TRIO mXTEND™ Antenna

The antenna selected for this reference design is the TRIO mXTEND^TM (NN03-310) provided by Ignion, which owns this disruptive Virtual Antenna^TM technology. This antenna is the only solution available on the market that is capable of managing three different radios (LoRa, multiband GNSS, and Wi-Fi/Bluetooth) at the same time, inside a single antenna package. Its miniature, off-the-shelf, multiband, high-efficiency, and tunable features make it ideal for use in combination with the LR1110.

The image below shows the TRIO mXTEND adjustable-length board.

Images courtesy of Ignion Antennas

The antenna performance has been assessed with different printed circuit board (PCB) characteristics:

• Adjustable length: the nominal size is 90mm x 50mm, in line with the ISO card standard. Vertical lines are placed on the silkscreen, as guides to cut the board and vary its length from 54 to 126mm. Five boards are equipped and optimized, highlighting the impact of board size on the tuning / efficiency for each band.
• Built-in module: an FMLR-1110-x-STL0 module from Miromico replaces a chip-down implementation. This module integrates an SP3T switch, and only three antenna feed points are available: LoRa, Wi-Fi and GNSS.
• Antenna space and keep-out are implemented on the left of the board.
• External matching is placed next to the antenna.
• Ground is laid out under the complete board, even though there are no active parts on the right-most half of the board; this is to emulate a large counterpoise of an IoT object.
• Jumpers, JTAG interface, and positioning holes are added for the power measurement, programming, and positioning of the device on its substrates.

Ignion Antenna Technology

Ignion owns a new and revolutionary antenna technology, called Virtual Antenna^TM.  Already installed in more than 25M edge devices, this technology can replace conventional and custom antenna solutions by a new class of so-called antenna boosters, delivered in the form of a new range of miniature and off-the-shelf chip antenna components. These new chip antennas are, by nature, multiband and multipurpose, so they fit in a variety of wireless platforms to provide a wireless link for many different communication services. By using a Virtual Antenna component, the design becomes more predictable than custom solutions, making the entire process faster, cheaper and easier.

Common techniques for designing small multiband antennas in wireless devices are based on the use of complex geometries, where the resonant modes of the antenna determine the frequency bands of operation, requiring a high level of expertise for correctly shaping the antenna geometry and for achieving acceptable behavior operating at a given frequency band.

The TRIO mXTEND chip antenna component used in this reference design is built on a glass epoxy substrate, and belongs to this new generation of off-the-shelf antenna solutions based on Virtual Antenna technology. It offers the advantage of being non-resonant. Its frequency-neutral characteristic allows the designer to easily select the operating frequencies according to their needs, since they are not set by the antenna geometry, unlike conventional antenna solutions. Such an antenna supports a number of applications and provides many benefits:

The TRIO mXTEND^TM chip antenna component offers the versatility of being usable in a single port or multiport configuration and the flexibility of being tuned to other frequencies by simply adjusting the matching network. The configuration for LoRa, GNSS, and Wi-Fi is illustrated here, but you can configure it to operate with any communication standards that fit your needs

What Makes an IoT Device Radiate Properly?

All well-designed “things” have good connectivity, and that means a better uplink (UL) and downlink (DL) packet success rate (PSR), if all of the following conditions are satisfied:

• The antenna matching is correct, ensuring that a vast majority of the incident power is radiated by the antenna, and not reflected back to the source
• The antenna efficiency is good, ideally 100 percent. This would mean that ALL the forward power injected into the antenna (and not reflected to the source), is effectively radiated (and not dissipated) by the antenna
• The RF source supplies the right power to the antenna through its matching network, meaning that the PA matching is correct

 

The effects of mismatching are as follows:

Most of the mismatch effects described in the previous table are estimated with a passive antenna efficiency measurement, where the “efficiency” number accounts for both the mismatch loss, and the intrinsic inefficiency of the antenna.

The graph below on the left shows the measured reflection coefficient (S₁₁) for the reference PCB size (90mm x 50mm). This value represents the amount of power injected by the RF module to the antenna system that is reflected. Generally, the lower the S_11, the better the antenna performance. This parameter can be easily improved through the proper adjustment of the matching network to effectively adapt the antenna performance to the environmental conditions. In the reference design presented herein, the three bands (LoRa, GNSS, and Wi-Fi) were properly matched with a reflection coefficient below –6dB. The associated mismatch loss can be computed as follows:

Another important parameter to consider is antenna efficiency, which considers both mismatch loss and the intrinsic radiation efficiency of the antenna. The graphs below show the radiation efficiency (Ƞ_r) and the antenna efficiency (Ƞ_a).

