna would have a length of 69.1 cm. That’s pretty long, and often too long to be handled.

To keep antennas practical, you can…

• Fractions: …use fractions of the wavelength λ: a quarter-wave (around 17 cm) or half-wave (around 34.5 cm) antenna may fit better into your device, at the expense of reduced efficiency.
• Coil: …wind up the wire to a coil (as long as the wire is insulated).

Coil Antennas (Space Constraints)

Coil antennas are very space efficient and often found in rubberduck-style antennas or DIY devices that have space constraints (including smartphones).

Coil antennas are less efficient due to the increased resistive losses in the wire, mismatched impedance, and the reduced radiation efficiency caused by the compressed geometry.

So if you have the space, a whip antenna (or a simple stretched out wire) produces much stronger signals. Geometry beats length: even a half-wave antenna wound up as a coil is less efficient than a quarter-wave antenna at full length.

Slow Website?

This website is very fast, and pages should appear instantly. If this site is slow for you, then your routing may be messed up, and this issue does not only affect done.land, but potentially a few other websites and downloads as well. Here are simple steps to speed up your Internet experience and fix issues with slow websites and downloads..

Comments

Please do leave comments below. I am using utteran.ce, an open-source and ad-free light-weight commenting system.

  Show on Github    Submit Issue

^{_{(content created Jan 15, 2024 - last updated Mar 15, 2025)}}