This article will focus on embedded antennas and some design improvement tips to boost antenna efficiency. We shall begin our discussion with a few general concepts when designing an embedded antenna.
In an ideal world for a wireless product designer, antennas would mount far from any disturbances. Unfortunately, the size requirements of wireless products are shrinking dramatically.
In some cases where external antennas can be used, it’s considerably easier to boost antenna efficiency to improve link performance by avoiding these RF disturbances.
Let’s start by looking at a few simple guidelines to reach peak antenna performance.
How do you improve an embedded antenna?
To start, some good rules of thumb to avoid implementation pitfalls and increase efficiency for embedded antennas are:
- Breakdown the RF network into more manageable blocks
- Use the antenna manufacturer’s recommendations when implementing the antenna in your product to avoid significant disturbances
- Consider the placement of the antenna with respect to the packaging and end use orientation
- Be aware of the ground plane and via fencing around the antenna and RF path
- Leave space and separate the power and digital lines from RF
Simplifying the Solution with RF Blocks!
RF design only requires breaking the problem down into more manageable blocks. The further the blocks are from each other, the less likely they are to interact with one another causing undesirable performance.
Typically, a simple RF chain can be broken into 4 elements:
- Matching Network
How do you create an antenna feed network?
RF sources and receivers can include WIFI, Bluetooth, LTE, ZigBee, or other radios. A Balanced/Unbalanced (Balun) transformer is only required if the radio only supplies balanced lines at the output and you need to transform the characteristic impedance to 50ohm.
For sub 3Ghz, lumped elements can be used to match the antenna. Components like capacitors and inductors can also be used to transform the line to single ended however they typically require a larger footprint than integrated Balun modules as shown in the following examples.
RF traces can be used for matching, however, they can require an even larger footprint depending on the operating frequency. It’s recommended to leave that to an experienced RF specialist with a simulator and fast optimization routines. It’s important to note that the manufacturing variation may be larger than the tolerance on lumped element components which may shift the matching resonances. Some manufacturers may offer impedance sensitive PCB materials to reduce the dielectric and substrate thickness variation and improve expected results but this will increase the cost.
An example block diagram antenna network could look like the following:
A good example is the nRF24L01 as shown in the datasheet example. Note that C5 also operates as a DC blocking capacitor, but it’s also used to match to the 50ohm RF I/O line.
The schematic follows the same rules in the block diagram. The 50ohm RF I/O could simply be a trace antenna as listed in their datasheet, a chip antenna, or an SMA connector for an external antenna. The following image shows the example top layer gerber for the nRF24L01, which implements a trace monopole as the antenna element. Not exactly compact!
Integrated components can be found on distributor sites like Digikey or LCSC. Searching those sites can save a lot of design time especially for filters, baluns, and other integrated components. Depending on the link requirements, other building blocks could also include
- Polarizers for Circular Polarization
- Beam Forming Network for Arrays
- RF Power/Low Noise Amplifier
How to select the antenna?
If you’re already at the point where you know you have a link issue due to poor antenna performance, no doubt that you have already selected an antenna. If not, there are lots of manufacturers for chip and embedded antennas that provide datasheets and guidelines to follow when implementing them. In some cases, it’s even possible to download an example layout!
A nice example is Johanson’s chip antenna for an 2.4GHz ISM module:
On top of an example layout, the manufacturer provides encrypted models to run in HFSS to simulate your products performance. Unfortunately, it requires an expensive license, an experienced RF engineer, and serious hardware to run the sim. There are open-source alternatives like openEMS, but this requires an even broader skill set.
It’s important to note the antenna pattern in the datasheet and not just the peak gain especially when the product needs omnidirectional connectivity. A smoother gain profile will help boost antenna efficiency when testing the product in different orientations. Plastics and other materials near or in contact with the antenna may degrade performance so be aware of everything around it!
Where do you place the antenna on the PCB?
Determining an optimal location for the antenna on the PCB requires several inputs like the 3d radiation pattern and how the PCB will be packaged. Normally, packaging will be a mixture of plastics (ie: dielectrics) and metals (ie: conductors).
It’s important to know exactly where lumps of metals will be since they will change the radiation pattern of the antenna. If you have a battery right beside your antenna, chances are this will destroy your expected pattern. It’s important to keep the antenna far enough away from these elements.
Plastics and other dielectric materials very close or in direct contact with the antenna can shift the resonance and/or disturb the antenna efficiency by reflecting power off the packaging surface. Exposing the antenna to air or thinning the plastics around the antenna will provide better performance.
How to take ground planes and walls into consideration?
Grounding is very important when trying to boost antenna efficiency to improve link performance. Not only will it shape the radiation pattern, but it can provide shielding for EMC/EMI especially along the RF feed network. If an antenna simulator is not available it’s recommended to add a via fence around the antenna interface and corresponding shared space. The antenna datasheet will have recommendations regarding the top and bottom layer ground planes, stay out zones, and via fence criteria. If space permits, adding multiple layers of fence will aide to assume a sufficient ground wall.
It is highly recommended to use a Co-planar Waveguide (CPW) trace to keep the path free from unintentional mismatches and keep to a constant 50 ohm characteristic impedance. There are lots of CPW waveguide calculators out there that can help you design your 50 ohm line to the PCB manufacturer you’ve selected. If you’re manufacturing in China, you can grab spec from a supplier like JLCPCB.
Those parameters can be entered into a CPW designer like wcalc to compute the required trace width. A link to the tool is give below.
Just keep in mind that the PCB manufacturing spec will most likely be different across PCB suppliers and would require recomputing the trace width.A helpful hint would be to add RF lines on the side of your PCB to measure actual line impedance. This can give insight to the real dielectric constant and substrate thickness originally assumed in the trace width calculation.
An important factor: RF Network Spacing!
Always consider a four layer board or at least make sure there is nothing around the RF path if you decide to stick to a two layer stack up. Four layer boards allow you to have RF top, solid ground, VCC plane, and Signal Bottom. This helps to debug or simply avoid EMC/EMI issues. Keep in mind that any signal lines near RF lines will couple and cause interference. Holes in the ground plane can also leak energy.
Another important factor is that RF lines have less loss on the top layer so try to avoid long stripline traces in the substrate. More loss equates to less gain so try to keep the antenna reasonably close to the radio. Vias can also add unwanted resonances at high frequencies so try to avoid them.
In this article, several design improvement tips were discussion to boost antenna efficiency. You can always contact us to consult on your wireless product’s antenna design if there is a need to further improve link distances or overall wireless efficiency. We would be glad to help.
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