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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