Member Since: December 30, 2010

Country: United States

  • Yes, I believe the RFM12B port is designed for ~50 ohm unbalanced feed, so a ¼-wave whip with proper ground plane or other antenna with impedance close to that should work best. Unfortunately, most users don’t provide a real > ¼-wave radius ground plane and just use the tiny PCB resulting in poor radiation efficiency. In such cases a simple 2 x ¼-wave dipole should work noticeably better, as you’ve observed. The free-space impedance of a thin-wire (relative to wavelength) dipole is ~73 ohms, but in practice will be closer to ~60 ohms at 433MHz with real wire, and so provides a fair match, though with the balun in place you’ll still have a balanced ant connected to an unbalanced port. Don’t use a folded dipole without external balun/xformer, though, as their nominal impedance is ~300 ohm.

    The ARRL Antenna Book is the bible for applied antenna design and I can’t recommend it too highly for those interested in learning about antennas and transmission lines as these issues are thoroughly covered.

  • Looks like the RFM12B module does, in fact, include a balun for converting the Si4421's balanced high impedance port to 50-ohm unbalanced. The "Differential antenna input" bullet in the description is inaccurate for the module as it only applies to the chip.

  • Has anyone figured out what the nominal impedance of the antenna should be for maximum range? The Si4421 data sheet suggests it's ~250Ω balanced, but I wasn't clear on whether Hope might've included a balun to 50Ω unbalanced. The single pad labeled "ANT" suggests maybe so? If not, I suppose a 300ohm folded dipole should match nicely.

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