An Evaluation of Multi-band Antennas for use with LoRa Edge™ [Part One]

Semtech’s LoRa Edge™ LR1110 integrates a LoRa® transceiver, a modem compatible with the LoRaWAN protocol, Semtech’s LoRa Cloud™ Device & Application Services, and a Wi-Fi b/g/n scanner and a hybrid GPS/Beidou scanner. Connected devices onboard the LR1110 must support at least three radio frequency bands:

Upper UHF bands (hereafter referred to as LoRa bands), where unlicensed LPWA connectivity is available—anywhere from 863 to 928MHz, depending on region.
GNSS bands: 1.575.42 MHz for the GPS L1 band and 1561.098MHz for the B1 Beidou band.
2400 to 2483.5MHz for the 802.11 Wi-Fi band.

The intention of this paper is to survey the available solutions for tri-band antennas. The industry, influenced by multi-band cellular technologies such as the last generation 4G smartphones, has developed multi-band antennas, and most of these concepts are applicable to our LoRa Edge platform.

In this document, we address the following:

Pros and cons of several antenna technologies
Common traps that are inherent to wireless product design, including the do’s and don’ts
Guidance for board size, matching requirements, and antenna placement as a means of reducing the risk of poor product design
Maximizing the chances of running successful proofs-of-concept
Optimized reference designs, including performance metrics

This is the first in a series of papers providing experimental results of the commercial, off-the-shelf (COTS) multi-band antennas we have analyzed, and is a starting point for companies wanting to integrate the LR1110 without needing to create an expensive study regarding custom antennas.

Overall Methodology

The antenna manufacturer websites offer baseline information for their antenna products: VSWR, gain plots, and efficiency, all measured in a specific test conditions. Most of the time the test board is specified, showing the antenna feed point and the board size used during the experiment. Because trackers may be size-constrained, highlighting the reasons why the antenna may not behave as specified is critical: for instance, when the antenna is “loaded” by specific materials in its proximity, or when it is de-tuned by a counterpoise of the wrong size.

During this study, the guiding principles were to explain how to:

Select an antenna based on its generic characteristics, such as size and efficiency. Of course, the focus is on tri-band antennas (wherever possible), since the LR1110 is optimized to provide radio functions across three bands.
Simulate or evaluate the performance that an antenna is expected to have for a given set of test cases. This test scenario can be re-produced for many antennas and compared easily to highlight the benefits of the various models.
Measure the antenna performance dependency on board size; given that some use-cases call for extremely small, highly integrated solutions. Our intention here is to re-iterate that the laws of physics demand a certain board size to maintain a correct efficiency, and the corollary that antenna tuning also depends on board size.
Estimate the detuning sensitivity of the antenna. Pet trackers work in close proximity to animal tissue, whilst cold-chain monitoring trackers operate close to metal, which has a substantial effect on the antenna performance.

In future papers, passive antenna tests will be augmented with active antenna tests.

The goal is to provide guidance for antenna selection, and to highlight the pitfalls related to sizing and detuning, which are common mistakes that can lead to underwhelming performance observed in the field or during qualification.

Why Should Antennas be as Efficient as Possible?

The bands in which LoRa operates, mostly between 863 and 928MHz, are employed around the world for unlicensed radio devices: garage-door openers, remote controls, and domestic alarm systems, to name a few. With the strong interest generated by the LPWAN market opportunity, traditional cellular network carriers have invested in nation-wide deployments, installing gateways with an average density of anywhere between one gateway for ~1000 km2 (rural deployment), to one gateway for ~5 km2 (dense urban deployment), and will densify networks in the coming years to absorb more traffic.

When rolling out a network, carriers use traditional propagation models and tools to simulate the coverage, making general assumptions such as:

Height of the gateways (Base Stations in cellular terminology)
Radiated power and sensitivity of the gateway
Effective radiated power of the connected IOT device
Radiated sensitivity of the IOT device

The first two elements in the list above are well controlled: gateways are industrial-grade equipment that can cost anything from a few hundred to a few thousand dollars, with well-placed and high-quality antennas strapped to a fixed point on a roof-top or on a telecom tower. The propagation models, although imperfect, benefit from decades of experience and refining from the cellular and pager industries, and are reliable.

More importantly, to guarantee coverage, carriers make an assumption about the actual power that the connected device radiates towards the network. In Europe, the effective radiated power (e.r.p.) is legally limited to 25 mW, and Telecomm operators will guarantee a certain coverage of territory assuming that the actual power going out of the objects will be 25 mW. Without a proper (device and) antenna design, the coverage maps and SLAs become irrelevant. This is why it is paramount to get the LoRa antenna design correct from the start.

For the assisted GNSS scanner onboard Semtech’s LoRa Edge products, the performance of the GNSS antenna is equally important. Our assisted GNSS scanner renders a sensitivity of approximately ‑141dBm. The US military guarantees that, at any spot on the planet, the GPS signal will be greater than ‑130dBm (https://www.gps.gov/technical/icwg/IS-GPS-200L.pdf). This represents some margin, but with IOT trackers being size-constrained, not necessarily well oriented, and typically without circular-polarized antennas facing the sky, every percent of efficiency of the GNSS antenna is important.

Wi-Fi band efficiency is somewhat less critical; with many Wi-Fi access points (AP) usually available in densely populated areas. Nonetheless, having a well-designed antenna ensures that as many access points as possible are being observed.