Build Your Own Patch Antenna for Your Next PCB
On the surface, devices with wireless capabilities seem like they would be pretty complicated. Once I started diving into designing wireless systems, I realized that incorporating these capabilities is only as complex as you make it. Thankfully, there are some simple solutions that you can use to add wireless communication to your next PCB.
If you’re looking to add GPS, WiFi, or other communication capabilities to your next electronic device, you might consider adding a patch antenna to your PCB. These inexpensive components are readily available and are easy to incorporate into your PCB. If you’re really feeling creative, you can add a microstrip patch antenna to your board, giving you wireless capabilities that are directly integrated on your PCB.
Incorporating an External Patch Antenna
External patch antennas come pre-packaged on their own PCBs, making it quite easy to incorporate these components in an electronic system. If you look for these antennas on the internet, you’ll quickly find an antenna that looks similar to a bow-tie. These antennas are inexpensive and are available from a number of distributors.
These antennas are placed directly on an FR4 substrate and connect to your mainboard using a small feeder cable. This makes it very convenient to incorporate a patch antenna into your device. The downside is that you will need to make room in your packaging for the antenna. The connection cable is usually several inches long and is directly soldered on the antenna board, so you can’t swap it out for a shorter cable. This means you will need to design your packaging to accommodate the size of the antenna and the long cable.
These antennas also come in the dual-band or multi-band variety, usually operating in the WiFi, GPS, Bluetooth, WLAN, and/or Zigbee bands. The geometry is usually optimized to provide desirable gain in each band, and they are typically impedance matched at 50 Ohms with a passive matching network. These are a real plug-and-play type of solution, making it easy to add wireless capabilities to your next device.
The receiver electronics required for separating signals into different data streams are not packaged on these antennas, so you will need to incorporate this functionality directly on your board. The external nature of these antennas makes it easy to add shielding to the main board so that you can protect other sensitive circuits from EMI.
Patch cable connection on a WiFi module
Designing an On-Board Patch Antenna
Placing a patch antenna directly on your PCB is quite easy when you use the right design software. You need to be able to define a custom shape for your ground plane and the shape of your conductor that will act as your receiver/transmitter. A patch antenna has a very simple geometry. A square or rectangular radiator is placed above a ground plane, and the two are separated on different layers.
If you examine the electromagnetic field distribution in the patch, you’ll find that the impedance of the patch actually varies of across the patch. In order to match the impedance of the antenna, you’ll need to connect your feed line at the point where the impedance distribution is a maximum. The distribution will be flat at this point, so the impedance seen by signals will not vary across the cross-section of the conductor in the connector line.
If you don’t have experience sizing a patch antenna and you don’t have access to microwave simulation software, you can still find the information you need to place a patch antenna on the internet. Many hobbyists have taken the time to prepare guides on designing patch antennas on simple PCBs, and you can easily adapt the geometry they use in your antenna.
Note that these antennas may not be impedance matched, they may receive in multiple bands, and they may not be optimized for your particular substrate. For ease of fabrication, hobbyists usually design these antennas to be quite large and bulky as this makes these antennas easier to work with by hand, and the antenna is really receiving in a higher order band.
When placing a copper antenna on a PCB, the size of the radiator should be equal to have the wavelength (sized in the conductor) associated with the signal frequency you wish to use for wireless communication. The edge of the ground plane should also extend out beyond the radiator by a distance that is equal to ~3 to 5 times the thickness of the substrate. This helps capture fringe fields from the radiator.
One should note that you should avoid the use of vias when working with an on-board patch antenna on traces that carry your RF signal. Each via will increase the impedance of the trace, depending on the diameter. If your antenna was already passively impedance matched to 50 Ohms, you may need to compensate this matching if any vias appear in the trace.
Find the software that can fit the antenna you need
Designing for Polarization
As you make the antenna progressively larger, it becomes possible to design slits in the antenna if you want to control linear polarization. This is a simple method for enforcing linear polarization but is somewhat inelegant. These slits can also cause electromagnetic waves to diffract, which degrades very faint signals. It also creates an odd impedance distribution if the slits are not arranged symmetrically on the patch.
If you design the antenna to resonate at the input signal frequency along only one dimension in the patch, then the radiated electric field will be linearly polarized. You can take advantage of this to enforce circular polarization. This can be done by taking advantage of the so-called “nearly square mechanism”.
First, design a square patch on the top layer and place two feed lines in the patch. If you visualize a pair of perpendicular lines that intersect at the center of the square patch, each feed line should be placed on one of these lines approximately halfway between the edge and the center of the patch. Finally, the signals in each feed line must be 90 degrees out of phase. The emitted radiation will be circularly polarized.
The extensive range of layout tools in a great PCB design package like Altium Designer® makes it easy to add wireless capabilities to your PCB. The smart design rule checking, component libraries, analysis, and simulation tools can help you build the best PCBs and verify their functionality. Download a free trial to find out if Altium is right for you.
Talk to an Altium expert today to learn more about Altium and how it may help you create your next wireless PCB.