Critique my chip antenna layout - Page 1
Forum section: electronics / rf-microwave

author: electroro
author_rank: Newbie
timestamp: August 02, 2021, 04:09:22 pm
content: Hey folks,I am a beginner in RF and designing my first board with an antenna on it.I have referred the datasheet as much as i could.It is a 4-layer board with signal(red)-GND(yellow)-VDD(pink)-signal(green) stackup. The transmission line is 0.34mm (50 ohm impedance)Would love to get your opinions on the design.I am using a small molex 2.4GHz ceramic chip antenna due to space and availability constraint.The IC is a NRF52840.datasheethttps://www.molex.com/pdm_docs/as/2030060001-AS.pdfhttps://www.molex.com/webdocs/datasheets/pdf/en-us/2030060001_ANTENNAS.pdf

author: E Kafeman
author_rank: Regular Contributor
timestamp: August 04, 2021, 06:15:34 am
content: Real opinions about RF design is not possible as it must include a more complete view of the project, PCB, enclosure and knowing RF functional goal.Selected type of antenna is basically a monopole, where second pole is mirrored in ground. Ground shape and size and closeby lossy material is therefor a part of the antenna.Second part of antenna part is a small chip antenna. This antenna is very critical in its ground and tuning for its performance and careful tuning is important to get good performance. A simple wire, 30 mm long will often be a decent monopole antenna in any environment and without tuning as it naturally will have a impedance not to long way of from wished 50 Ohm. These small chip-antennas have without tuning a very poor impedance and tuning need to be rather precise as they tend to be a bit narrowband. It depend on your need, is it long range or is distance not critical. If distance is within armlength do it probably work with or without tuning.Your transmission line is short relative wavelength and its impact on performance is in this case less important but as information, transmission line should be matched with its end terminations is the basic idea to avoid reflection losses. Your antenna impedance is not 50 Ohm where it is connected the transmission line so in this case will transmission losses increase if its characteristic impedance is mismatched which it also is in opposite end, against the tuning components. Notice also that in your linked pdf is matching components placed very close to antenna.Actual losses in transmission line is probably not important but it impacts value of matching network components due to change in phase relative antenna port. Is good RF coverage important must tuning be done with aid of a VNA. Can you live with less optimal performance is it probably good enough as it is.Regarding matching shouldn't NRF52840 have a T-network for tuning its transceiver for best performance? Can not see these components in your design. Two serial matching networks, one for the radio and one for the antenna can of course be combined to reduce number of components and reduce component related losses.If battery consumption also is an important parameter, with more effective antenna feeding can you get same coverage area using less RF power. RF power is programmable for your selected chip or look on it from another view, with increased antenna space can battery space be reduced at no increased total needed space and no loss in performance and it will have less battery cost, or need to be charged less often.Tune the antenna and it will probably be good enough for your needs.

author: MarkF
author_rank: Super Contributor
timestamp: August 04, 2021, 10:20:31 am
content: Question:Would not the PCB antenna on the nRF24L01+ modules work in this case? And easier to implement.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: August 04, 2021, 10:40:10 am
content: Quote from: MarkF on August 04, 2021, 10:20:31 amDesigning a (good) on-PCB antenna requires simulating it which in turn requires EM simulator tools. It's also going to be bigger - this ceramic packaged antenna will make use of the higher dielectric constant inside the packaging material to shrink the actual size