Virtual Antenna™ for easy design of IoT devices with embedded antennas: Page 6 of 8

September 20, 2019 //By J. Anguera, A. Andújar, C. Puente, Fractus Antennas, Barcelona, Spain
Anechoic chamber for testing Virtual Antenna™
Virtual Antenna™ technology enables one antenna component to cover all IoT frequency bands. This article describes the main features of Virtual Antenna™ technology and shows a simple design flow comprising only three simple steps.

#Step 3: The third step in the design flow is to test the device. Once the matching network has been implemented into the device’s PCB, VSWR and efficiency must be tested. VSWR can be tested with a vector network analyzer (VNA) providing information about VSWR (or S11). By measuring VSWR (see examples at Figure 5 and Figure 6), the wireless designer knows how well the matching network and the antenna booster behave. Usually the VSWR results of less than three are preferred across the frequency bands of operation. Compact VNA are available in the market for testing VSWR such as those offered by Rohde&Schwarz.

Once, the VSWR achieves a specified target, total efficiency must be tested which is carried out with the device inside an anechoic chamber (Figure 7). Total efficiency is the ratio between the power radiated into space (Prad) over the available power of the radiofrequency module (Pavs) – (Figure 4). Although VSWR measurement provides a good sense on how well then antenna system is behaving, total efficiency will inform on how much power from the module is radiated into space and how much is lost in nearby components, materials, and components of the matching network. Total efficiency is, therefore, a relevant figure of merit to be sure that the device will be competitive in the market. Moreover, when total efficiency is measured taking the full device parts (the PCB including the antenna booster, the matching network as well as battery, displays, casing, etc.) provides information about the TRP (Total Radiated Power) which is used in many wireless device certifications (eq. 1) such as PTCRB certification. TRP is linked to total efficiency as shown in eq.(1):

TRP(dBm) = Pout(dBm) + 10·log(total efficiency)       – eq(1)

where Pout is the nominal output power from the radiofrequency module.

For example, imagine we have a radiofrequency module with a nominal output power of 23 dBm and to certify the product, the TRP should be above 18 dBm, this means that the total efficiency must be above 31.6%.

Figure 7: Anechoic chamber for testing total efficiency - full 3D pattern is measured.

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