Optimizing Antenna Integration in ISM LPWA Devices

The continued expansion of the internet of things (IoT) in industrial, consumer, and medical devices, plus emerging smart cities and smart buildings, is driving the rapidly increasing use of low power wide area (LPWA) wireless networks. That is especially true in the industrial, scientific, and medical (ISM) radio frequency (RF) bands of 915 MHz in the U.S, 868 and 169 MHz in Europe, and 433 MHz in Asia, that support wireless protocols such as LoRa, Neul, SigFox, Zigbee and Z-Wave.

LPWA devices continue to shrink and need inexpensive and compact antennas with superior performance. Antenna ground plane issues can be especially troublesome in 868 and 915 MHz ISM bands. They can be dealt with using additional circuitry, increased device integration, and more precise frequency tuning, all of which can add to development time and costs. Designers need antennas that minimize ground plane concerns. In addition, LPWA devices are often battery-powered and require maximum energy efficiency. The selection and integration of the antenna is a critical aspect of an efficient design. A less-than-optimal antenna solution can reduce battery life and result in poor overall system performance.

An optimized link budget is one key to a reliable and efficient wireless communication interface. Antenna selection and integration have a significant impact on the link budget. But designing or selecting an efficient and high-performance antenna that addresses both link budget and ground plane concerns is a complex process. Antenna specifications that impact the link budget include impedance, return loss, voltage standing wave ratio, gain, radiation pattern, and more. Identifying easy-to-integrate, compact, and high-performing antennas that minimize ground plane issues can significantly reduce engineering time and improve overall system performance.

This article describes a basic link budget model, reviews key antenna specifications that impact the link budget, and presents examples of antennas from Molex that can overcome ground plane issues and help optimize link budgets in LPWA devices.

Basic link budget
A link budget in a wireless system measures the effective RF energy that arrives at the receiver. The equation starts with the transmitted power in decibel-meters (dBm), adds any gains in decibels (dB), subtracts losses, also in dB, and arrives at the received power in dBm. In a practical design, there are numerous contributors to gains and losses.

Taking a deeper dive into link budgets
Antenna performance is the only factor impacting gains and losses in a link budget. Antenna efficiency, gain, and radiation pattern are three important aspects of antenna performance, and they are often measured using an over-the-air (OTA) chamber (Figure 1). Other factors that can impact link budgets are return loss (the S11 parameter) and voltage standing wave ratio (VSWR).

Image of antenna efficiency, gain, and radiation pattern are measured using an OTA chamber
Figure 1: Antenna efficiency, gain, and radiation pattern are measured using an OTA chamber. (DUT in image refers to Device Under Test) (Image source: Molex)

Antenna efficiency determines the emissivity of an antenna. Average efficiency is often used, but efficiency is not a single number. It’s a curve that can be more or less flat, depending on the specific antenna being considered (Figure 2). Often an antenna with a flatter efficiency curve will have a lower maximum efficiency than an antenna with a more peaked efficiency curve.

Image of antenna efficiency curves can vary widely (click to enlarge)
Figure 2: Antenna efficiency curves can vary widely: the antenna on the left has a flatter efficiency curve, but the one on the right has about 10% higher peak efficiency at 915 MHz. (Image source: Molex)

Like efficiency, antenna gain can be measured as an average or peak/maximum value. At a given frequency, the average gain is measured across all the angles in three-dimensional space, while the maximum gain is a single operating point. In general, the higher the average gain, the better.

The radiation pattern of an antenna is an important factor in determining the gain. A theoretical antenna that radiates the same energy in all directions is called an isotropic radiator and has a gain of 0 dB (unity). Real antennas, even so-called omnidirectional designs, have non-isotropic radiation patterns and can be more or less directional as measured in 3D planes (Figure 3). An antenna with a gain of 3 dB is twice as effective in a given direction as an isotropic radiator. It doubles the transmitter’s power, or the receiver’s sensitivity, in that specific direction.

Image of radiation patterns differ for various antenna designs (click to enlarge)
Figure 3: Radiation patterns differ for various antenna designs and can be important in link budget calculations. Both of these antennae are specified with omnidirectional radiation patterns. (Image source: Molex)

Antenna design and the surrounding environment affect the radiation pattern. Typical datasheet measurements use a free space environment with no surrounding interference. In actual implementations, the peak gain will be reduced by 1 to 2 decibels relative to isotropic (dBi) since the radiation pattern will change due to the surrounding components.

Return loss (S11) and voltage standing wave ratio (VSWR) are related measurements of the amount of energy reflected from the antenna back to the RF circuit, and smaller values are better (Figure 4). S11 ≤ -6dB or VSWR ≤ 3 are often considered to be minimum acceptable performance levels. If S11 = 0 dB, then all the power is reflected, and none is radiated. Or, if S11 = -10 dB, when 3 dB of power is delivered to the antenna, -7 dB is the reflected power. The antenna uses the remainder of the power.

