Tracki Cellular Antenna Design
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Tracki makes a cellular GPS tracker that is fairly small, about 47mm long as shown in Figure 1.
 Figure 1. The Tracki Device is Only 47mm Long. This device has GPS, WIFI, BT, and Cellular (4 LTE bands). The lowest cellular frequency is 4G Band 17 LTE, which is 704-746MHz. The interesting thing here is that antennas want to be about a half-wavelength long to have high efficiency and bandwidth. At 704MHz, a half-wavelength is about 213mm, so the device here is about an eighth of a wavelength long. Hence, it will be interesting to see how this antenna performs, particularly at the lowest frequencies. From searching the FCC ID, I can see that the cellular bands supported by this device are: The cellular bands are divided into a lowband (<1 GHz), and mid-high band (1710-2700MHz) which is fairly typical these days for cellular supporting LTE or 5G. Generally, it is more difficult to cover the required bandwidth in the lowband than it is the highband, because the volume (measured in wavelengths) is smaller at the lowband. Hence, while the highband has higher total bandwidth, the fractional bandwidth is higher for the lowband and is correspondingly more difficult. Note that the Tracki has 4 antennas as shown in Figure 2: (1) WIFI, (2) Bluetooth, (3) GPS, and (4) Cellular. On this page we will focus on cellular as it is the most complex.
 Figure 2. Opening Tracki To Reveal the Antennas. The tracki cellular antenna has a single spring from the main logic board (MLB) to the antenna, which is a flexible pcb (called a "flex"), which is essentially a meandered dipole. The flex is one arm of the dipole, and the ground of the MLB forms the rest of the dipole. This is mounted on a piece of plastic that fits into the outer plastic shell when the battery and cover are removed:
 Figure 3. A Flex PCB Forms the Cellular Antenna. The MLB of the tracki device is shown in Figure 4. The feed spring is at the top, which directly connects from the radio (via a transmission line on the board) to the cellular antenna flex. Note that the antenna flex is half of the meandered dipole; the other half of the dipole antenna is made up of the ground plane of the MLB itself. This is a re-use of the MLB for part of an antenna that is very common in all of consumer electronics.
 Figure 4. MLB of the Tracki Device showing the Cellular Antenna Spring. In Figure 4, you can also see that the antenna designers left room for an adaptive tuning element on the right side of the MLB (often just a switch to change the impedance). This is done to adaptively tune the antenna depending on which frequency band is currently being used. This technique is required on modern smartphones in order to cover the wide bandwidths needed with a small antenna. However, in this case, it appears the designers removed this to reduce cost and/or complexity. To measure the cellular antenna's VSWR, efficiency, and gain, I removed the test connector along the RF transmission line and attach a coaxial cable at that point. This is shown in Figure 5. I use an SMA connector on the end of a coaxial cable to enable easy measurement of the antenna with a Vector Network Analyzer (VNA).
 Figure 5. Applied Coaxial Cable to Antenna Feed to Measure Antenna Properties. The VSWR is shown in Figure 6, plotted from 700MHz to 3GHz:
 Figure 6. VSWR of Tracki Antenna. In Figure 6, I identify the relevant cellular bands that the Tracki supports (which can be easily obtained from the FCC website, as tracki has FCC ID 2AAI6-TRKM010B). The bands are B17 [700MHz], B5 [850MHz], B4 [1700MHz], B2 [1800MHz], and B7 [2500MHz]. From the VSWR measurmeent, it can be seen that the device has poor matching in all bands. In particular, at the lowband (B17 and B5), the designers tuned the antennas between Band 5 and Band17, resulting in suboptimal matching for both bands, in order to achieve the design without an adaptive tuner. The impedance at the mid- (B4/B2) and high bands (B7) is similarly poor. A decently matched antenna in consumer electronics has VSWR < 3, which represents about 1 dB of impedance mismatch loss. Next, we measure the antenna efficiency in an anechoic chamber. The results are shown in Figure 7:
 Figure 7. Antenna Efficiency of Tracki Antenna.
I also have a youtube video to describe the tracki antenna if you prefer to learn that way:
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