December 30, 2010
Assembly, C, C#
Sensors, applied RF, uCs
Aviation, Amateur radio, weather, astronomy
about 2 years ago
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.
about 4 years ago
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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