Simplest loop antenna? - Page 1
Forum section: electronics / rf-microwave

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: March 10, 2025, 10:23:50 pm
content: I'm designing a PCB with the Si4012https://www.silabs.com/documents/public/data-sheets/Si4012.pdf.It uses a loop antenna. I'm hoping to use the same PCB for both 315MHz and 433MHz with as little difference as possible.Could I achieve my goals with a wire antenna, cut to lengths and tuned for 315/433 (two different lengths depending on application), and possibly a single capacitor change?For reference, here is what one of their sample designs uses (PCB trace antenna)My impression is that for RF, 315/433 is pretty "low" frequency that should be decently forgiving? I've built a few designs already using a single (433) band that have worked well using a helical antenna and ground plane (ANT-433-HETH). I'd love to do a PCB trace antenna for this new design, but that would make experimentation much slower (and expensive).

author: A.Z.
author_rank: Super Contributor
timestamp: March 11, 2025, 09:01:00 am
content: https://www.changpuak.ch/electronics/Butterfly-Antenna-Designer.phpif more gain is needed, add a corner reflector

author: A.Z.
author_rank: Super Contributor
timestamp: March 11, 2025, 09:06:49 am
content: https://www.changpuak.ch/electronics/Butterfly-Antenna-Designer.phpif more gain is needed, add a corner reflector

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: March 11, 2025, 12:18:00 pm
content: Hmm that would be pretty sweet. Unfortunately the size needed for 433 even would be much larger than the PCB itself. Would there be other wide band antennas I could consider that would be a bit smaller?Does this app note influence anything?

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: March 13, 2025, 03:40:42 am
content: This chip expects to see a 600 ohm differential antenna load, your sample reference in the original post didn't work for me so maybe try attaching it to the post instead of hotlinking. But usually these are keyfob style transmitters that use a loop antenna much smaller than 1/4 wavelength, brought to resonance by a capacitor opposite the feedpoint. Think of it like a single turn inductor with a capacitor (LC tank circuit).Substituting a wire antenna cut to 1/4 wavelength will give you a 35-50 ohm, or a dipole will be 72 ohms. Either one is a pretty significant impedance mismatch so it will work poorly. Coupled to that is that these chips usually have a really ugly drive waveform. An LC tank only resonates at one frequency, so you can drive it with a square wave and "only" the fundamental gets out. But a 1/4 wavelength element is also resonant at 3/4 wavelength and so on. So your harmonics can be free to cause mischief.Quote from: joeyjoejoe on March 11, 2025, 12:18:00 pmYes, a termination resistorSize, bandwidth, efficiency. Pick 2.

author: A.Z.
author_rank: Super Contributor
timestamp: March 13, 2025, 08:44:01 am
content: https://www.silabs.com/documents/public/data-sheets/Si4010.pdfworth a check, maybe

