Shockpad antenna considerations

Now that battery life has been addressed, it's time to consider two other key features of the product: the buttons and the antenna. Because the product IS a set of wireless buttons, these elements simply MUST be correct.

With the proliferation of wireless consumer devices lately, I'm assuming most readers have a rudimentary understanding of radio waves. RF energy propagates best through open air, with direct line of site from the transmitter to the receiver. In a home/office environment, wireless signals bounce around and generally reach their destination, albeit at reduced strength. And finally, radio waves fare particularly poorly inside metal boxes, underground, or in the water.

It's metal that I'm concerned about in this case. For both rugged performance and appealing looks, metal buttons would be an appealing choice for Shockpad. But, would the addition of these metal elements have a negative effect on the product's wireless performance? I set out to answer this question using Ansys HFSS simulation software.

First, I needed a simulation-ready antenna. Fortunately, Texas Instruments provides 100% of the native design files for their CC2541-based Advanced Remote Control (ARC) reference design.

Because Ansys HFSS cannot directly import Allegro design files, or gerbers for that matter, I needed to perform some file translation first. Using Allegro PCB Editor, I opened the provided PCB design file (*.brd) and exported the layers to a *.dxf drawing. I then imported the dxf layers into HFSS. Before/after images are shown below:

Before: Allegro view of ARC; an outline of the antenna is seen in the upper left.

After: HFSS view. Instead of replicating the entire ARC design, only the antenna structure was retained. A large uniform ground plane was created as a stand-in for the remainder of the ARC device.

As a baseline check, it's always good to confirm that the antenna is functioning as expected. To this end, I simulated the design, and produced an S11 plot.

Antenna S11, original length

This was not good. The antenna was functioning well, but at 3.1 GHz instead of 2.4 GHz! The antenna was too short. Digging deeper, I traced this unexpected performance back to the limited geometry which I chose to import. As shown below I was missing a small initial section of the antenna.

Satisfied that I understood the source of the discrepancy, I chose to fix it in a field-expedient fashion. My end-goal was to observe the effect of metal buttons upon an antenna, not to perfectly re-create this particular antenna. So, I simply increased the length of the antenna at its tip.

I iteratively added length, and checked the S11 plot, until I had the return loss centered at 2.4GHz. The optimal length addition was 6mm, and the corresponding S11 plot is below:

Now that the antenna is working at the desired frequency, let's examine its radiation pattern.

This image above shows us two things. First, the radiation pattern is fairly balanced; a few nulls exist, but nothing major. Second, note that the red regions exhibit positive gain of 3-5 dB. Now, I turned my attention to buttons. I enlarged circuit board (more FR-4 substrate and more copper ground plane) and then I created six stainless steel buttons. Each button was 20mm in diameter, 4mm thick, and hovered 3mm above the copper ground plane. The revised model, and the resulting radiation pattern are shown below.

It looks like those buttons really made a difference! Radiation in the +Y direction is visibly diminished. But, notice that buttons were not the only change since the prior results; I also extended the ground plane by 100mm in the +X direction. To investigate whether the change in pattern was due to the buttons or the ground plane, I removed the buttons and re-simulated.

Aha! Now, it's clear that the change in radiation pattern has more to do with the ground plane than the buttons. I hear the echoes of Rob and Kate, two excellent antenna engineers I worked with a few years back, saying "the whole thing is the antenna". If the antenna structure were dangling off a 3 story steel skyscraper, then the antenna would truly be the only thing radiating at 2.4 GHz. But, the thrust of modern embedded electronics is to be small. Consequently, the ratio of "ground plane" to "antenna" shrinks; as a result, both the ground plane and the antenna become radiating elements. If the ground plane is part of the antenna, rather than an idealized equi-potential plane, then it follows that geometric changes to the ground plane become relevant to the product's RF performance; for example, this F antenna required a 6mm lengthening to center its resonance at 2.4 GHz.

So, I returned to the simulation and made a few changes. First, I chopped a corner off the ground plane. My intuition was that the "tip" of the antenna should be in free space, not near other copper, because I want to send RF energy into the air, not back into the copper. Furthermore, this seemed to be the area in need of improvement because it is where the radiation pattern was dramatically reduced, as compared to the original simulation. With this many changes going on, I re-checked the S11 plot, and I found that the resonant frequency of the antenna had indeed shifted up to 2.6 GHz. This was just further confirmation that the ground plane plays a significant role in this antenna's performance. My previous 6mm addition was no longer adequate - I added another n mm for a total of m mm beyond the original baseline. As shown below, the revised model exhibits a much improved radiation pattern. It is more uniform, and eliminates the loss of performance in the +Y direction.

Now, with the ground plane "damage" repaired, let's check again to see if those buttons make a difference.

As seen above, the radiation pattern does not suffer noticeably with the addition of 6 stainless steel buttons. Stainless steel buttons may be used for Shockpad, without impacting RF performance, if they will improve the durability and/or aesthetic appeal of the product.

In conclusion, we've demonstrated the power of RF simulation software to both inform product design decisions and reduce prototype iterations. What-if scenarios can quickly be explored, and two alternatives can be compared side-by-side. For Shockpad, the stainless steel buttons remain a viable design option; if used, they will not negatively impact the RF performance.

Further thoughts:

Re-simulating this model, on a modest desktop machine, takes ~10 minutes at zero cost. In contrast, building new PCB prototypes could take 1-4 weeks at a cost hundreds or thousands of dollars. This also provides a practical example of why good antenna designers prefer to make their antenna extra long for first prototypes. On the computer, it's equally easy to add/subtract geometric length to achieve resonance at the desired frequency. But, on physical prototypes, only subtracting can be done easily and reliably. First, build the most accurate simulation possible, but then add a couple of mm for safety.

The power, and limitations, of reference designs are also demonstrated. Thanks to Texas Instruments, I was able to get off to a quick start by downloading their ARC design files. But, because I didn't use them exactly as provided, the antenna didn't perform as expected. I remain a big fan of reference designs, but their limitations need to be understood. Leveraging a reference design can shorten your design cycle, reduce errors, and increase your odds of success. But, the designer must still think critically and apply good engineering to produce a proper design.