WiFi ANtenna
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I was recently asked to fix the broken capacitor connection on this Raspberry PI-Zero. The fix was easy (at least it was once under a high magnification) , just folding ithe capacitor back on the track where it had been wrenched off, a dab of solder on the other side. Then a touch of adhesive to strengthen it. The 'PI now works fine, apparently. But I'm puzzled by that antenna - the triangular gap surrounded by grounding holes. The underside is free of copper, so it's some kind of two-dimensional cavity. Anyone know how it works? Don't know if it's 2.4 or 5.8GHz, or even both. That's a very thin microstrip track feeding it, the two capacitors are 0402, or perhaps even smaller - soldering needed a very fine tipped iron.  Andy |
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Colin G4EML
Hi Andy, I found this article. I don't know how valid his analysis is though. https://www.antenna-theory.com/design/raspberry-pi-antenna.php I also seem to remember that Raspberry Pi made a posting about it when it was released.i can't remember where that was though! Colin G4EML On 3 Feb 2023 10:03, Andy G4JNT <andy.g4jnt@...> wrote:
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That's neat. The whole groundplane forms the antenna and the little triangle is a matching network into it Andy On Fri, 3 Feb 2023 at 10:35, Colin G4EML <colin@...> wrote:
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Jules - M0UGA
Here's another page which includes radiation pattern tests and perhaps why this particular antenna solution was chosen.
https://embeddedcomputing.com/technology/analog-and-power/power-semiconductors-wireless-charging/a-lesson-in-wireless-engineering-from-the-raspberry-pi It doesn't detail why it works though ;-) |
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I did a bit of research on it. The antenna was licensed from ProAnt of Sweden. ProAnt calls it a notch antenna, their brand name is Niche. The descriptions referred to above mention the Vivaldi antenna, which is a notch antenna (also referred to as a tapered slot antenna). The video and associated paper description develop from a Vivaldi to an inductively loaded Vivaldi to a shorted Vivaldi. For me, the path around the triangle is quite short, and the spacing of the vias is quite close, so it's quite a stretch from a Vivaldi and the darned thing looks just plain shorted to me! In this article, the Pi designer says it's a cavity antenna, and the capacitors capture the radio signal: |
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Dave G8KHU
Link to the US patent application
https://patentimages.storage.googleapis.com/39/da/99/a6a16469f1c9ba/US10910715.pdf |
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Robert G8RPI
I agree with Jim. Even before I looked at other replies it was apparent to me that this was a cavity backed slot / notch antenna. These are used, on larger scale / lower frequency on aircraft. Typically built as part of the structure. |
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Marcus Walden
The measured impedance without a matching network looks remarkably like that of an electrically small loop antenna. I don't buy the Vivaldi/tapered-slot argument because these antennas are generally large compared to the wavelength at their highest frequency of operation. I agree with Andy that it's a way of exciting currents on the main PCB ground. If you model this structure, the majority surface currents align with the long dimension of the PCB. In other words, and considering the PCB length is about 6 cm, it behaves more like a half-wave dipole at 2.4 GHz. In academic papers and books, the buzzwords to look out for are 'characteristic mode analysis' (CMA). CMA can be used to identify effective ways to excite large structures as antennas (e.g. vehicles, aircraft, etc.). 73 Marcus G0IJZ
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Marcus Walden
I forgot to add the measured impedance taken from the earlier reference.
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