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. 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 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). 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 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.
That all sounded great. 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. After spending hours trying to figure out how some of these NEC2 front-ends work here is what I found.
I started with a dipole (dipole structure along the z axis on the left, radiation pattern on the right):
The length of the dipole is 152mm and I’m focusing on 912Mhz. The simulated gain and impedance (71Ohm) look correct.
Long and skinny board with AAA
I then created a model for the long and skinny board:
I modeled the PCB as a grid of wires and then there’s the monopole sticking out at the center top. I picked an 80mm board length (really PCB + battery) and tuned the monopole length! Surprise… the overall length top to bottom comes out to 152mm.
The gain is a tad higher than for the dipole, the impedance dropped a little, vswr is a tad worse.
Now on to the CR2032 sized board:
You see how the board is much shorter. Well, for tuning, guess what, the monopole needs to be much longer to arrive again at 152mm total overall length.
Gain dropped a bit, vswr is better, impedance is around 55Ohm.
CR2032 board with helical antenna
Playing around a bit… This is the small CR2032 sized board with a small spiraled up antenna wire:
I found that very difficult to tune, small changes in almost any dimension throw it way off. Poor vswr, awful impedance… if these simulations are really representative of reality, it’s eye opening…
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. The helical antenna seems like a recipe to be guaranteed something mediocre.
I hope I didn’t make a stupid mistake somewhere, but it all seems to hang together…
I read in Wikipedia that “If a half-wave dipole is driven at a point other the center, then the feed point resistance will be higher.” but I don’t know where/how that would show up in these simulations.
I’m thinking of making a few physical mock-ups using some FR4 and/or copper tape and then measuring using my el-cheapo VNA, but before I go there, I’d love to get some feedback.
NB: is there any better simulator than NEC-2 that is affordable?