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Imaginary impedance track for adapting antennas.

oscargomezf , 03-24-2022, 01:10 PM
Hi Robert,

I have a tricky question related to impedance track for antennas.

In the company I work we usually deal with adapting antennas for: NB-IoT, LTE-M, LoRa, Sigfox and 2G. In order to get a good adaptation between the module communication and the antenna, it is very important to achieve a track with 50 Ohms. First, we use a free SW as TX Line to get theoretical track dimensions according to a PCB stack-up.

Therefore, the first task I do before getting a new PCB is to check the track impedance. The main goal is to be able to get 50 Omhs +/-10%. But sometimes, I've got some extra complex impedance, like this one:

Z (870 MHz) = 51.05 + 4.96 j
Z (966 MHz) = 51.63 + 5.53 j
Z (1705.0 MHz) = 53.29 + 7.24 j
Z (1885.0 MHz) = 53.86 + 7.11 j

The real impedance is within the range 50 +/-10%, but in some case, I've got +7.24j.

Despite this imaginary impedance, I am able to adapt the antenna correctly, but I don't know if it is acceptable or not.

Do you know what role is going to play the imaginary impedance in the process of adapting the antenna?

Best regards.
robertferanec , 03-26-2022, 02:30 AM
@oscargomezf this is not my area. Maybe someone else can answer?
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