The Challenge of Mobile Phone and IoT Antennas

By: 
September 13, 2015

Antennas are a crucial component of any connected device such as a mobile phone or IoT product. But choosing the right antenna for an application presents a key design challenge. Wireless devices use several radio bands, and reliable radio links are important. Creating effective antenna performance in mobile phones and other IoT devices requires engineers to examine a number of factors including antenna size, from what is needed to what is possible, antenna shape, and placement.

Mobile phones can contain anywhere from four to 13 different antennas. There are at least four radios (transmitters and/or receivers) in mobile phones made today: cellular, Wi-Fi, Bluetooth, and GPS. Some phones will have three more radios: 802.15.4 (930 MHz and lower), FM radio, and magnetic Near-Field Communication (NFC).

By contrast, most existing or envisioned remote IoT devices usually have one, maybe two, antennas. This makes sense given most existing or envisioned IoT devices are smaller than even mobile phones. Think smart watches, health and fitness monitors, skin moisture monitors, skin moisture monitors, vibration sensors, broadcast chips in retail store products, and many others that need a radio link for sending information to a mobile phone or the Internet.

What Is the Right Size for an Antenna?

Given the space restrictions in a mobile phone or a remote IoT device, the more appropriate question is, "How small can an antenna be and still work well?" A quick answer is: about one-quarter of a wavelength. Half a wavelength is the optimum length (see figure below) because it looks like a pure 73 ohm resistance (close to the 50 ohm resistance in most radio circuitry), but one can cut the size to one-quarter wavelength and it will still work well.

Additionally, there are tricks one can apply to reduce the antenna size such as using the ground plane of the circuit board or device cover, or zigzagging the antenna trace on the board. These tricks can make the antenna much smaller while giving performance close to the effective length of one-quarter wavelength. Antennas smaller than an effective length of one-quarter wavelength will work, but the signal strength will drop roughly with the area of the antenna, and the available frequency bandwidth will shrink.

So what are the wavelengths of all these radio bands? Two simple rules can help:

  1. The wavelength of a radio signal with a frequency of 300 MHz is 1 m (or for Americans, the wavelength of a radio signal with a frequency of 1 GHz is 1 ft).

  2. The wavelength is inversely proportional to the frequency and will decrease as the frequency increases. So the wavelength at 2 GHz is half a foot.

If we disregard the NFC frequency, which is really a magnetic field link, all these radios operate from from 88 MHz (FM radio) to almost 6 GHz (high Wi-Fi band). The most widely used radios in a mobile phone (the phone itself, GPS, Wi-Fi, Bluetooth) operate from about 800 MHz to 5.5 GHz, which converts to a wavelength range of 15 in. down to a little over 2 in. One-quarter of a

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