What is the correct antenna matching network for 915 MHz?

[Adam Haun]

The xDot Developer Guide has two contradictory reference circuits for antenna matching at 915 MHz.

The first circuit, from the Antenna Matching Network section on page 33, shows a pi network made up of R30, L1, C34, C23, and C24. R30, L1, and C24 are labeled “OMIT”. There’s also C26, which is connected from the antenna to ground, forming another pi network. This network consists of a 7.5 pF capacitor on the left leg, a 6.8 pF capacitor in the middle, and an 82 pF capacitor on the right leg.

The second circuit, from the developer board schematics on page 52, shows R30 as a zero-ohm resistor shorting out L1 and C24. This results in three parallel capacitors with a combined capacitance of 102 pF, since C23 and C24 are 10 pF in this version.

What is the actual matching network we need? Our current board has just an 82 pF capacitor connected between ANT1 and ground, which passes FCC testing with a U.FL antenna. Is this okay? We’re reusing the design on a new board and I want to make sure there aren’t any hidden problems.

Thanks,
-Adam

[Steve Kovarik]

Hello Adam

The actual matching network you need is C22(4.7pf) and C26(82pf).
It would be our recommendation to add C22(4.7pf) to your design.

-Best Regards

[Adam Haun]

Steve,

That’s for the chip antenna on the developer board, right? On our board, we’re using a Laird TRA9023 antenna (datasheet here)connected to the U.FL connector on the xDot. It’s a 50 ohm antenna, so do we even need a matching network?

Thanks,
-Adam

[Steve Kovarik]

Hi Adam

You are correct, if an external antenna is used interfaced to the U.FL
connector of the xDot, no antenna matching network is needed. It is
recommended to remove C23 to remove the chip antenna from the circuit.

-Best Regards

[Author]

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