How to calculate the capacitor value for wire antenna tuning in a TPMS?

Kafei42

Kafei42

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I’m developing a TPMS transmitter, and we initially used a chip antenna. It performed well in field tests and even had a reasonable transmission through a metal container. However, when I installed it on a truck, the transmission became terrible.

After researching, I found that wire antennas are better suited for this type of application, so I’m testing a copper wire antenna, drilling through the A1 pad and adapting the PCB for it. During my research, I noticed that many circuits use a capacitor in series between the outer antenna end (which is usually left floating) and the ground plane.

My questions:​

  1. What is the technical name for this capacitor? Is there a specific designation for this component in this context?
  2. How do I correctly determine the capacitor value?
    • Should I measure S11 with a NanoVNA and adjust based on the reactance jX?
    • Are there general guidelines or typical values for 434 MHz?

Additional information:​

  • The RF trace includes a choke inductor and a DC-blocking capacitor near the MCU.
  • The PI circuit for the MCU has already been calculated so that the Z1 pad is at 50Ω.
  • I am not an RF engineer, but I have moderate knowledge of the subject.
  • I own a NanoVNA, and I know how to match impedance and tune the antenna based on measurements, but any additional help is appreciated!
  • I’m open to any other suggestions as well!
Any insights or reference materials would be greatly appreciated!
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My PCB
 
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Hello,

I don't know if I am right but here is what I think :

You can use an antenna which uses the metallic part of the container as a ground plane (monopole, IFA like) for better results

If you replace the antenna A1 by the wire, the capacitor can be a way to miniaturize the antenna (Scardelletti, M. C., Ponchak, G. E., Merritt, S., Minor, J. S., & Zorman, C. A. (2008, January). Electrically small folded slot antenna utilizing capacitive loaded slot lines).
If it is the case, you should measure the antenna at its terminal (port 1) and at the capacitor (port 2). You can add this S2P in a circuit and vary the capacitor value placed at port 2 to see its impact on the resonance. You can mathematically formulate this using characteristics modes analysis

Do you have references to the circuits where you saw this ?
 

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The capacitor will shift a bit the resonant frequency but will affect the efficiency of the monopole antenna.
Definitely the antenna efficiency will be better without capacitor.
Meanwhile, I think the circuit have too many matching components. Probably, two, maximum three, should be enough.
 

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