Designing and optimizing an Antenna is not an easy task. If you want to build an industrial IoT Antenna, you can use a reference design as a starting point, but what can you change to optimize the design when you want to implement it into your final product?

You might think that an experienced designer can easily design and optimize an Antenna. After all, they are experts who have been doing this for a while and have learned over a period of time from failures they saw or made happen. But technology standards are improving quickly, and young engineers do not have the time to make mistakes. They also need to design “first time right” in a shorter time to market. The solution is EDA software, in which the electronic design is automated to a certain extent.

The goal of this industrial IoT project was to integrate a Bluetooth Antenna into the product to remove the display and configure the device with an app on a smartphone. This is a common task and can be used for a variety of products.

We searched the web and found a Bluetooth reference design from Cypress Semiconductor. If you read the spec, it says that this design is just for reference and should not be used as a module in products. It is designed to work in the lab, and you can develop your software before your own PCB is designed.

The design data for the schematic, BOM and PCB layout was available in Cadence Allegro format, so we used it as a starting point for our optimization. The reference design has large connectors to connect in the lab.

Miniaturization and optimized Antenna performance were the two goals of the project. Miniaturization of the form factor can be achieved when connectors are replaced with a Rigid flex pcb. But does this have an impact on the Antenna performance? We made a series of “what if” analyses to understand the impact of these changes and come up with the right strategy to achieve both goals.

The import and setup of the design data in Microwave Office was easy. The ports were set up in an automated manner and we had to enter a few parameters for the mesh size. We entered the real material values for our production PCB with the right thickness and dielectric values (FR-4: standard Isola 370HR, ε_r = 4,0).

In the first analysis, we looked at the length of the meandered inverted-F Antenna and the Impedance-matching network between the chip and the Antenna structure.

A sweep through a combination of various Antenna lengths provided us with the right length for the Antenna return loss at our desired frequency of 2.45 GHz.

Return loss of different Antenna lengths (Source: AWR Microwave Office)

Another automated sweep of many analyses with a combination of available discrete components in our part library improved the performance of the Impedance-matching network in combination with our values for the PCB material we want to use. Just these few analyses would result in an Antenna performance improvement of 2 dB. But we have not yet investigated miniaturization.

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Automatic optimization Impedance-matching network

Antenna miniaturization is a problem because Antennas need enough ground (GND) to perform. If you reduce the form factor, the ground plane in the PCB will get smaller. The ground system in the reference design consists of two planes, several vias and an external cable connected to GND.

We analyzed the little piece of the PCB, where the Antenna and circuit are located. As a result, we found that the ground in the rigid area would not be enough for the Antenna to perform as desired, so we need to provide additional ground area on the flexible part of the PCB. If this is implemented as a solid plane on the flex part, it would be easy and quick in simulations. But if you apply a hatched structure to allow bending without breaking the copper, the mesh size and simulation time will increase.

But a comparison between various plane shapes and sizes showed how much we could simplify the structure for our “what if” analysis without increasing errors, although the final simulation should always be done as accurately as possible.

Another sweep analysis investigated the minimum distance between the product housing made of plastic and the Antenna. In our case, we saw that after a spacing bigger than 10 mm, the Antenna will no longer change its behavior.

The next question was how big the impact is if we change the shape of the flex part by designing different shapes in 2D or by folding and bending in 3D. After the mounting location in the product was confirmed and the length and shape of the flex part were determined, the next question was how the rigid piece of the PCB would be mounted.

Another sweep analysis of screw locations made clear that the best mechanical location for a screw would have a very negative influence on the Antenna performance. Looking for alternative mounting possibilities, a snap-in solution was preferred. This would require cutouts in the PCB.

Current-density distribution of modified PCB shapes

Visualization of current density in the PCB showed that cutouts with 90° corners or mounting holes will have high currents in the corners, which will lead to EMI issues. Several changes were made to minimize the EMI issues.

In the end, we could reduce the size to 53% of the original size of the reference design, the bandwidth was wider and we could increase the PCB Antenna performance by 6 dB.

Our takeaway was that Antenna design becomes critical if you miniaturize it, and you have to pay attention to mechanical mounting constraints like mounting holes, spacing and material of housing and EMI issues. Finally, the design was “first time right” and the time to market was shorter compared with producing only one extra prototype redesign.

RF project of Nordcad, Rohde & Schwarz and FlowCAD: power amplifier and Antenna RF design flow, from design to measurement verification

Nordcad designed together with FlowCAD a state-of-the-art Antenna amplifier and an inverted-F Antenna module. Schematic entry and PCB layout were done with Cadence’s Allegro pcb designer.

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For RF simulation of the pcb design data, the engineers loaded all parts (Antenna amplifier, Antenna design and Impedance-matching network) into Cadence’s AWR Microwave Office.

Components have been optimized and Impedance finally matched. With Rohde & Schwarz’s Vector Network Analyzer, the manufactured physical design has been measured and compared with the simulation results.

This great example of collaboration and teamwork of expertise proves that theory, simulation and measurement correlate very well and enable efficient and predictive RF designs.