Integrating IoT and Mobile Device Antennas

The wireless world would not exist without antennas, but selecting the best antenna for a wireless device represents a challenge. One can pick from several radio bands and it is crucial the chosen link is reliable.

Mobile phones have multiple antennas, sometimes up to 13 different ones. A minimum of four antennas are present in every smartphone today, including GPS, Bluetooth, Wi-Fi and cellular. Conversely, IoT (Internet of Things) devices are considerably smaller than cell phones and only contain one or two antennas.

IoT devices, like broadcast chips in store products or tags for livestock, pose a challenge due to their size. The size restrictions have made researchers wonder how much smaller they can make an antenna that functions well. The simple answer is the antenna size is optimum at half a wavelength, but you can probably go as small as a quarter wavelength and still get a decent functioning antenna.

Additional tricks manufacturers can implement to shrink antenna size include zigzagging the antenna’s trace on a circuit board, or utilizing the ground plane of a board or gadget cover. These strategies allow you to produce smaller antennas, while receiving performance close to quarter wavelength-sized antennas.
Going smaller than a quarter wavelength for an antenna will cause the strength of the signal to be reduced in accordance to the antenna’s area and the frequency bandwidth will be narrower.

Two rules can help determine the wavelengths of these radio bands:

1. A radio signal with a 1 GHZ frequency has a wavelength of 12 inches.

2. The frequency and wavelength are inversely related. As the wavelength decreases the frequency increases. For instance, a wavelength of six inches will be associated with a frequency of 2 GHz.

In general, radio frequencies vary between 88 MHz (FM radio) and 6 GHz (high-speed Wi-Fi band) if we discount NFC frequency. The frequencies used in smartphones typically range between 800 MHz and 5.5 GHz. This means mobile phones have wavelengths from just over two inches to about 15 inches. At this range, a quarter of a wavelength will be between half an inch to approximately four inches. This implies the only frequency that fits well for an average mobile device is a 5.5 GHz band. Most other antennas crammed in such a limited space will not work as efficiently as they could otherwise. FM radios emit a wavelength of approximately 10 feet so a tiny antenna will perform horribly. Fortunately, the majority of commercial radio stations sound out around 100kW.

Antenna Performance
Although the coverage area of an IoT device is restricted by its size, it is not a dramatic restriction like a factor of five, in usable range. This is because of radio signal drop offs at the fourth power of the separation distance due to signal fluctuation or blockage and from the delay spreads caused by numerous reflections in an area. Dropping half of radiated power due to small size of an antenna might only lead to a 15 to 30% drop in usable range.

Another important issue to work around is to find real estate in tiny IoT devices to put down an antenna, especially if you want to include more than one antenna in a tiny device.

Mobile Phone Antennas
Antenna technology has evolved substantially from the days when you had little antenna scoping out of mobile phones. Nowadays, mobile devices incorporate tiny chip antennas as small as 0.3 square inch that are printed on circuit boards.

IoT Device Antennas
Iot devices need a connection to the Internet as well as an antenna. The major problem with IoT devices is they have even less space available for antennas than mobile phones.

Antennas for frequencies less than 5.5 GHz, in which case a one cm antenna represents around 0.2 wavelengths, will be considerably inefficient in small sensor packets. Therefore, developers of IoT devices using power similar to that in mobile phones will get even less range. Frequently, 1 GHz frequency bands will not even be an option in the smallest of devices. In effect, this only leaves frequencies at 2.4 GHz and 5.5 GHz as the two available options.

Future of Antennas

There are proposed standards for frequencies approaching 60 GHz with corresponding wavelengths of only 5mm that will greatly improve the efficiency of micro-antennas.

The new tiny IoT hardware can be viewed as the evolution of mobile devices that started off with a network connected via cell phone towers linking to mobile phones. At one end was the internet and at the other were the phones. This evolved to Wi-Fi networks that created hotspots at home, office and public places. Next, designers were able to incorporate Wi-Fi radio in smartphones to allow cell phones to connect to the Internet via traditional cellular networks or through Wi-Fi hotspots.

As these wireless networks evolved, so did diffuse wireless connections with tiny access and relay points, often called “mesh” networks, for home automation and industrial purposes. Our environment is constantly adding small and inexpensive radios to allow instantaneous access to other devices or the Internet.