Embedded Antennas – how they work
A dipole antenna uses two radiators to operate, but an embedded chip antenna has only one. For an embedded antenna, a surface of the PCB becomes the second radiator. This explains why, if the length of the PCB is too short, the antenna will not operate efficiently.
The resonance of an antenna is directly related to its wavelength. The antenna must resonate at whole number multiples or fractions of the wavelength, with the shortest resonant length being a quarter of the wavelength.
A full-wave antenna at the 916MHz frequency would need to be approximately 327mm long, which is not practical for an embedded antenna, but a quarter-wave version is practical at a ground plane length of 87.2mm. This will be coiled up across the copper traces and layers that are hidden within a tiny surface mounted chip antenna.
Antenna designers get around this limitation by using the ground plane as the missing half of the half-wave dipole, so a quarter-wave monopole antenna radiates against the ground plane. Therefore, the most popular embedded antennas in small wireless devices tend to be quarter-wave monopole antennas.
Ground plane length
For an embedded antenna to work efficiently, the ground plane must be at least a quarter wavelength of the antenna at its lowest frequency. Accordingly, at the lower frequencies the design will be much easier when the ground plane is 100mm or greater.
The performance of an embedded antenna is directly related to the length of its ground plane, so allowing for the ground plan to be the correct length is the greatest challenge for smaller designs.
Figure 1 shows the trade-off between ground plane length and antenna efficiency from 794 MHz on the left to 2.69 GHz on the right.
These results show clearly how the antenna efficiency drops for small ground planes at frequencies below 1GHz. These results were obtained for a 3G/4G chip antenna operating at frequencies 791-960MHz, 1710-2170MHz, 2300-1400MHz and 2500-2969MHz.
Generally, the ground plane would need to be 100mm or more for a device using the frequencies below 1GHz. In the USA, the 4G frequencies use bands as low as 698MHz or even 617MHz as with T Mobile’s B71 band requiring a ground plane even longer than 100mm.
Positioning the antenna on its PCB
Next, we should consider the position of the antenna on the PCB and its placement in relation to other components. The antenna should be placed in the best position in the overall RF layout and PCB stack-up to allow it to radiate effectively.
Each individual antenna is designed to work efficiently in a few places on a PCB. This is often the corner or an edge, however each antenna is different, so it is important to select an antenna that fits into the design and place it according to the manufacturer’s recommendation for that antenna.
Figure 2 shows how the antenna is placed with its clearance area in a small device such as a wearable product or watch.
Figure 3 shows a suitable antenna placement for a watch design. The design maintains the recommended clearance specified above and below this antenna, which is shown in red.
Do not place noisy components, such as a battery or an LCD close to the antenna section. Antennas are passive components that receive energy and will pick up noise radiated from the noisy components, and transfer that noise to the radio, degrading the received signal. The antenna should also be placed away from the human body to improve RF performance, this is the distance marked in blue in Figure 3 above.
The arrangement of the RF feed and the ground connections are critical to the function of the antenna. With small embedded antennas in small PCBs, the copper tracks etched on the PCB may form an integral part of the antenna so care should be taken to follow the manufacturer’s specification or reference design.
Overall RF layout and PCB stack-up
You can maximise the performance of antenna by giving careful consideration to the layout of the RF elements in the design. The copper ground plane should not be cut up with traces or arranged over more than one layer, then the ground plane portion of the antenna will be able to radiate more effectively.
It is essential to keep components such as LCD or batteries clear of the antenna area in the PCB layout, as these can interfere with the way the antenna will radiate.
For multiband frequencies, we suggest a PCB layout with a minimum of four layers.
Figure 4 shows how the top and bottom layers provide ground planes, while the digital signals and power which need to be away from the ground plane, run in the space between these.
Tuning the antenna for performance
For those cases where the ground plane is shorter than ideal, a designer can look at other techniques to increase the performance of an embedded antenna.
One way is to tune the antenna for its country of operation. The 4G frequency range is a wide one, spanning from 698MHz to 2690MHz, but each different world region uses just a portion of this band, and an antenna can only operate on one frequency at a time. This means that when a product is to be used in one geographical region, it can be tuned to operate in a narrower section of the frequency band. This will boost the performance of the antenna.
Another technique is to include an active tuning network, effectively an additional RF switching circuit, which will help to get over the bandwidth reduction caused by a smaller ground where the host PCB is less than 75mm. A PI matching circuit is added close to the antenna feed point, to fine tune the antenna and boost up performance. The design of the matching circuit will usually need some assistance from an RF specialist.
Figure 5 shows a matching circuit on an antenna evaluation board.
Designing the transmission line
Once the material for the PCB has been chosen and its thickness and dielectric constant are known, a co-planar transmission line can be designed using one of the commercially available RF trace design software packages. This will use the PCB thickness, the copper layer separation and substrate dielectric constant to calculate the optimal width for the transmission line and the appropriate gaps on either side to achieve a co-planar transmission line of 50 Ω.
All transmission lines should be designed to have a characteristic impedance of 50Ω, and the other parts of the RF system, such as transceivers or power amplifiers should also be designed with an impedance of 50Ω.