Wearable Antennas are essentially any antenna that is specifically designed to function while being worn. Examples include smartwatches (which typically have integrated bluetooth antennas), glasses (such as Google Glass which has WIFI and GPS antennas), GoPro action cameras (which have wifi and bluetooth antennas, and are often strapped to a user to obtain their footage), and even the Nike+ Sensor (which communicates to a smartphone via bluetooth, and is placed in a user's shoe). Wearable antennas are becoming increasingly common in consumer electronics, and as such this page is dedicated to describing the unique difficulties involved in wearable antenna design.
Figure 1. Examples of Wearable Antennas.
Wearable Antenna Challenges
There are 2 challenges to wearable devices that make antenna design particularly difficult:
1. Proximity to the Human Body. The human body is a lossy material for electromagnetic waves. This means the body converts Electric Fields into heat; put another way, the body absorbs energy from electromagnetic waves. Consequently, when an antenna is placed near the body, the result is a large reduction of the antenna efficiency of your wearable antenna. For example, if you design an antenna and measure an efficiency of -3 dB (50%), when placed on the body the efficiency may easily drop to -13 dB (5%). This is a huge hit to the performance of the wireless system.
2. Very limited Volumes . Wearable devices must be as small as possible. No one wants a watch with a big dipole antenna hanging out the side. Space is at an extreme premium on wearable devices, particularly for anything near the face (such as google glass). As such, industrial designers and product designers often give very little space for the antenna, which further makes the antenna design problem more difficult.
A Wearable Antenna Study
To help give an understanding of the way the human body affects antennas, we will do a study on multiple antenna types and the effect an absorbing material placed near the antenna can have. To start, we'll take a 3.5" * 1" slab of copper tape, and construct 3 planar antennas. The first, on the left in Figure 2 is a wideband dipole antenna. The center shows a slot antenna, and on the left is a loop antenna.
Figure 2. Three Planar Antennas. Left: dipole. Center: Slot. Right: Loop.
Our goal is to compare the free space (no absorbing material) antenna efficiency with that of the antenna efficiency in the presence of an absorbing material (similar to the human body). First, the VSWR of the 3 antennas above are measured across a frequency range of 1 GHz to 6 GHz, as shown in Figure 3:
Figure 3. VSWR of Wearable Antennas Shown in Figure 2.
The antenna efficiency is then plotted, along with the mismatch-loss compensated efficiency. Note that in mismatch-loss efficiency, we simply are removing any loss due to impedance mismatch, so that we would know how efficient the antenna was if it had perfect impedance matching. This is shown in Figure 4:
Figure 4. Left: Raw Antenna Efficiency. Right: Mismatch-Loss Compensated Efficiency.
To show the effect of absorbing material, we use a lossy block of material from the Speag corporation, which was specifically designed to be similar to the human body. The parameters of interest are the permittivity (also known as the dielectric constant), and the electric conductivity, which is a measure of how lossy (Electromagnetic Wave absorbing) the material is. The parameters for the block of material is given in Table 1:
The antennas are placed 2.7mm above the lossy block, as shown in Figure 5:
Figure 5. Antenna Placed Above a Lossy Material For Measurement.
We first compare the VSWR of the antenna for the Free Space (no material) and wearable case (on lossy material). The results are very different:
Figure 6. Left: VSWR of Antennas for Free Space. Right: VSWR of Antennas on Lossy Block (wearable condition).
Figure 6 shows that the the VSWR uniformly drops for all antennas. This means that there is less mismatch loss for the antennas. This might seem good, but it is actually bad. There is less mismatch loss because more of the energy is being absorbed than would otherwise occur due to radiation (which are the intended losses) loss. Hence, the lossy block (or the human body) acts as a nice magnet for electromagnetic waves. This will reduce our antenna efficiency, as we will see.
We then measure the antenna efficiency 2.7mm above the lossy block. The results are shown in Figure 7, which show a very strong loss in antenna efficiency. In Figure 7, we again plot the raw (directly measured) antenna efficiency on the left, and the mismatch-loss compensated efficiency on the right:
Figure 7. Left: Raw Antenna Efficiency. Right: Mismatch-Loss Compensated Efficiency.
Figure 7 shows that there is about a 10 dB loss in efficiency due to the absorbing material. This is what makes wearable antenna design formidable. It is bad enough to make a good antenna with little volume; adding the guaranteed loss is not a great thing for the antenna designer.
Further, note that the antennas of Figure 7 perform fairly similar in the wearable (lossy block) case. However, the dipole antenna is several dB superior to the slot antenna at GPS frequency (1.575 GHz), and generally performs better than the other antenna types. Hence, this study shows little benefit of using a loop or slot antenna over a dipole.
As you can see, wearable antennas are a significant challenge, because the body directly degrades the performance of the antenna. The best method of mitigation is to maximize the distance between the antenna and the body. This is a bit obvious, but also critical. In addition, it should also be noted that GPS antennas mounted to the body are particularly limited. GPS/GNSS/GLONASS antennas are best designed to have a wide field of view of the sky, so that multiple satellites can be seen. However, when an antenna is used as a wearable antenna, the body obstructs about half of the field of view for the antenna. This makes GPS performance on wearable devices particularly challenging.