Understanding Cell Phone Technology: Antennas
I thought I had finished talking about cell phones. Then a colleague of mine asked when I was going to talk about cell phone antennas. So we’ll talk a little about cell phone antennas today and move on to a new topic next week.
Every cell phone has multiple antennas because every cell phone has multiple radios in it. Radio? What does radio have to do with it?
Radio is the technology that allows us to signal and communicate using radio frequency (RF) waves. Some people use the word to mean those signals that are broadcast to the receivers we have in our cars and homes dedicated to receiving radio signals, or “radios.” These receivers originally included AM and FM radio, but now we’ve added digital radio and satellite radio. Every receiver is designed to receive RF at specific frequencies. Today, many receivers are embedded in complex devices with multiple capabilities, like computers and cell phones.
Every cell phone contains at least four radios. These include radios for the primary cellular communications, GPS, Wi-Fi, and Bluetooth. Every radio needs its own antenna, so there are at least four antennas in your cell phone.
Now you’re saying, “Wow! That’s amazing! I thought an antenna was usually a big thing?! What gives?” It turns out that the size of the antenna depends on the frequency/wavelength of the radio wave. Remember that the frequency nu and wavelength lambda are related. If we take their product, we get the speed of light c:
nu x lambda = c = 3 x 108 meters/second.
The size of the antenna is directly proportional to the wavelength of the signal. In general, the optimal antenna size is ½ a wavelength. Given the space constraints of the cell phone (tiny!), you can actually design a ¼ wavelength antenna that will work just fine. You can also play some other tricks with antenna design – like configuring the antenna in shapes besides a straight line – in order to pack a powerful antenna in a small cell phone case.
Today’s primary communications radios in cell phones are starting to operate in the new “5G” bands. Two U.S. 5G bands are 28 GHz and 39 GHz. These correspond to wavelengths of
3 x 108 / 28 x 109 = .0107 meters = 10.7 millimeter = .42 inches
3 x 108/ 39 x 109 = .0077 meters = 7.7 millimeter = .3 inches.
Aha! Now we see why these are called millimeter-wave communications! We can also see how small an antenna this could be, one which would easily fit inside a cell phone today.
What about antennas for the other bands? The GPS L1 band operates at 1575.42 MHz, making its wavelength
3 x 108/1575.42 x 106 = .19 meters = 190 mm = 7.48 inches.
One U.S. Wi-Fi band is 5 GHz. To use this band, your cell phone’s antenna must be able to receive a wavelength of
3 x 108/ 5 x 109 = 0.06 meters = 60 mm = 2.36 inches.
In the United States, Bluetooth uses the 2.4 GHz frequency band that corresponds to
3 x 108/2.4 X 109 = .125 meters = 125 mm = 4.92 inches.
So for all four radios, we’ll be able to fit a suitable antenna into our cell phone cases.
These ideas about cell phone antennas only scratch the surface of antenna design. In general, engineers talk about antenna parameters like gain and polarization. Antenna gain describes how much better an antenna receives a signal in specific directions than others. Antenna polarization describes how the electric field corresponding to the antenna’s transmitted signal is oriented. It can be oriented vertically, horizontally, at an angle, or even circularly! Transmitting and receiving antennas reduce the noise on their communications channel by aligning their polarizations.
One thing engineers struggle with is bringing together the theory with the reality. For example, there is a model that says the power density PD of the signal at a distance R away from a radiating antenna with gain GT as:
PD = PT GT / 4πR2.
It’s a nice model, but this is not what radiated power from an antenna looks like in the wild. Engineers spend a lot of time collecting empirical data, comparing it with the models, refining the models, collecting more data, etc. They apply this scientific method iteratively so that communications systems will work correctly and reliably for customers.
Today we’ve developed a little intuition about cell phone antennas and antennas in general. Antenna design is a pretty complicated subject, so if you want to master it, you’ll have to study it in depth. In the meantime, you can get your feet wet by finding a local amateur radio club (http://www.arrl.org/find-a-club) and talking to some of its members. Amateur radio is a popular hobby and service by which people communicate anywhere in the world without cell phones or the internet on designated amateur radio frequencies. Amateur radio operators must be licensed. In the United States, the FCC issues these licenses. All FCC-licensed members of your local club know a lot about how radios (and antennas) work. Most are eager to share that knowledge. If you’re persistent, they’ll even help prepare you for the licensing exam so you can become an amateur radio operator yourself!