Page 29 - EE Times Europe Magazine | June2020

EE|Times EUROPE 29

WIRELESS
How to Get Better Wireless Performance for

Mobile Devices with Small PCBs

By Geoff Schulteis

he demand for smaller wireless devices is growing, for use in PCB is too short, the antenna will not operate efficiently.
consumer applications such as wearables, medical devices, The resonance of an antenna is directly related to its wavelength.
and trackers as well as in industrial applications such as The antenna must resonate at whole-number multiples or fractions of
T lighting, security, and building management. It follows that the wavelength, with the shortest resonant length being a quarter of
smaller electronic devices will require smaller PCBs, meaning the the wavelength.
antennas must work with shorter ground planes; if they are battery- A full-wave antenna at the 916-MHz frequency would need to be
operated, power is also a factor because the device must not consume approximately 327 mm long, which is not practical for an embedded
too much power. antenna, but a quarter-wave version is practical at a ground plane
This presents quite a challenge for the product designer. The end length of 87.2 mm. This will be coiled up across the copper traces and
design will need to be submitted for formal network and government layers that are hidden within a tiny surface-mounted chip antenna.
approval before the new product can be used on the carrier networks, Antenna designers get around this limitation by using the ground
and the design is likely to fail if the antenna doesn’t perform correctly plane as the missing half of the half-wave dipole, so a quarter-wave
or if the device creates radio interference by re-radiating noise. It fol- monopole antenna radiates against the ground plane. Therefore, the
lows that it is even harder to get carrier approval for a smaller product most popular embedded antennas in small wireless devices tend to be
because it is more difficult to achieve wireless performance that is good quarter-wave monopole antennas.
enough to pass minimum
The design challenge is transmit and receive levels. GROUND PLANE LENGTH
For an embedded antenna to work efficiently, the ground plane must
This is particularly true in
to give enough space for the U.S., where a design must be at least a quarter-wavelength of the antenna at its lowest frequency.
the antenna to perform meet strict criteria to gain Accordingly, at lower frequencies, the design will be much easier when
the ground plane is 100 mm or greater.
network approval.
correctly and still fit all It is a fact that for elec- The performance of an embedded antenna is directly related to the
the components into a trically small antennas to length of its ground plane, so allowing for the ground plane to be the
correct length is the greatest challenge for smaller designs. Figure 1
operate at frequencies below
smaller PCB. 1 GHz, they ideally need shows the tradeoff between ground plane length and antenna efficiency
ground plane lengths of
from 794 MHz on the left to 2.69 GHz on the right.
100 mm or more to achieve These results show clearly how the antenna efficiency drops for
good performance and efficiency. If the antenna efficiency should drop, small ground planes at frequencies below 1 GHz. The results were
it will cause issues with power consumption and achieving network
approval for the finished product. This means that the challenge for a
product designer is to create a design in which there is enough space
for the antenna to perform correctly and still fit all the components
into a smaller PCB.
This is especially true for antennas operating at frequencies below
1 GHz, which are typically used for products such as internet of things
devices, product trackers, fitness devices, and similar small devices.
Wearable devices and medical devices that are used close to the
human body present a special challenge. The human body restricts RF
signals, so the designer should consider how the antenna will radiate and
be certain to place the antenna in such a manner that the human body
will not obstruct the signals. Wearable devices can be as small as 50 mm
or even less. And some of them may use more than one antenna.
There are several factors that affect the performance of the antenna
in a small device, and this article will address them in turn. The first
and most important is the ground plane, which, in many cases, is
essential for the antenna to radiate. But this is not all; the designer
should place the antenna correctly and consider the other components
and the position of these in relation to the antenna to ensure that
nothing noisy or metallic lies in the antenna’s path. Finally, the casing
for the device can make a difference, and we will outline the main
materials to avoid.

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 Figure 1 (Image: Antenova Ltd.)

www.eetimes.eu | JUNE 2020