Antennas are not like other components. Their behaviour is more difficult to predict, because an antenna will perform a certain way in a test laboratory with an evaluation board prototype, and then behave differently when it’s finally integrated into a device. The differences in performance depend upon the other components around the antenna and materials close by that might absorb the antenna’s signal or cause it to reflect in a different way. This means that it is particularly difficult to design wireless devices to work close to the body – for example fitness devices or medical devices or other wearable electronics – as the human body absorbs signals. It is also challenging to integrate antennas into device with metal housings as the metal shadows the signal in the same way that a tree blocks sunlight from hitting the ground. The radio must rely on receiving a reflected signal which is usually significantly lower in power.
We should also mention that an antenna will perform one way in free space or in test conditions, and differently when it is embedded into a device.
If a design uses an embedded antenna, there will come a point in creating the design when the antenna needs to be matched to the other RF components in the device, to refine its performance and to be sure that the antenna will radiate at the correct frequency, and perform at the specified range. The matching process is not so well understood and is one of the more challenging aspects of creating a wireless design.
This article explains the steps in this process.
Most embedded antennas are designed to reflect against a ground plane (this is called reciprocity), and the ground plane needs to be a certain length to allow the antenna to operate correctly. This is likely to be a factor in the initial consideration of the design and layout of the PCB, as the design must satisfy the ground plane requirements of the antenna and allow enough space and length for it to operate. Although some antennas may not require a ground plane, this is a key characteristic to consider when selecting an embedded antenna.
For antennas that are ground plane dependent, the PCB actually becomes the ground portion of the antenna. This means that the lower layers of the PCB may affect the antenna’s performance, so it is important not to place a battery or an LCD near the antenna in the PCB stack-up.
The ground plane size should also allow for any wires used for communication to the device and the batteries or power cable that power the device. If the correct ground plane size is used, this will ensure that design allows enough space, and the cables and batteries connected to the device have less influence on the antenna.
Antenna performance is measured in terms of efficiency. It is the phenomenon where other objects close by alter the electromagnetic fields of the antenna’s radiation, and cause the antenna to perform less efficiently, and maybe not as was specified for the design. Efficiency takes into account how well the antenna was matched in order to deliver all of the energy the radio is transmitting to the antenna, and how well the antenna trace design captures that energy, allowing it to radiate away from the device.
Voltage Standing Wave Ratio (VSWR)
The Voltage Standing Wave Ratio, or VSWR is a measurement of the return loss showing how much energy is either being transferred through or being reflected back along the transmission line, which are both detrimental to the performance of the antenna.
VSWR is an important value and understanding it will help to build a successful wireless design. If the VSWR can be kept low (at least between 2:1 and 3:1) in most cases, it helps to compensate for lower efficiency in the antenna structure. A low VSWR means that the antenna is receiving more power which is preferable. A VSWR value smaller than two is usually considered to indicate a well-matched antenna.
The transmission line is the copper trace line that carries the signals to and from the antenna. It is in this trace that there can be a very high level of resistance if it is not designed properly, and there may be a loss of signal, even as high as 50% caused by the energy coupling to ground instead of transferring to the antenna. The impedance value for an antenna’s trace line is an impedance of 50 Ohms, and the other RF elements (such as the radio) in the design should also be set to 50 Ohms.
Transmission lines that carry RF energy to and from an antenna are a key factor in determining wireless performance. A poorly optimised transmission line can cause wireless performance to drop by as much as 50%. The transmission line ideally carries 100% of the power to the antenna, although in reality this is difficult to achieve. This is due in part to the absorptive loss of the materials used, sharp bends in the trace and reflected energy due to impedance mismatch.
The dimensions and length of the trace and PCB stack should be calculated to minimise the VSWR as far as possible.
The old way to calculate antenna impedance was to use Smith Charts, which show the correct value for impedance in relation to frequency. The centre of the circle represents a perfectly matched antenna which receives 100% of power, and the outer circles show the maximum reflection co-efficient where all the power is reflected back to the source.
Figure 1 shows an example of a Smith Chart. (Source: Antenova Ltd)
Today there are software calculators that give instant results for GCPW, basing the calculation on the thickness of the PCB, the thickness of the copper, and the dielectric constant of the PCB substrate. Both the thickness and dielectric constant of the PCB play a key role in limiting the return loss from the antenna. There is a free software tool for this at www.antenova.com.
