Antennas for Z-Wave
Discover the basics of Z-Wave antenna design, from antenna sorts and impedance matching to RF tuning and greatest practices for optimizing wi-fi efficiency, vary, and reliability in IoT units.
Introduction
The antenna is the most essential factor of a wi-fi IoT product. A nicely designed and tuned antenna means the product “simply works” in most clients’ residence. A poorly designed antenna that has not been tuned will lead to one-star evaluations and shortly get on the “merchandise to keep away from record”. Happily, there are loads of assets to assist engineer an antenna that matches the wants of your product. This submit goes over the numerous antenna choices and among the greatest practices to get the most effective RF efficiency.
Hub Antennas are Vital
Early in my Z-Wave consulting profession I assisted an organization who had not too long ago added Z-Wave to their Good House controller utilizing a
plug-in enlargement board. The tiny board with a 500 sequence Z-Wave chip had a 20-pin header to the motherboard. The issue with this plug-in board was that the antenna was positioned within the heart of the 8×6 inch motherboard. The antenna was additionally only a easy 7 mil hint across the fringe of the PCB with the bottom airplane simply a few millimeters away and had no impedance matching circuitry. This was about as unhealthy an antenna as I’ve seen. Their clients complained the controller had solely about 10 toes of RF vary which I shortly confirmed. The CEO of the corporate said that the RF vary didn’t matter since Z-Wave is a mesh so it would discover a path to all the opposite units within the residence. Whereas the mesh will assist, it does imply that at the least one system with an honest antenna was required to be positioned subsequent to the controller. The poor vary meant each command needed to be repeated, leading to latency and extra Z-Wave site visitors that tended to clog the community when many instructions have been despatched in sequence (for instance at dawn or sundown). My resolution was to flip the PCB round and place the antenna towards the sting of the plastic enclosure and enhance the scale of the bottom airplane of the plug-in board. As soon as I tuned the PCB antenna, the RF vary met the Z-Wave certification requirement of 40 meters and buyer complaints dropped dramatically. Sadly, the poor RF vary gave Z-Wave a nasty repute within the distributors neighborhood. The issue was their poor antenna design and never the Z-Wave protocol itself. Key take away: At all times design the perfect antenna doable particularly when designing a controller. See the House Assistant ZWA-2 for the most effective antenna design I’ve seen to date.
Physics of Sub-GHz Antennas
Z-Wave units typically use monopole antennas the place the bottom airplane supplies the reference. Z-Wave chips have separate RF out and in pins. The separation of the RF in/out supplies paths for filtering and impedance matching. The facility output of the RF Out pin is sort of excessive whereas the RF enter wants excessive sensitivity and is thus switched out at any time when the chip is transmitting. A set of passive parts really helpful by the chip vendor supplies impedance matching to 50 ohms and a few filtering. It’s important to implement these circuits as carefully as doable to the silicon distributors’ suggestions. At one level, the 2 alerts are on the desired 50-ohm impedance and are joined collectively after which linked to the antenna. The important thing metric of any antenna is its effectivity – how a lot of {the electrical} power is transformed into the electromagnetic waves transmitted into the air – normally denoted because the Q issue.
Identical to a guitar string, an antenna has a resonant frequency. The antenna is best at its resonant frequency, so you will need to tune it as shut as doable. The size of the antenna is given by the equation the velocity of sunshine divided by the frequency: wavelength=3×108/900MHz or 0.33m. Happily, for monopole antennas a ¼ wavelength is all that’s wanted which is 8.3 centimeters or 3.2 inches. That is simply the theoretical place to begin and including an enclosure, batteries, the PCB materials and close by atmosphere conspire to change the theoretical size of the antenna. Strategies for making the antenna even shorter are required as most IoT merchandise are a lot smaller than 3 inches on a aspect.
Antenna Varieties
I typically quote that even a paperclip soldered to the pin of a Z-Wave module will meet certification necessities of 100 meters of RF vary. Whereas I haven’t truly examined this, I’ve labored on so many antennas that it does appear it would work. There are lots of varieties of antennas and beneath is only a partial record. Do you’ve gotten a singular antenna? I’d wish to see it! Ship me an electronic mail with a photograph and the way far it could actually attain.
PCB
PCB antennas are the most typical Z-Wave antenna resulting from their low value and easy design. All you want is slightly further PCB house and to comply with a couple of easy design guidelines. PCB antennas embody easy L, meander, inverted F, fractal and greater than I can probably record right here.
