PCB antenna trace impedance
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Bob Kruijer 1 week ago
I have been using these 2.4ghz transmitter previously in my arduino projects, with good success. One of my latest PCB design incorporates the schematic of the chip used on these boards. It is working but I've narrowed an issue down to the pcb antenna, and the manufacturing of that. Basically I'm having intermittent signal loss and other weak signal artefacts. My question is, knowing that certain factors of the radio signal and the 'ideal' antenna will depend on the final casing and other components, how do I make sure I can get the trace impedance to be 50 ohms? THis is required by the chip manufacturer. Assuming the microstrip impedance is the way to calculate this (I used this one [https://www.eeweb.com/tools/microstrip-impedance](https://www.eeweb.com/tools/microstrip-impedance)) for a 'standard' board, 1.6mm thickness, 1oz trace thickness and substrate dielectric of 4 (which I believe is FR4) I get a trace thickness of 3.2mm. However, the PCB on most of these off the shelf transmitters (they are around 1-1.1mm thick) have antenna traces of 1mm width. I am puzzled as to which is wrong here. Based on the info that I know, should I go with the 3.2 mm wide trace antenna? Or should I rather 'copy' the layout of these existing boards, even though I cannot know the trace thickness and thus my information to calculate their exact impedance is incomplete? ![Image result for nrf24l01+](https://www.makerfabs.com/image/cache/makerfabs/NRF24L01+%20Module/NRF24L01+%20Module_1-1000x750.JPG)
Comments
martin 1 week ago
On your design, how long is the trace from chip to antenna? If the trace to the antenna is less than roughly a tenth of the wavelength (in this instance, the trace would have to be less than 1.25 cm), if it not considered a transmission line, and its impedance is not a factor. Depending on your answer, it seems likely that your antenna isn't properly matched to 50 ohms. In order to properly match your antenna, as you pointed out yourself, you have to take into account a number of external factors, and it is imperative to place footprints for this match, usually in the form of a pi network. Then, you have to use a VNA to match the antenna. There are too many factors that impact an antenna match to think that it's possible to just plug in values for the filter based on another design. Personally, I wouldn't copy the design of these modules. I have not been impressed with their performance.
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Bob Kruijer 1 week ago
@martin thanks for your answer. I measured the trace length from antenna to chip and it comes to some 3.1 cm total. There is some components in between (that I believe make up the pi network) and those are connected by traces as well, so I just added them all up. Based on your comments, you think if I can get the antenna withing the 1.25 cm range of the chip I can mitigate the impedance factor? I understand there would still be other factors involved after that, but unfortunately I do not have access to an analyzer device that would allow me to tune it further :/ I'm just trying to get it good enough, and these off the shelf ones have worked for me before at least.
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andyfierman 1 week ago
@b.kruijer, When it comes to doing the PCB, JLCPCBs information about controlled impedances might be helpful: [https://jlcpcb.com/capabilities/Capabilities](https://jlcpcb.com/capabilities/Capabilities)
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martin 1 week ago
Bob, if you can shorten the trace, it's one less factor you have to deal with, so I would definitely do that (I do on my RF boards!). In terms of it being good enough, that is really hard to say. In my experience, tuning has been 100% necessary to get good performance. Trying to guess the right LC values for your filter is basically impossible. You might be able to borrow VNA time at a university, or you could do like I did and buy a "good enough" VNA if you're willing to spend a few hundred dollars. I got this guy: [https://xaxaxa-dev.com/xavna.cppsp](https://xaxaxa-dev.com/xavna.cppsp)
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martin 1 week ago
Oh, another thing I'd try: don't populate the filtering network. Random values are most likely making the problem worse.
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