You’ve probably seen old-time photos of radio setups with the station operator fiddling with the dials on the wireless radio setup, and also adjusting a large, wirewound (or copper tubing) coil. From the earliest days of wireless, it was understood that matching the transmitter’s output impedance to the antenna’s input as load was needed for efficient power transfer. The principles behind VSWR (voltage standing wave ratio) soon became well established, and it became a general guideline that a VSWR reading below 2:1 was a good operating objective, although 1:1 would be the desirable ideal.
Fast-forward to the 21st century, and VSWR hasn’t gone away. Instead, it now appears in new guises and with new challenges. The original VSWR setup “back in the day” was usually a static issue of matching the unknown PA output to either an antenna which was property tuned and resonant for the band in use, or to deal with an antenna that was less than optimum. In contrast, today’s matching problem is often dynamic and related to mobile or random-like use (smartphones, wireless IoT devices, and more). Unfortunately, antenna and matching performance has gotten worse, as the article “Reversing 25 Years of Antenna Degradation” makes clear.
There are several drivers of this dynamic scenario. First, even a well-matched antenna in a device such as smartphone is detuned by the user’s hands, body, and nearby objects, all which are constantly changing in use. Second, the device may have to handle multiple bands which a single antenna can’t cover, so multiple antenas are need, and there simply isn’t room for a properly resonant antenna for each of these bands. That’s why there’s a need to dynamically match the PA to the antenna. Finally, as devices get smaller and more is packed into the device (larger screen, for example), there isn’t room for a decent antenna. Tuning via changing of matching element can be done using silicon switches, but their overall RF performance is only adequate, especially at higher frequencies; and while all-mechanical switches have excellent RF specs, they are obviously too large, slow, and costly for portable devices.
Once again, though, MEMS technology and materials science is offering some innovative solutions. There’s something almost other-worldly about how humble, abundant silicon can become the solution to so many electronic, mechanical and electromechanical problems. What started out as a mass-market technology for triggering air-bags and providing accelerometers has not only conquered those markets, but is moving on to new ones, such as gyroscopes and more.
Recent introductions from different vendors are now providing true mechanical switching but in IC format. Using these switches, the system can implement dynamic autotuning in one of two ways. It can switch in different matching elements for a given antenna, striving to keep VSWR as low as possible, or it can change the configuration and size of the antenna itself by switching new radiating elements in and out of the array.
There are tradeoffs to each approach, as in most system designs. Regardless of approach chosen, each needs one critical function: the ability to measure VSWR, usually done via a directional coupler which senses the back-flow of energy from the antenna to the PA. So that’s another component that has to be added, but may be worth the cost is performance, space, power and other attributes.
A few recent examples show what is available:
- Wispry has digital software-controllable (tunable) MEMS device, the WS1040, which contains four individually controllable series capacitors, each digitally adjustable from 0.3 to 2.9 pF
- Analog Devices recently introduced the ADGM1004, a MEMS-based single-pole, four-throw (SP4T) electromechanical switch for operation from 0 Hz (DC) to 13 GHz, Figure 1.
The ADGM1004 from Analog Devices is silicon based and provides true mechanical switching of wideband RF signals, thus avoiding the issues associated with all-electronic RF switching. (Image source: Analog Devices, Inc.)
MEMS technology is not the only approach. Among the other options are antenna tuners and tunable capacitors in the three families (STPTIC, STRAFT, and STHVDAC) from STMicroelectronics based on a ferroelectric material composed of barium strontium and titanate (BST), Figure 2:
STMicroelectronics’ families of ferroelectric-based devices includes tunable circuits and capacitors as well as tuner modules. (Image source: STMicroelectronics)
Taken as a group, these MEMS-based and other solutions are providing designers with new options for optimizing matching and minimizing VSWR in an increasingly hostile antenna environment.
What’s been your exposure to, or experience with, dynamic antenna tuning?