//Antennas in plastic devices, what could go wrong?

Antennas in plastic devices, what could go wrong?

Any wireless device needs antenna(s). The antenna can be internal or external. When it comes to internal antennas, an internal antenna can be implemented using an SMD (Surface Mounted Device) component or it can be designed on a PCB (Printed Circuit Board). Internal antennas can also be integrated with the device’s mechanics. For any off-the-shelf antennas, the antenna component manufacturer provides a test board indicating the environment where the antenna is designed to work. This information can be found from the reference design document.

This article analyses an off-the-shelf antenna component (SMD chip antenna), its performance in plastic surrounding and compares it with a custom PCB trace antenna. We will be using antenna simulation method in this study due to its cost-effectiveness and accuracy. If you are interested to know more about antenna simulations and simulation-driven approach, we have written a blog about this topic here.

The environment

Let us choose a common form factor for our sample device. Most of the IoT devices and many wearables are enclosed in plastic covers. But let’s take one step further and make the device so compact that the cover is touching the antenna. Size of the PCB and antenna GND clearance follow the test board of antenna component manufacturer.

Picture 1: Plastic coated

Performance

The first parameter to look at is Return Loss or VSWR. We have discussed different methods to measure this parameter in our earlier blog here. In Picture 2 below, 4 different results are presented. Let’s go through them one by one for clarification. The black curve “S1,1 Chipant No Plastic” shows the return loss without the plastic cover. This is the expected return loss that the antenna manufacturer intended. The antenna is designed to operate at 700MHz-960MHz frequency band and it has a healthy -3dB matching and a good -6dB efficiency for the majority of the range.

The Blue curve “S1,1 ChipAnt Plastic RefComp” shows what happens when the antenna is covered with a plastic surrounding, using reference matching component values. Antenna ends up tuned to a completely wrong frequency band of 550MHz to 700MHz. This behaviour is something antenna manufacturers do warn you about! You can not use the reference matching component values in your real product as they are dependent on surroundings.

Picture 2: Antenna measurement results and matching components

Let us introduce another measure to look at more deeply what happened. Picture 3 below shows total antenna efficiency. Introduction of the plastic surrounding changed a good chip antenna with -2dB efficiency into an antenna that has lower frequency at -15Db efficiency. The blue curve shows that the performance is tolerable when the tuning shifted to a new frequency band but that is not what this antenna is originally planned for.

What can be done to fix this? Antenna manufacturers have given the instructions in their technical data sheet or reference design board. You need to change the matching components. In this example, we will try two ways to do that.  First, we use the same component topology but only change the values of the components, and later we come up with a new component topology. Both are quite fast to accomplish with simulation tools but they might take several trials and errors with prototypes.

The pink curve “S1,1 ChipAnt Plastic RefComp Tuning” in Picture 2 shows what can be accomplished with the first approach. There is a clear improvement of 1,5 dB in matching level at 700MHz but no improvement at 960MHz. A similar result can be seen when we use a new matching component (Red curve). 1dB improvement at 700MHz but nothing at 960MHz which means that, when considering performance over the entire frequency band, the worst performance level can be beyond -15dB.

Picture 3: New matching component

Next, let’s have a look at a new matching component topology that was designed to optimise entire frequency band, the pink curve in Picture 3 “ChipAnt NewComp Opt”. Matching level is quite flat at -2dB but there is an improvement of 1dB at 960MHz which is a good improvement when compared to the previous cases. This matching level leads to about 1dB worse efficiency at 700MHz but clear improvement above 800MHz and more than 1dB at 960MHz. This means that the worst efficiency level over the 700-960MHz is about -14dB. Doesn’t this improvement feel quite small? You are absolutely right. Let’s dive into that now. We’ll introduce a solution to this problem.

Solution

The reason behind the poor performance of the antenna above was that something more fundamental than matching was wrong. The component itself was no longer functional due to the plastic surrounding. In product development, there are two options for how to move forward: either change the product form factor based on what antenna requires or customize the antenna. Here we are focusing on the second option.

Unless you are ordering a custom antenna component from an antenna manufacturer, usually the easiest solution is to start from the scratch. Generally, a PCB trace antenna can be designed for same space that requires for a chip antenna, but it is always a custom design and thus requires some more work.

Picture 4: PCB antenna model

Picture 5 shows how a PCB trace antenna (Green curve) that has been designed for the plastic environment outperforms the chip antenna reference component topology (Red curve). Efficiency is up to 20 times better at 960MHz. In fact, this performance level matches the ideal test board situation for the chip antenna without plastics.

Picture 5: PCB antenna performance

Conclusion

It is important to keep in mind that the antenna performance is dependent on its surrounding. A good antenna in free space does not guarantee the same performance level in your applications. Fine-tuning and matching are always needed. One way to ensure the wireless performance of your product and save in R&D costs is to test the performance with the simulation tool.

Radientum can help you to integrate antenna components from selected vendors using our simulations into your specific product. We can also develop custom solutions to PCB, FPC, LDS, etc. Read more about our services from here.

Disclaimer: The views and opinions expressed in this article are those of the author. It is intended only as a sharing of antenna design knowledge for educational purpose.

By | 2018-09-24T13:40:19+00:00 September 24th, 2018|Blog|

About the Author:

Olli is a Senior Antenna Specialist with more than 20 years of experience in antenna design in mobile devices. During his career, he has filed numerous patents and patent applications in antenna & RF field.