Parent Category: 2015 HFE
By: Matthew Meiller
Abstract: Wireless device range can be the pivotal make or break characteristic of a successful end product. This paper will dig into the mystery and explore the mechanisms by which wireless range can be reduced or optimized through RF and antenna design. The discussion is relevant to board and system- level circuit and antenna design. The useful rule of thumb that every 1dB of additional RF loss reduces wireless range by 10% is presented.
Index Terms— Wi-Fi, Bluetooth, BLE, Zigbee, RFID, GSM, GPS, MBAN, HBAN, UWB, CDMA, Chip Antenna, Circuit Board Antenna, Wireless Range Reduction, Wireless Range Optimization, Radio Module, 802.11 and 802.15.4
Any RF engineer who has optimized RF or microwave system hardware in the lab will agree that squeezing out the last 1 or 2dB from a design can be the most challenging aspect. After reading this paper you may better appreciate the value of such rigor. This is where the rubber meets the road for applying the art and science of RF design to the development of wireless products. At this point the product requirements may be defined, the theoretical path loss calculations may be complete and you want to ensure execution of the hardware development goes smoothly. Or, the product may be designed and prototypes delivered and debugged, but questions are being asked regarding the wireless range or lack thereof. This article will help the reader understand quantitatively how much wireless range may be lost if the antenna tuning and match steps are neglected, there is more RF loss in the design than anticipated or a related aspect of the design is out of control.
Unintended Loss in the Design
There are many possible sources of insertion loss, mismatch loss and general degradation of antenna gain. These are RF signal losses resulting from product design decisions and features. Collectively we will refer to these as unintended losses and all can have identical impact, which is to reduce the range of wireless products. By referring to them as unintended losses, we mean that they are a consequence of poor RF layout or antenna design and were not factored into the link budget calculation, which can be used early in the design to predict the range of a wireless device.
The RF engineer can prevent these problems and their disastrous consequences by optimizing the performance critical aspects of the design before the prototypes are built, and continuing the optimization and performance assessment in the lab when the hardware is available. This is not a long and drawn out process. It is a matter of simply involving the right expertise with access to the proper design, simulation, and test and measurement tools at the right times. The end result will be a product which provides the best possible wireless performance for your customers and shareholders, with predictable cost and schedule.
Common Sources of Unintended Loss
The contributions from all sources of unintended loss are cumulative, including the separate losses of each of the 2 radios participating in a wireless link. For example, if we have 2dB of mismatch loss and the antenna gain is degraded by 2dB due to the layout, the impact of 4dB must be considered. If two such identical radios are communicating, then the total impact of 8dB must be considered.
Antenna match refers to optimizing the impedance matching network classically located close to the antenna using a piece of test equipment called a RF Vector Network Analyser. The impedance matching network is typically composed of lumped element capacitors and/or inductors, which has values that must be chosen or transmission line stubs which must be trimmed. Once the impedance matching network is tuned based on precision laboratory measurement, subsequent product may be built using the values determined. The purpose of matching the antenna is to force it to resonate over the appropriate range of frequencies for the radio, and to couple as much energy as is possible between the 50 ohm antenna and transmit/receive circuitry.
Circuit Board Layout
If the antenna is mounted on or integrated into a circuit board, careful attention must be given to the layout and the Gerber files reviewed. Often times the antenna used is really only half of the antenna capability since the circuit board RF ground plane plays a key role in the antenna performance. Without the presence of the ground plane and proper control and checking of all the geometric positioning of the antenna and the matching and feed network, the design may be destined to provide poor wireless performance before it is fabricated. The board layout team must be given detailed guidance and instruction, including the positioning of vias critical to RF performance. Simulation tools as well as theoretical knowledge as to how signals behave on circuit boards are needed to get this part of the design right.
Integration of Antenna into Operating Environment
Your end product may use more than one circuit board or contain other large conductive objects such as shielded LAN or USB connectors, transformers or discrete wires and cables. All of these can profoundly impact the performance of your antenna as can proximity to materials such as plastics and conductors. The typical use case should be evaluated, including accessories. Proximity to the human body must be considered if the device is handheld or body worn. Integration of the antenna into the product enclosure refers to evaluating the entire product design with respect to the antenna(s), retuning the impedance matching network in the final assembled product since everything mentioned above can impact antenna performance. Tuning the board used for laboratory development is often different from the final product tuning!
Quantify Impact of Loss on Wireless Range
Free Space Path Loss
Once prototype hardware is built and the wireless link functioning in the lab, the easiest part of the link budget to modify is often the physical separation between the two radios. Technically, we are changing the free space path loss (FSPL). The FSPL gets smaller (less loss) when the radios are moved closer together and vice versa. Here is a handy version of the equation for FSPL:
The distance between the two radios is d (meters), and the frequency of interest is f (Hz).
If we plot path loss vs. separation distanced, the slope of the line is 20dB/decade or 6dB/octave for any range of separation distance d. Figure 1 shows the path loss in dB for 3 different commonly encountered frequencies and a single decade of distance d in meters from 100 to 1000 meters.
Figure 1 • Path Loss Over 1 Decade of Frequency.
Loss Compensation by Range Reduction
If the RF design has unintended loss not accounted for in the link budget, without changing any other variable, we can move the two radios closer together (reduce separation distance d) until they can maintain a wireless radio link. The effect of moving the radios closer together is to compensate for unanticipated loss by reducing the free space path loss defined earlier with an equation. Through inspection of the graph or mathematical analysis of the equation, we determine an approximate rule of thumb that regardless of the source of the loss or separation distance,
Every 1dB of unanticipated loss
Reduces wireless range by 10%!
We are making a linear approximation to quantities plotted on logarithmic scales, and this approximation is reasonably accurate for the final 5dB of link budget power while investigating the maximum separation distance. For example, you expected 300 meter range but your antenna gain is 2dB low, the 2 dB translates into an approximate 20% loss of or wireless range so you measure a range of (300 meters)*(80%)=240 meters. This is a range reduction of 60 meters. If the range is 50% of what you expected, you are compensating for exactly 6dB of unintended loss.
Other Loss Compensation Techniques
Standard coping mechanisms include turning up the transmitter power to compensate for an underperforming RF design. This may appear to work well in the lab, however as we increase transmit power, we also increase the amplitude of spurious emissions and harmonics which often lead to failure when the FCC or ETSI compliance tests are performed. This is similar to stepping on the gas if you have a flat tire. You may move forward for a while, but you will get emissions that you weren’t counting on such as your tire flying apart. If you do not have timely access to RF and antenna engineering capabilities when you need it, Peak Gain Wireless is ready to help with the expertise and equipment to solve these types of problems the right way. We can prevent these problems if we are involved early in the design or define and solve the problem if hardware is already complete.
What does this all mean? Many factors impact the wireless link budget. Examples include antenna selection, design, impedance matching and final product integration. If an antenna has not been properly designed, tuned and optimized in the final product enclosure, it is not uncommon to have a total unintended loss of 2 to 6dB. Since the impact is 10% range reduction per dB loss, this translates into a 20% to 50% range reduction. These types of problems can often be predicted, understood and designed out through EM simulation or the knowledge and insight of an experienced RF engineer with access to the right tools.
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