You mentioned that loop antenna's are very narrow banded and I'm wondering how much of the design can be easily reproduced by others.
I agree. Kind of hard to tune it without VNA. On the other hand once I have a working design I could see others build it based on exact specifications. Maybe I would have to laser-cut the cap tape so it is just right.
BTW, I measured a monopole on the same Tiny board yesterday, too. And it behaves very similarly when you make it very short. It's a well know fact of antenna design that small efficient antennas are very narrow band. To really make them work well they need be tuned.
Loops are less susceptible to the environment since they are magnetic antennas. Which is one of the reasons why I think you'd rather want a tuned loop than a tuned stub.
Comparing your design with common HF designs I noticed that your inner loop has a relatively large circumference.
Yeah, typically one uses 1/5th of the greater loop size. Now whether I have to adjust for the object inside the loop - I'm not clear. I think probably not though. I do think that having the mote inside the loop will affect efficiency significantly. The changing magnetic field will induce eddy currents in the ground plane and the coin cell. Which is why I would prefer to mount the pcb vertically - but even with cr1632 that's 1.6 cm of height, yuck.
A big question is whether it's better to have a smaller loop with pcb outside or the larger one as I've done it now. The upside of the current design is that you use every square mm of area to maximize loop size. But eddy currents are going to be larger inside because of the larger field strength. It will require more testing.
Also, the tuning capacitor would have to be tiiiiny and may already be detuned by changes in temperature or humidity.
Luckily plastic film capacitors are among the most temperature stable, so hopefully at least on that front we should be good.
Being fixed on one frequency brings a lot of disadvantages obviously: No frequency hopping, no switching bands for listen mode or OTA etc. There are now some low loss digitally tunable capacitors (
http://www.psemi.com/products/digitally-tunable-capacitors-dtc for example). But they are super expensive and still too high ESR for the low resistance resonant loop. So I just don't think they will work.
I also think these lastertrim caps are kind of cool (
http://www.johansontechnology.com/lasertrim-rf-tuning). Although probably a bit too involved for a hobby setup and frankly also not high q enough.
You can always add some resistance to make the antenna more broad band. I think that's what Felix has done in his design. Unfortunately you pay for it in efficiency.
I look forward to further posts as I've been toying around with making dipoles on pcb's just so that they're reproducable and on a rigid material. Oh, well.. back to 4NEC2.. :-)
Definitely report on your results here! I use a dipole on the gateway and it works great. But I've also thought about making it on a PCB for manufacturability and as you say reproducibility. The antenna will probably become much smaller due to the dielectric constant of the pcb.
Comparing your design with common HF designs
BTW, that's a key point: consensus is that small loops are inefficient. Yet it is a antenna that HAM's build with an efficiency approaching that of a dipole in HF. Radiation resistance is proportional to A^2 / l^4, so as long as proportions are the same relative to wavelength it should not change in UHF. Only problem is skin-effect and proximity effect. They do cause losses that grow with sqrt( f ). This is why I think a good sized copper tape has a better chance of success than a pcb trace.
Joe