It is worth mentioning here that the final PCB must be manufactured from the same material as was used during the development of the design – or the differences will change the impedance values for the antenna, and the antenna may not perform as expected.
What is Antenna Matching?
Antenna matching is the process of adjusting the design to ensure that the impedance of the antenna (chip or module) is correctly aligned with the other RF circuitry on the PCB. The aim is to create a design where the impedance is as close to 50 ohms as possible, to create an antenna that performs well, bearing in mind that the frequency of an antenna can shift if other components close by on the PCB or housing are causing interference.
One way to do this is to add an impedance matching circuit to the design. Antenova recommends a pi (TT) matching circuit that will allow the antenna to be tuned and optimised. The pi matching circuit may consist of three components (inductors and capacitators) for a single band antenna, or more for a multi-band antenna. This is a useful way to tune an antenna to perform better within the design, and especially in real-world conditions where the antenna is required to operate in less optimal positions, for example hand-held, or worn on the body.
With an embedded antenna which is positioned on a PCB, it is usually recommended to use a grounded co-planar wave guide, (GCPW). This means that there is no need to connect to components on the underside of the PCB using vias, which is preferable as vias add loss to the transmission line or added inductance to the matching components causing inaccurate tuning values.
The GCPW is the conventional way to calculate the trace lines for an embedded antenna. It shows the optimal height to leave between the ground plane and the component layer in order to maintain a 50 ohms system.
GCPWs are configured in what’s known as a ground-signal-ground configuration on the top layer. Additionally, the signal is isolated further with an additional ground plane beneath the signal.
Figure 2 shows a grounded coplanar waveguide cross-section. (Source: Antenova Ltd)
This configuration lends itself well for high frequency applications. The G-S-G configuration offers good isolation from interference and generally minimises radiation losses, when designed optimally.
Another advantage of this arrangement is the flexibility it offers designers. The dimensions and physical layout of a transmission line will define the characteristic impedance of the transmission line. With no fewer than four dimensions to customise, designers are afforded plenty of options when it comes to accurately matching for impedance.
Figure 3 shows the four components of the coplanar waveguide. (Source: Antenova Ltd)
There are four different parts of a CPW, these are:
- The conductive strip
- Surrounding ground planes
- Isolating gaps
- Dielectric substrate layer
GCPWs require consistent and careful design to function as intended. As with other types of transmission line, their dimensions and length are key. The longer a transmission line is, the more interference the signals will be subjected to as they travel along the line. It is important, therefore, to keep the length of the line to no more than 10% of wavelength, especially when designing for high-frequency applications.
It is essential when using coplanar waveguides that the width of the isolating gaps is kept consistent along the whole length of the line. The gaps must also be slimmer than the surrounding ground planes to avoid diminished performance. The width of these gaps can alter the characteristic impedance, so maintaining the uniformity of these lines prevents impedance mismatching. Lastly, the dielectric substrate layer should be twice the thickness of the conductive strip’s width.
Figure 4 shows the layout of the GCPW. (Source: Antenova Ltd)
GCPWs are a good choice when interference is likely, when the cost is less of a priority, or in ultra-compact, miniaturised designs. While they are more expensive than alternatives such as the microstrip line, their performance in most situations is far stronger. For larger applications, the coaxial cable might be a better choice due to its cost-effectiveness, ease of use, lower loss and ability to avoid interference. However, the GCPW still comes out on top for small, compact systems where coaxial cables are unable to be used.
When the design is for a small device where the size of the PCB is restricted, and other components are placed closer to the antenna, it becomes more crucial to match the antenna correctly, to be sure of good RF performance.
With the trend to smaller devices, the issues of space on the PCB, ground plane, and the proximity of other components do become more challenging. For smaller or more complex designs, it may be easier to utilise the services of an RF specialist to help with antenna testing and matching.
Once the device starts to transmit and receive signals it is recommended that the matching component values be adjusted up and down a value or two while measuring the radiated power to optimize for the best possible match possible. The reason for doing “active” tuning is to optimize the impedance of the trace line between the radio and the antenna without the presence of a test coax.
|Geoff Schulteis is Senior Antenna Applications Engineer with Antenova Ltd. Geoff has more than 20 years of experience in designing, integrating and testing antennas, and he currently heads technical support for Antenova’s customers in North America. He is an antenna engineering professional with more than 20 years’ experience designing, integrating and testing antenna systems for consumer products from R&D through manufacturing and commercial deployment.|