Image courtesy of Ignion Antennas

The radiation efficiency indicates the proportion of power that would be radiated to the space if there were a perfect match (mismatch loss = 0dB), whereas the antenna efficiency represents the proportion of power actually radiated to space once mismatch losses are considered. It is computed through the following expressions.

 

The following table shows Return Loss, Mismatch Loss, Radiation Efficiency, and Antenna Efficiency.

 

It is important to understand that highly-efficient and non-linear power amplifiers used in LoRa devices, specifically for the sub-GHz LoRa bands, are sensitive to their mismatch and may offer different output power, harmonic content, and intake power consumption, when their load changes. This effect isn’t evaluated here, as it requires an active antenna measurement method, but will be documented further later in this series.

The image below describes a principle and is not the result of any actual measurement. It shows how a Power Amplifier (PA) maintains its stated power over a certain zone of proper matching. Beyond this zone, the PA induces some losses, is misloaded and, therefore, has a slightly degraded performance.

Mismatch Loss: Understanding the Quantities

Most of the following results are expressed in terms of reflection coefficient. They are published with a scalar plot, with frequency on the horizontal axis, and a negative dB value on the vertical axis. The lower the number in dB, the lower the amount of power reflected back to the source; conversely, the more power is absorbed (and radiated) by the antenna.

The following graph represents the return loss of the TRIO mXTEND^TM antenna in the Wi-Fi band, for the standard board, with no material loading on the antenna.

Just to put the numbers into perspective, the mismatch loss induced by a certain “miss” on the load impedance, is given below:

This is why the ‑6dB bar is displayed in the coming plots and is  used as a performance indicator; keeping ‑6dB of return loss, indicates that no more than 1.25dB are lost during the power transfer, which is a reasonable target in extreme conditions, even if a perfect matching of 50Ohms with no reflected power is desirable.

Board-size Impact: Matching and Resonance

With the virtual antenna technology, as well as with any other conventional antenna design, the board size impacts the resonance of the object. Conversely, the antenna matching networks (in an LR1110 design, there are three: LoRa, GNSS, and Wi-Fi) must be modified to ensure that the RF power delivered by the LR1110 is effectively radiated.

As you can see in the table below, all elements of the matching network must be modified as a function of board size, to maximize return loss and therefore good performance:

All matching networks depicted below were designed with the objective of obtaining the lowest S₁₁ and the highest antenna efficiency in the frequency bands required to cover the whole spectrum, from 863-928MHz for the LoRa case, to, 1561-1606MHz for GNSS, and 2400 -2483MHz for Wi-Fi. The PCB size affects the performance of any antenna, not only in terms of radiation efficiency, but also in terms of impedance. This may mean detuning, but can easily be solved by readjusting the matching networks properly. That is why each PCB size has its own matching network to optimize the performance in the three bands. A minimum number of components is used for this purpose, to reduce the associated losses as much as possible. The use of high quality factor (Q) and tight tolerance components is recommended to avoid efficiency losses in the matching network, and to ensure repeatability of the solution. Unlike classic resonant antennas, which are specifically designed to work on certain bands, Virtual Antenna technology can be used for all bands. The antenna element remains the same and a readjustment of the matching networks means the antenna can work on the desired bands with the maximum performance.

The following table shows the TRIO matching elements for each board size.

The Notch filter indicated below is used to guarantee a certain isolation between LoRa, GNSS, and Wi-Fi. In the same way, the first two components of the Wi-Fi branch (Z7 and Z8) are part of another notch filter that isolates Wi-Fi from GNSS. The other components of each branch compose the matching networks used to minimize the S₁₁ and maximize the efficiency of the antenna.

Image courtesy of Ignion Antennas

The network diagrams below illustrate the differences among LoRaWAN, GNSS, and Wi-Fi networks.

LoRa Matching Network

GNSS & Wi-Fi Matching Network

Images courtesy of Ignion Antennas

 

	The antenna tuning components must be optimized for the board size. Likewise, if the board shape is different, these elements may have to be tweaked as well.
	As part of the free-of-charge NN Wireless Fast-Track service, Ignion can assist in designing or optimizing your own matching network. Visit: https://ignion.io/fast-track/

 

The Importance of a Wideband Design

Unlicensed bands in different regions of the world are not harmonized and span from 863MHz (bottom of the European band) to 928MHz (top of the US band). The LR1110 Power Amplifiers are capable of sourcing a carrier at any frequency across these 65MHz, at high efficiency and at the legal level of power. As seen in the next graph, when properly tuned, the TRIO mXTEND^TM antenna offers a wide bandwidth, allowing for a single tuning for all frequency bands across the different PCB sizes.

