I hope I didn’t make a stupid mistake somewhere, but it all seems to hang together… The helical antenna seems like a recipe to be guaranteed something mediocre. So really what’s going on is that each time by tuning the monopole I end up with a half-wave dipole that, in some cases, is fed off-center. Poor vswr, awful impedance… if these simulations are really representative of reality, it’s eye opening… Summary I found that very difficult to tune, small changes in almost any dimension throw it way off. I started with a dipole (dipole structure along the z axis on the left, radiation pattern on the right): After spending hours trying to figure out how some of these NEC2 front-ends work here is what I found. So I figured I should try to see whether I can simulate the various options to determine how much I stand to gain in terms of TX/RX. (I guess that makes it really a dipole where one half contains the circuitry and battery.) From a physical design point of view, the whole thing can go into a 13mm tube and look quite neat. By putting them back-to-back I can get my quarter wavelength ground and have a quarter wavelength wire stick out at one end for the antenna. I did some brainstorming around the notion that the length of the PCB should be at least a quarter wavelength for good RF performance and came up with an alternate design, which is to use a AAA LiFePO4 battery (41mm long) plus a long and skinny PCB (about 12x40mm). I’m working a small LoRa temperature/humidity sensor and started out with a PCB the size of a CR2032 holder, approx 35mm x 20mm and was wondering what to do about the antenna… I’ve been toying around a bit with small PCBs and small antennas as a follow-on to the discussion in Ceramic Antennas vs Stamped Metal Antenna.
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