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Helical PCB Antenna for 868MHz

ueihttam
Prodigy 20 points

Other Parts Discussed in Thread: CC110L

Hello,


I worked on CC110L with the helical PCB Antenna (Design Note DN038).

I would like to redesign it to fit in a small board with a PCB thickness : 1.55mm.

In the Design Note, it is said that the length of the antenna is a main topic to match with the frequency. If I measure the length of the antenna in the DN, it is approximately 120mm. However, if I calculate the wavelength for 868MHz, it should be 8.63cm.

What is the total length I have to keep if I want to modify the value of H1 (page 3 in DN038) ?


Thank you for your help.

  • 0
    TI__Mastermind 34890 points

    Hi,

    The quarter-wave length for 868 MHz is around 8.6 cm. But the capacitive coupling between the tracks in the PCB helical design causes capacitive coupling which reduces the effective length. The exact length of the antenna is also determined by the size of the GND plane.

    As an example, the new CC-Antenna-DK2 kit uses this antenna and the length of the antenna was extended by an extra link. Then the natural resonance of the antenna was at 868 MHz without any matching components. If the GND plane would be smaller, then the antenna length would need to be increased.

    If you are making are PCB, then for the first prototype the antenna length can always be made too long and then reduced (cut). When measuring your antenna, make sure that it is loaded in the same manner as the final product. That is, display, plastics, batteries etc. Any loading of the antenna will shift the resonance frequency down and this will also effective the natural resonance (antenna length).

    Regards,

        Richard

  • 0 in reply to RGW
    Prodigy 20 points

    Hello Richard,


    Thank you very much for your answer which is very helpful.

    I understand the length of 8.6 cm is not for PCB antenna ; is it valid for a wire antenna ?

    If I understand you well, when you design a PCB with the helical antenna, you need to find the optimum length for the PCB. But how can you reduce the length of the PCB antenna ? Do we have to reduce 1 mm by 1 mm and test the link quality (RSSI ?) ? Do we have to reduce the antenna length by cutting properly the track ?

    Thank you for your help.

  • 0 in reply to ueihttam
    TI__Mastermind 34890 points

    Hi,

    The length of the antenna can always be reduced and compensated by extra inductance in the matching network.

    To measure the resonance of the antenna, measure the antenna with a network analyzer. Measuring the RSSI is not accurate enough.

    Regards,   Richard