The first graph below illustrates the return loss for 90mm Board in Air. The second graph shows the reflection coefficient for all Board in Air sizes.

 

 

Image courtesy of Ignion Antennas

Board Size Impact: Antenna Efficiency

The ability of an antenna to radiate the power it is being fed is typically expressed in terms of efficiency, either in dB or as a percentage:

The Total Radiated Power (TRP), shown in the table above, expresses the sum of all power radiated by an antenna, connected to an RF source and integrated in all directions. It is not to be confused with e.r.p., which is the observed power in one direction. Regulatory laws typically request that the peak e.r.p. be below a certain threshold, whilst the overall performance of an omnidirectional antenna, such as the ones being measured here, is better described in terms of TRP (indeed, the orientation of most IoT devices with regard to the network they are broadcasting to, is generally an unknown).

The graph and table below show a summary of the efficiency numbers obtained for all frequency bands of interest, and all PCB sizes that were tested:

Image courtesy of Ignion Antennas

The efficiency numbers stated in this report include mismatch losses induced by any detuning of the antenna. On the Wi-Fi band, the antenna efficiency remains high, and even increases as the board gets smaller. At these frequencies, the radiation is the contribution of the longitudinal and transversal radiating modes of the PCB. The resonant frequency of the longitudinal mode increases as the PCB length reduces, being closer to the Wi-Fi operating frequencies, thus providing better performance.

However, for the LoRa bands where the wavelength is much longer (about 33 centimeters), the main resonance is obtained on the longer board edge, which gets shorter and shorter as the board is cut. Efficiency drops from 76 percent to about 36 percent on average, this is a 3dB hit on the antenna performance (UL and DL) for the smaller object.

The TRIO mXTEND has the advantage of embedding three antennas in a single component, simplifying the integration and bring-up of the IOT device.

	The efficiency in the LoRa band might be optimized with the smaller board sizes by selecting an independent antenna for that frequency band.

 

Board Size Impact: Radiation Pattern

The TRIO mXTEND antenna has an omnidirectional pattern for all board sizes and frequency bands. This is an important feature when, typically, the orientation of the device is unknown. The only exception to this would be the GNSS antenna radiation pattern where, irrespective of the device location on the planet, the satellite signals will always be the most powerful when the SV is “over” the device (at least in line-of-sight conditions), meaning an elevation of 90 degrees from the horizon.

See the measured radiation patterns below for the reference PCB size (90mm x 50 mm). The three main cuts, as well as the 3D illustrations, are represented for the central frequency of each frequency band.

Image courtesy of Ignion Antennas

The following diagrams show 2D radiation plots for a 90mm board.

Images courtesy of Ignion Antennas

The images below show 3D radiation plots for a 90mm board:

Image courtesy of Ignion Antennas

The main radiation pattern cuts for GNSS frequencies remain omnidirectional for all PCB sizes, so are preferable for those devices in constant movement where the direction of the incoming waves is unknown.

Image courtesy of Ignion Antennas

Based on the results above, we can conclude that the TRIO mXTEND is a suitable antenna capable of working at LoRa, GNSS and Wi-Fi bands at the same time. The larger the PCB, the higher the performance that is expected for the LoRa band. Apart from that, any change in antenna impedance due to PCB size or changing environmental conditions can easily be compensated through the matching network. This is one of the advantages of Virtual Antenna technology: it can work at any standard and under different conditions using the same antenna component, by just adjusting the matching network.

Antenna Detuning with Select Materials

IoT use-cases are diverse: Smart Home, Smart Utility, Smart Farming, Asset Tracking etc. In any of these scenarios, the connected device is somehow anchored to an asset. Trackers may be placed on top of a shipping container, or may be integrated on a wooden or plastic pallet. Connected thermostats may be placed on a wall made of concrete, plaster, or on a metal beam in an industrial building. Pet trackers are placed near animal tissue. The list goes on.

Test Setup

It is important to estimate and even better, anticipate the material that will surround the device, to ensure that the antenna has the best possible performance in its use case.

Tracker on a Wood Pallet	Tracker on Concrete
Tracker on Metal	Tracker on Body Phantom

Images courtesy of Ignion Antennas

 

Multiple experiments show that the underlying material and, more importantly, the distance to it, plays a critical role in the antenna performance, both in terms of mismatch losses, and in terms of efficiency:

Image courtesy of Ignion Antennas

 

The following graphs show the impact of the substrate on antenna resonance per frequency band.