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: March 13, 2025, 01:02:52 pm
content: Indeed, looks like Si4010 is a much more productive search term and I have found some open source designs for it including an antenna.From what I can see, perhaps I am better to aim for an external antenna, and impedance match 600ohm to 50ohm since the off the shelf (eg ANT-433-CW-RCS) are 50ohm?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 13, 2025, 01:53:15 pm
content: Loop antennas are tuned specifically to one frequency. They have very sharp frequency response. Any wide banding is a result of internal resistance in the circuit and is responsible for low efficiency.All loop antennas are really just LC circuits, as has been pointed out by Geoff.If you try to get two frequencies to work as you have written, the efficiency of the antenna will be horrible.The efficiency of a loop antenna is determined by the radiation resistance vs the internal resistance. The radiation resistance is VERY small and determined by the actual size area of the radiating element. Any internal resistance is to be eliminated as much as possible. In HF loops even soldering is not acceptable, any joints are usually welded or silver soldered.If you somehow convert the output of your transmitter from 600 to 50 ohms, there will also be losses associated with your impedance match.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: March 13, 2025, 02:21:00 pm
content: The Si4012 was chosen since it is crystal-less and supports both bands. Designs in the past I've used the MICRF112, but that requires different crystals depending on the TX frequency.I did findhttps://e2e.ti.com/support/wireless-connectivity/other-wireless-group/other-wireless/f/other-wireless-technologies-forum/270474/cc1101-design-that-can-operate-at-two-frequencies-315-and-433mhz-what-circuit-design-should-i-usewhich describes my goal - the author notes a decrease in range using the same design, with the proper design @ 433MHz hitting 250 metres, and the same board at 315MHz only reaching 70M. That would be acceptable in my case.I am a beginner (or lower) with RF. It sounds like I should first create a test PCB with just the Si4012 and components, and perhaps some different designs as well as a capacitor at the end. Not sure if empty matching (0R) components would be useful.I could also look at putting two antennas on the same PCB allowing for antenna selection depending on the freq.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: March 14, 2025, 12:35:30 am
content: Si4010 datasheet references the following app note for antenna design, which includes a single-ended 50 Ohm matching network:https://www.silabs.com/documents/public/application-notes/AN369.pdfThere are considerable difficulties taming harmonics with that approach, as expected. A narrowband matching network is the only practical solution which means a BoM change for each band.QuotePCB loop antennas are invariably undersized compared to the wavelength. The smaller you go (relative to the wavelength) the less efficient it gets. 315MHz is difficult to get good performance out of.QuoteAbsolutely, play around with it. I'd also recommend a NanoVNA and learning how to use it correctly. SimSmith/SimNEC is also a great program for visually seeing how impedance matching works.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: March 14, 2025, 01:39:43 am
content: I do have a nanoVNA actually! What's not clear to me is, can I use it for non 50-ohm testing? Can I create my own 500 ohm load for it with a resistor? (open and short loads remain unchanged)

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: March 14, 2025, 05:02:44 am
content: In theory, yes. In practice, probably not.A VNA will, to the best of its ability, use your definition of a load as the "perfect match" (centre of smith chart, and in S11 calculations). I've seen comment elsewhere that the NanoVNA might have the centre of the smith chart hard-coded to 50 ohms and not the load you calibrate it with - so be careful of that possibility eventuating. In practice there's a limit to how far away from 50 ohms you'll be able to push without getting misleading results.When making cal standards and measuring circuits you also need to consider how to transfer between the cal standard and the circuit without adding anything "else" to your setup. Everything has RLC parasitics, even very short lengths of wire will wreck the measurement.In this situation they've glossed over some details by saying "500 Ohms" - the transceiver output is not purely resistive it's reactive as well, so a perfect conjugate match also needs to match the reactive portion.Added to that, the 500 Ohms they've specified is also a differential impedance (both lines driven) and the NanoVNA is single-ended (signal vs "ground"). So it's not really 500 Ohms it's 2 lots of 250 Ohms.If you might want to sell this, you also need to care about harmonics so that it passes EMC certifications. Your calibration now has to be good up to a few GHz.It's all getting pretty hairy.The best way out of this is to use the example 50-Ohm matching network in AN369 Section 4. Follow the parts choices and the layout gerber exactly. They've done the hard work for you (and in this case, it is a lot of hard work). Then go forth and use your NanoVNA to design the antenna part starting with a 50 Ohm system.The fun thing about RF is that you can throw just about any antenna on any transmitter and it'll work a little bit. But making it workwellrequires a bunch of learning.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: March 14, 2025, 02:43:08 pm
content: I will probably dust off the W3AEW channel this weekend to review a few things as well.I'm torn between the 50-ohm match, which AN369 says is very narrow band, or yolo a PCB antenna. I suppose I can do both on the same PCB, with a 0-ohm jumper to try them both out!

author: A.Z.
author_rank: Super Contributor
timestamp: March 24, 2025, 11:46:46 am
content: Quote from: joeyjoejoe on March 11, 2025, 12:18:00 pm433MHz means a Lambda (wavelenght) of about 70cmwilling to reduce the antenna size, consider to use either a meander or a fractal antenna; performance will probably be a compromise but then it's the price to payexamplehttps://www.semanticscholar.org/paper/Miniaturized-meander-PCB-antenna-for-433MHz-Tripathi/de626cb4a3dbb84280d4243fa54f3fd187fc44b7/figure/0