The early Zensys builders equipment really helpful a easy L formed antenna proven right here from the datasheet. This antenna works nicely
and simply achieves the required efficiency for Z-Wave certification. The one problem with all PCB antennas is that the radiation sample shouldn’t be uniform and infrequently has nulls and achieve in sure orientations. Keep in mind that the regulatory necessities will determine the orientation with the strongest sign. If that orientation has antenna achieve, you might have to cut back the transmit energy to remain beneath the regulatory necessities. Decreasing the transmit energy will scale back the RF vary in all different instructions. Z-Wave certification will examine the RF vary in all 6 orientations. You could want to revamp your PCB antenna to make the radiation sample extra uniform. Antenna redesign generally is a time consuming and costly course of.
The Inverted F antenna is in style with 2.4GHz antennas because it has good efficiency and is small. However within the 900MHz
band the antenna is giant as proven right here. When you’ve got PCB actual property, it may be a very good possibility. The factor I’ve a tough time wrapping my head round is that the antenna is linked on to floor. Looks like that might quick it out, however as a substitute it anchors the resonant standing wave so it really works however must be fastidiously tuned.
Meander antennas are generally utilized in house constrained purposes. A meander antenna merely folds the wire forwards and backwards quite a lot of instances to compress the PCB house required. The Silicon Labs BRD4205 has an instance meander PCB antenna which is selectable by putting in a 47pF capacitor and eradicating the one for the SMA connector. The PCB proven right here is from a distant management that has a singular spiral antenna that has nice RF efficiency.
Wire
Wire antennas fall into two broad classes: versatile and stiff. Versatile antennas are just like the one proven
right here which, because the identify implies, is an easy stranded wire lower to a particular size. The wire is ideally held in a identified orientation utilizing plastic standoffs however there are various merchandise which might be stuffed right into a wall field with the antenna stuffed alongside all the opposite wires. The issue with versatile wires is that it’s unattainable to account for the impedance shift resulting from adjoining supplies. The size can also be onerous to constantly handle in manufacturing as even a one-millimeter variation can change the antenna efficiency. A easy wire is low-cost and works fairly nicely particularly for house constrained purposes.
The second class is a stiff wire or stamped metallic. The wire is often pre-cut and bent on the 
manufacturing facility with precision after which hand soldered in place. This ends in a repeatable and predictable atmosphere that means it’s doable to precisely tune the antenna for optimum efficiency. The opposite benefit is that the antenna is normally held vertically off the PCB floor airplane yielding a greater radiation sample than a PCB antenna. The antenna will be stamped metallic in any form permitting for extra flexibility in optimizing efficiency as proven right here.
Helical
Helical antennas will be fairly small however can have respectable efficiency if correctly tuned. The Z-Wave UZB USB stick controller makes use of
a small helical antenna as proven right here. The problem with making the antenna so small is to know that the bottom airplane can also be a part of the antenna. Thus, a tiny floor airplane reduces efficiency. The opposite drawback with USB sticks is that they’re typically plugged immediately right into a PC or a Raspberry Pi. A pc has many parts and oscillators that generate a big quantity of RF noise. As well as, the orientation could also be lower than optimum. The advice is to place the USB stick on a USB cable of at the least one foot in size. The cable isolates the USB stick from the noise from the pc and permits extra optimum orientation.
I made a prototype PCB with a form of helical antenna that used the two layers of the PCB and loads of vias. Nonetheless, it was troublesome to tune and didn’t have good efficiency. I consider the issue was the plated thru-hole vias have been vital impedance mismatches. Thus, I don’t advocate attempting to make a PCB helical antenna.
Chip
I personally haven’t had good expertise with chip antennas, however I’m conscious of firms which have used them efficiently. Chip 
antennas are sometimes the smallest antenna thus they’re typically utilized in very house constrained merchandise. The problem is that they’re impacted by the close by atmosphere. The antenna have to be fastidiously tuned with the enclosure, battery and mounting floor included within the tuning.
Exterior
Exterior antennas, typically referred to as whip antennas, are normally linked through an SMA or U.FL connector on the skin of the enclosure. Exterior antennas can yield the most effective efficiency as a result of the antenna is omnidirectional and has a donut
formed radiation sample leading to some achieve. They’re sometimes the most costly possibility and are usually solely utilized by the controller or by a tool that wants lengthy vary. Don’t be fooled into believing the declare that the antenna is 50-ohms and “tuned” in your chosen frequency. You should nonetheless tune it. I examined all of the antennas proven right here they usually range broadly in efficiency. The dearer antennas weren’t the most effective. In truth, one of many cheaper antennas practically matched the efficiency of the most costly. Nonetheless, one of many in style antennas turned out to be horrible. The House Assistant ZWA-2 makes use of an enormous screw-in antenna and has by the far the most effective RF vary of any system I’ve examined.