The vertical dotted lines indicate the extremes of the frequency bands of interest.

Legend:

Measurements performed by Semtech. Image courtesy of Ignion Antennas

 

The S11 = -6dB performance target is displayed, guaranteeing mismatch losses of less than 1.25dB.

Measurements performed by Semtech. Image courtesy of Ignion Antennas

Measurements performed by Semtech. Image courtesy of Ignion Antennas

Note that the matching network considered for the material impact in this analysis differs from the reference matching network selected for maximizing performance in LoRa bands. Nevertheless, the qualitative results and conclusions extracted apply to both. Details are available in Appendix A6: Alternative Matching for the 90mm Board.

Summary of our experimental learnings:

1. The TRIO, generally speaking, behaves well when loaded with these materials, at a distance of 5, 10 or 15mm. In the nominal case, it guarantees less than 1.25dB of mismatch losses by maintaining the return loss below –6dB.
2. Metal loading has a dramatic impact for the LoRa and GNSS bands, at all distances. Specific recommendations are discussed later in this paper. Metal loading has an acceptable impact to the detuning in the Wi-Fi band.
3. Human Phantom loading has a bearable impact, except for the LoRa band when the antenna is too close, with a 5mm gap. Increasing the gap to 10mm appears sufficient to keep a reasonable detuning.

Detuning Results: Efficiency and Mismatch Losses

In addition to detuning, the materials in the vicinity also induce a dissipation effect which depends on the electromagnetic characteristics of each material; some materials cause more loss than others. The antenna efficiency has been measured for the reference board size (90mm x 50mm), placed 15mm from the material in the vicinity, to illustrate this effect.

When studied at the 15mm distance, the antenna impedance is generally robust to any detuning effect, except for the metal case in LoRa frequencies, where S₁₁ < -3dB. In this case, a retuning to improve S₁₁ values is possible using the virtual antenna technology to simply retune the matching network to optimize antenna performance (which is more difficult to attain with other antenna technologies where the operating frequencies may be determined by antenna geometry). The required bandwidths for all other cases are always covered completely with S₁₁ < -8dB and good antenna efficiency.

Images courtesy of Ignion Antennas

The recommendations to preserve good performance in these environments are as follows:

1. Increase the distance between the device and the material in the vicinity as much as possible. The recommended distance depends on the material. For all the scenarios above (wood, concrete, body phantom) except metal, a distance of 15mm is enough to guarantee very good performance in all bands.
2. In metallic environments, align the device as closely as possible to the edge of the metal (ideally with the antenna area protruding the metallic environment) while maximizing the distance between the device and the metal, with a minimum distance of 25mm. See Appendix A5: 10mm Placement Tests for Metal Case for images.

 

TRIO mXTEND^TM Conclusion

The conclusions extracted from the analysis above can be summarized as follows:

1. TRIO mXTEND^TM is currently the only antenna available in the market capable of handling three radio bands (LoRa, GNSS, and Wi-Fi) inside the same single and compact antenna package, thus reducing integration complexity.
2. TRIO mXTEND provides high performance in these three bands for the reference PCB size (90mm x 50mm). As a general rule, the bigger the PCB, the better the performance for LoRa channels.
3. The performance starts to degrade as the PCB shrinks, such as in the lower frequency bands where the board dimension is much lower than the wavelength, as happens with other conventional antenna solutions. The advantage of the TRIO mXTEND is that the detuning caused by PCB size reduction can be compensated by matching network adjustment, meaning that no customization of the antenna is needed. This adjustment is more difficult to attain with conventional antenna solutions where operating frequencies are determined by the antenna geometry.
4. The radiation patterns are omnidirectional in all cases, which is preferable for devices in constant movement, where the direction of the incoming waves is unknown.
5. The materials in the vicinity affect the radiation from two perspectives: antenna detuning and power absorption.
6. The TRIO mXTEND is robust in terms of detuning by the proximity of materials in the vicinity. The largest impact on detuning occurs in metallic environments, followed by human body interactions.
7. If a detuning occurs, it can be compensated by adjusting the matching network.
8. As a general rule, the larger the distance from the material in the vicinity, the better the performance. For the most critical case, the metal scenario, place the device as close as possible to the edge of the metallic section, at a minimum distance of 25 mm. Even better performance is expected if the antenna area protrudes the metallic area.