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PCB Antenna Impedence measurement setup

SaiTeja

Hello, I want to measure the impedence of PCB antenna designed for nRF52832 based board. I have the following quires.

  1. Is it required to isolate antenna from NRF52832 matching network to take measurement? My matching network contains shunt capacitor(0.8 pF) and series inductor(3.9 nH) as suggested in the datasheet .

2)Is it required to solder all the components on PCB board before taking impedance measurement?

3)How to decide whether the length of antenna should be increased or decreased to match the impedance to 50 ohm?

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    Hello SaiTeja

    1. Yes, it will be necessary to isolate the antenna, and its matching network if you're measuring from the 50 Ohm environment, from all other circuitry and PCB traces. To measure the antenna you should therefore cut the trace leading towards the nRF matching network. The cut should be made as close as physically possible to the point where you have connected the VNA.

    2. Preferably the PCB should be as complete as possible, and the test should be performed while the board is in an environment similar to where the final product will be used. Do keep in mind the antenna will be affected by pretty much any, and all, things in its immediate vicinity, so enclosures, hand-holding, obstructions, walls etc. should preferably be included where necessary.

    3. An antenna is a LC network, where L stems mainly from the length of the trace(longer traces have higher inductance), and C stems from the trace's relation to the ground plane (ground plane size in relation to antenna, spacing between antenna and ground plane). For the quarter wave monopole antenna, you want a length that ensures resonance at 2.45 GHz. The shorter the antenna is, the higher the frequency it will resonate for. Design the antenna a bit longer than what you need, this will make it resonate at a lower frequency than what you want. Then cut the antenna in steps until it resonates at the wanted frequency.

    Take a look at this whitepaper for more information on monopole antenna design and tuning.

    Best regards

    Jørn Frøysa

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  • 0

    Hello SaiTeja

    1. Yes, it will be necessary to isolate the antenna, and its matching network if you're measuring from the 50 Ohm environment, from all other circuitry and PCB traces. To measure the antenna you should therefore cut the trace leading towards the nRF matching network. The cut should be made as close as physically possible to the point where you have connected the VNA.

    2. Preferably the PCB should be as complete as possible, and the test should be performed while the board is in an environment similar to where the final product will be used. Do keep in mind the antenna will be affected by pretty much any, and all, things in its immediate vicinity, so enclosures, hand-holding, obstructions, walls etc. should preferably be included where necessary.

    3. An antenna is a LC network, where L stems mainly from the length of the trace(longer traces have higher inductance), and C stems from the trace's relation to the ground plane (ground plane size in relation to antenna, spacing between antenna and ground plane). For the quarter wave monopole antenna, you want a length that ensures resonance at 2.45 GHz. The shorter the antenna is, the higher the frequency it will resonate for. Design the antenna a bit longer than what you need, this will make it resonate at a lower frequency than what you want. Then cut the antenna in steps until it resonates at the wanted frequency.

    Take a look at this whitepaper for more information on monopole antenna design and tuning.

    Best regards

    Jørn Frøysa

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