Design & SimulationMarch 23, 2020

Fast and Easy Design of a Key Fob Antenna

Cabin Comfort Solution Problem Statement: Have you ever wondered how the doors…
Katie Corey

Cabin Comfort Solution

Problem Statement:

Have you ever wondered how the doors of your car unlock with just the click of a button, even at a distance of more than 30 feet? The remote keyless entry device, commonly known as “key fob,” is an engineering marvel that fits into the palm of your hand. Now try to imagine carrying a key fob the size of an iPad. That would be pretty inconvenient! This could have happened, if the designers of the key fob antenna gave up at an early stage in the design process.

A key fob operates at 434 MHz, and an antenna at such a low frequency is generally as big as an iPad. If we try to reduce the size, the frequency increases. And this is exactly the problem engineers were facing while designing an antenna that fits into a small 6 x 4 x 1 cm key fob casing for the CATIA Bleu car. For such small dimensions the antenna started resonating at around 3GHz. How did engineers fit an antenna into the small key fob casing while still meeting the lower frequency requirement?

  • Since Antenna Magus didn’t have an antenna that fulfilled the constraints, we designed a parametric patch antenna in CST Studio considering the physical constraints of the casing. Simulation revealed that the antenna resonated at a little above 3 GHz.
  • Next, we introduced meander lines into the patch to increase antenna length, which further reduced the operating frequency to around 2.8 GHz. However, this was still far from the required 434 MHz, but as low as we could get with the limited space.
  • We replaced the schematic matching circuit with lumped elements in the 3D geometry and re-simulated to verify the antenna resonated at 434 MHz.
  • The last step was to optimize antenna width to maximize radiated power.

And voila! It worked like a charm.

Thus, we have designed an antenna with the size constraints and resonating at a lower frequency of 434 MHz.

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