Graphs of return loss of the high-efficiency antenna vs low-efficiency antenna (click to enlarge)
Figure 4: The return loss of the high-efficiency antenna (right) is about -14 dB at 915 MHz, while the return loss for the lower efficiency antenna with the flatter efficiency curve is about -10 dB at 915 MHz. (Image source: Molex)

VSWR is a function of the reflection coefficient. Like return loss, a smaller VSWR indicates a better antenna. The minimum value of VSWR is 1.0, where no power is reflected from the antenna. Impedance matching can be used to minimize S11 and VSWR. Impedance matching involves modifying the transmission line between the antenna and the RF circuit to improve the maximum energy transfer. An impedance mismatch results in part of the RF power not being accepted by the antenna. An exact match between the transmission line impedance and antenna impedance results in all RF power being received at the antenna.

Some antennas have an impedance of 50 Ω and do not need a matching network. Most antennas require an impedance matching network in the transmission line to optimize the antenna performance. Matching networks are generally required with antennas that support multiple frequency bands. A matching network can consist of various combinations of capacitors, inductors, or resistors when needed.

Enhancing antenna performance
A basic antenna consists of a conductor of a given length, but additional elements can be added to enhance antenna performance. One example is the MobliquA™ antenna technology from Molex that includes bandwidth enhancing technologies (Figure 5). MobliquA technology is designed to improve the range of frequencies over which the return loss is acceptable, often referred to as the ‘impedance bandwidth.’ This technology can improve the impedance bandwidth by 60 to 70 percent without compromising the radiation efficiency or increasing the size of the antenna. An ISM antenna designed for 868 MHz and 915 MHz using MobliquA technology can have up to 75% less volume than conventional designs and eliminate the need for expensive circuits and frequency tuning required to address ground plane dependence issues.

Image of Molex’s MobliquA technology
Figure 5: Molex’s MobliquA technology is designed to improve impedance bandwidth and provide a high degree of immunity towards the insertion of metal objects into the antenna volume. (Image source: Molex)

MobliquA technology enables the use of RF decoupled or grounded parts, such as a grounded connector housing. It provides good immunity from the insertion of metal parts into the antenna volume. Its unique feeding techniques combined with a direct ground of the antenna elements provide enhanced electrostatic discharge (ESD) protection for the RF frontend.

Antenna integration
While all of the electrical specifications discussed above are important aspects of antenna integration, there’s also the issue of mechanically connecting and integrating the antenna into the system. There are multiple possibilities. For example, some antennas are designed to be soldered into the system, and others include a coax cable and connector attached to the system. The following two sections present some of the specifications for each omnidirectional antenna.

Flexible ISM antenna with coax and connector
For applications that need an 868/915 MHz dual band ISM antenna, designers can turn to the model 2111400100 from Molex (Figure 6). This monopole antenna measures 38 x 10 x 0.1 millimeters mm, is made from a flexible polymer material, and has a 100 mm long micro-coax cable with an outside diameter of 1.13 mm and a U.FL connector that is MHF compatible. It is ‘peel-and-stick’ and will attach to any non-metal surface. It can handle 2 W of RF power and has an operating temperature range of -40 to +85 °C. Other antennae in this series have 50, 150, 200, 250, and 300 mm cable length options, and custom lengths can be fabricated.

Image of Molex 2111400100 dual band ISM antenna is flexible
Figure 6: This dual band ISM antenna is flexible and is mounted in the system using a ‘peel-and-stick’ adhesive. (Image source: Molex)

Some key specifications include:

Efficiency: >55% at 868 MHz, >60% at 902 MHz
Peak gain: 0.3 dBi at 868 MHz, 1.0 dBi at 902 MHz
Radiation pattern: omnidirectional
Return loss (S11): < -5 dB
High-efficiency ceramic ISM antenna solders to the PCB
When the need is for higher efficiency, designers can use a 2081420001 ceramic antenna that’s specifically designed for ISM applications (Figure 7). Different matching networks can be used in two different frequency bands; 868-870MHz and 902-928 MHz. Rated for operation from -40 to +125 °C, it measures 9 x 3 x 0.63 mm.

Image of Molex 2081420001 ceramic antenna
Figure 7: With different matching networks, this ceramic antenna can be used in two different frequency bands; 868-870MHz and 902-928 MHz. (Image source: Molex)

Some key specifications include:

Efficiency: 70% at 868 MHz, 65% at 902 MHz
Peak gain: 1.5 dBi at 868 MHz, 1.8 dBi at 902 MHz
Radiation pattern: omnidirectional
Return loss (S11): < -10 at 868 MHz, < -5 at 902 MHz
Antenna optimization and integration into LPWA ISM applications, including LoRa, Neul, SigFox, Zigbee, and Z-Wave IoT protocols, is an important and complex task. Optimizing the link budget is necessary to ensure good wireless performance and long battery life. It includes numerous electrical operating specification tradeoffs and the development of an effective impedance matching network. The antenna selection process also must consider the operating environment and the mechanical and interconnect requirements of the device.