Much of the success or failure of a wireless product depends on the performance of the antenna. Too often, the antenna tends to be an afterthought and is added on towards the end of the design phase. While RF Design can be a challenge, following best practices and carefully considering which antenna type is best for your design can help you bring a winning product to market. Read this excerpt from the RF Design Considerations white paper to learn about some of the trade offs involved in the antenna selection and design process. Antennas play an important role on RF system performance. You’ll want to consider which antenna type will work best with your design. Antenna types include:
- Monopole Antenna
- Dipole Antenna/Sleeve Dipole
Sensitivity to board size is an important factor when choosing an antenna. For example, a monopole antenna is an antenna working with an ideal, relatively large ground which is considered part of the antenna. This makes the monopole antenna sensitive to ground size and shape, meaning that the ground can greatly affect the antenna’s performance. Inverted-F antennas, also called PIFA antennas, have an arrangement that makes them less sensitive to ground. Lastly there are dipole or sleeve antennas. These antennas have a positive current on one side and negative current on the other, thus establishing their own ground reference. Of the three antenna types listed here, the dipole is least sensitive to ground. All antennas require some amount of space for placement. When deciding on antenna placement, the surrounding materials must be considered, particularly conductive materials, as they affect the performance of the antenna. Antenna selection also depends on the system in which the antenna will be used. Here is a list of systems and their ideal antenna numbers and placement:
- Single-input, single-output (SISO) - This system uses only one antenna. SISO systems are usually quite sensitive to location. Performance is easily affected by the multipath effect. In a SISO system, some locations generate what is called a constructive effect and other locations generate a destructive effect. For example, a car’s FM radio is usually a single antenna system. As the car moves along the road, you may receive a clear signal one location and static noise in another. SISO systems with a single antenna are the easiest to design and are inexpensive.
- SISO with antenna diversity – In this configuration, the system has two antennas. A SISO system with a single antenna can only receive a signal at one point in space with no redundancy. However, a SISO system with antenna diversity support has two antennas, either one of which can be used at any point in time. This allows the system to switch antennas if the performance of one antenna is lacking. The system always switches to the best antenna to overcome the multipath problem. If your system supports antenna diversity, it is better to use two antennas. The rule of thumb is to place the antennas at least a quarter of a wavelength apart. As a rough estimate, a quarter of a wavelength is three centimeters in the 2.4 GHz band and 1.5 centimeters in the 5 GHz band.
- Multiple-input, multiple-output (MIMO) – These systems use multiple antennas to receive and transmit concurrently. For example, if you are using a 2X2 MIMO system, you need two antennas; this configuration is called a two data stream system. MIMO systems must have adequate isolation between each antenna. Typically, approximately 25 dBm isolation gives you better signal quality and thus better throughput. How can you achieve higher isolation? The first and easiest method is to increase the distance between the antennas. Move the antennas as far away from each other as possible. Longer antenna distance provides better antenna isolation. The second method is to adjust the antenna polarization. For example, if you have two dipole antennas, you can adjust them so that they form a 90-degree angle, one in in the horizontal polarization and the other in the vertical polarization. This way, even at very short ranges, you can still achieve 25 dBm isolation.
- Multi-com with multiple antennas- Multi-com systems use two different standards, such as Wi-Fi and Bluetooth, in one product. Multi-com systems require multiple antennas. Wi-Fi and Bluetooth operate at the same frequencies so adequate isolation between the antennas helps avoid interference and makes for better multi-com coexistence. For example, Wi-Fi products can handle a maximum of -20 dBm input signal and a maximum transmit power (TX) of approximately +20 dBm. Normally, Bluetooth can receive a maximum input signal of -10 dBm and TX power is typically limited to approximately 4 dBm. Therefore, you need approximately 25~30 dBm isolation between Wi-Fi antennas and Bluetooth antennas. This provides increased performance when Wi-Fi and Bluetooth operate concurrently.
To ensure a successful design, it is best to use the knowledge and experience of radio module manufacturers. For more information on this topic, LSR is hosting a free webinar and Q&A Session titled, The Engineer's Approach to Antenna Selection and Implementation. Antenna design experts from LSR will walk through a number of practical tips and best practices to keep in mind when selecting and implementing an antenna solution into your next design, including:
- Defining antenna requirements
- Deciphering antenna datasheets
- A typical antenna design and evaluation process