Module or SoC
The module variations of Z-Wave chips typically embody the impedance matching circuit within the module itself. Solely a easy PI community is required to tune the antenna. The SoC model requires extra matching circuitry however has the advantage of extra transmit energy and extra versatile filtering.
Impedance Matching Circuit
The output of the Z-Wave SoC chip is high-impedance however for optimum antenna efficiency, the impedance must be reworked 
to 50-ohms. Every silicon vendor has their very own really helpful circuit to match the chip to the 50-ohm antenna. Silicon Labs 800 sequence integration information (UG517) has the circuit proven right here. After this circuit, one other Pi network is used for filtering and tuning the antenna. The filtering is important to make sure regulatory necessities (FCC within the US) are met.
Tuning the Antenna
Tuning the antenna begins by taking a naked PCB, attaching a coax cable or SMA connector to the entry into the Pi community with a 47pF 
sequence capacitor. Calibrate a Vector Community Analyzer (VNA) to take away the parasitics of the cable/SMA. Then join the PCB to get the Smith chart of the antenna. Subsequent, use the device from everythingrf.com to design a Pi circuit that can tune the antenna to 50 ohms. The really helpful circuit is an IDEAL circuit utilizing very best units. Nonetheless, we stay in the true world and should use actual units which have parasitics that can skew the consequence. Select RF parts as shut because the really helpful values as doable and set up them one by one. I attempt to keep away from element values beneath 3pF or 3uH as different parasitics can detune the antenna with these small values. Measure the impedance at every step which ought to transfer alongside the trail proven however gained’t be actual. Strive totally different values till the VSWR is beneath 2.0, ideally beneath 1.0. When you’ve got a wire or PCB antenna you may shift the frequency by shortening the antenna however you may’t make it longer. I normally make a PCB antenna 10% longer than the calculated worth to permit for trimming throughout tuning. Reduce simply 1mm at a time as it could actually make fairly a distinction. Tuning is often a half-day effort of trial and error and generally entails a whole redesign of the antenna itself.
Can One Antenna Work for All Areas?
Sure. One antenna can work for all areas – from 868MHz to 920MHz. However you want an antenna with ample bandwidth to do it. For optimum efficiency, barely totally different Pi community element values are required for every finish of the spectrum. The advice is to optimally tune the antenna for the first goal market with maybe a shift in direction of a secondary market. For instance, if the first market is the US (908-920MHz) and the EU is secondary (868MHz), tune the antenna for 908MHz or barely decrease. Even when the antenna isn’t optimally tuned, it would nonetheless work if the bandwidth is broad sufficient.
Antenna Finest Practices
- No metallic inside 6mm of the antenna
- Floor airplane, batteries, enclosure
- Impedance matched and tuned for goal market with enclosure
- Together with typical set up floor inside 6 inches
- VSWR beneath 2 at goal frequency
- Preliminary PCB antennas are 10% longer than calculated to ease tuning
- Steady floor airplane(s) with array of vias
- RF alerts on a single layer and surrounded with floor vias
- Measure the radiation sample in all orientations
- Forestall robust achieve in a single orientation leading to failing regulatory testing
Subsequent Steps
Half 9 of the Z-Wave Developer’s Journey discusses the regulatory course of for subGHz units within the US. I’ll current the assessments it is best to carry out earlier than going to a take a look at home and the instruments you’ll want. As we proceed alongside the Z-Wave Developer’s Journey, I welcome your feedback and questions. Please be happy to achieve out to me immediately through electronic mail.
In regards to the Creator
Eric Ryherd has been on the forefront of Z-Wave innovation since 2003, starting as a marketing consultant and later serving as a Discipline Utility Engineer at Silicon Labs. Over the course of his profession, he has contributed to the design and growth of a variety of Z-Wave merchandise, together with sensors, distant controls, motorized window shades, and in-wall dimmers, lots of that are in the marketplace in the present day.
Though he “retired” in 2022, Eric stays deeply engaged in embedded programs and Z-Wave growth via his weblog, DrZWave.weblog, and ongoing IoT consulting tasks. He’s additionally a well-recognized face at Z-Wave Alliance Unplug Fests, the place he ceaselessly serves because the lead coordinator, supporting interoperability and developer collaboration.
