Comments on a 2.4 GHz PCB antenna + feed? - Page 1
Forum section: electronics / projects

author: Kremmen
author_rank: Super Contributor
timestamp: March 20, 2013, 06:27:43 am
content: Some days ago i wrote about an IR touch probe for a cnc mill. This time it is the same but phase 2, with radio as datalink to the receiver/controller. The protocol would be Zigbee over a 2.4Ghz PHY link, generated by an Atmel AT86RF231 transceiver. I would appreciate any comments you care to make regarding the RF feed to the PCB antenna, as well as the antenna dimensions. The antenna is an inverted-F or IFA, originally designed for a small USB dongle by TI. The original PCB is four layer, with layer thicknesses .25, .5, .25 mm for a total of 1 mm FR4. My design is for a 2 layer 1 mm FR4. Accordingly, the ground plane will be separated 1 mm instead of .25 mm. That would impact the dimensions of the antenna and feed, but exactly how much?Additionally, i haven't really "designed" the feed other than copy the recommended circuits from Atmel and Johanson datasheets. The attached image shows the transceiver, its RF interface and the feed components. The (relatively) bit footprints are 0603 DC filter caps, the 6 pad footprint is a Johanson balunhttp://media.digikey.com/pdf/Data%20Sheets/Johanson%20Technology/2450FB15L0001.pdf. The circuit topology conforms to the data sheets but any comments regarding the layout would be welcome. An additional issue is that basically the antenna is encased in a Faraday cage created by the touch probe body. There will be a slit cut through the sidewall for the antenna - a question would be what is the decent minimum size of that slit to get some signal out and in? The antenna doesn't need to be super efficient, it is enough to carry the signal a few meters because the receiver will be in the near vicinity in all cases. The receiver antenna can be a much more efficient design in both transmitting and reveiving, but that is another story.Feel free to suggest alternative antenna designs better suited for these conditions, as long as they can be fit into the area visible in the attached image. You get a rough idea of the area from the dimensions indicated.

author: jahonen
author_rank: Super Contributor
timestamp: March 20, 2013, 07:16:38 am
content: Well, any metal or alien dielectric even near the antenna will cause it to detune. I think that it is not practical to try to guess what the effect will be, or if the slot is enough to get acceptable signal strength. Most critical area is directly above or below the antenna (a plastic window there perhaps?). My only guess is that make the slot as large as possible.For line widths, if you want 50 ohm with microstrip configuration on FR4, you can get very near the correct line width by making the line width double to the FR4 dielectric thickness. For better estimation, try sonnet lite or similar EM-simulator (you could simulate the antenna too with it but that may be out of reach of the free version). So if your PCB dielectric thickness is 1 mm, then your 50 ohm microstrip is about 2 mm wide.Regards,Janne

author: Codemonkey
author_rank: Regular Contributor
timestamp: March 20, 2013, 07:39:12 am
content: One thing I'd add is a lot of ground plane stapling via's, especially all along the edges of the ground plane near the antenna and feed (its not clear if your image is showing all of your via's so apologies if you've already done this).

author: Kremmen
author_rank: Super Contributor
timestamp: March 20, 2013, 07:59:54 am
content: Quote from: Codemonkey on March 20, 2013, 07:39:12 amNo apologies needed, that is a good comment and a good catch - thanks. I should have included those in the image from which they are actually still missing. But yes, the intention is to do just that.Quote from: jahonen on March 20, 2013, 07:16:38 amYes, a plastic window was in the plans. The PCB will be horizontal so i assume "above" and "below" mean towards zenith and nadir. I will have to see then where the threshold of pain lies as far as window size is concerned.QuoteThanks, i will give Sonnet a go.Do you have a ballpark guess how the antenna dimensions should be adjusted? One thing that i am thinking about is that there is no ground plane under the radiator so is it just the amount of FR4 next to the conductor that impacts dimensions there, or is something else involved as well?

author: KJDS
author_rank: Super Contributor
timestamp: March 20, 2013, 08:22:28 am
content: There are plenty of accurate calculators available for transmission line widths. Txline is one example. Google will find you a copy.

author: RobB
author_rank: Contributor
timestamp: March 20, 2013, 08:28:04 am
content: With regard to the slot may I suggest you do a search on "slot antennas".If the slot is not tuned to length correctly the RF transfer will not be great if the slot is too small, There are plenty of online designs for WiFi slot antennas. Just pick up the principals of operation and you should be good to go.Also have you considered possible interferance from wireless networks. There is some usage of wireless networking to CNC machinery these days.Maybe even look into 5,8Ghz, tiny antennas which is good for your application and no likelyhood of interferance.

author: jahonen
author_rank: Super Contributor
timestamp: March 20, 2013, 08:50:00 am
content: Quote from: Kremmen on March 20, 2013, 07:59:54 amThat is my understanding too. As long as the dielectric is similar (same e_r) and dielectric thickness does not change significantly, then it should not affect the antenna operation very much. Increasing line width reduces losses there but that's about it.Also, now when I think it again, it might be possible to use the metallic enclosure itself as an antenna with a slot of suitable length:No PCB antenna at all in this case.Regards,Janne

author: Kremmen
author_rank: Super Contributor
timestamp: March 20, 2013, 10:39:49 am
content: The slot antenna would be an interesting alternative to consider. A question about the picture though: are you sure the connection should be asymmetrical (as in coax) and is the slot impedance really 50 ohms in the middle?What about the slot dimensions then? What is the primary factor affecting width/height of the slot? Wave propagation speed in steel?Sorry, question questions. I can try to find the answers myself as well, but if you/someone already knows, maybe this is the shorter route.

author: jahonen
author_rank: Super Contributor
timestamp: March 20, 2013, 10:47:55 am
content: Might be better to do it this way but at the middle (sorry for the large image):Also, the slot could be filled with some low-loss dielectric material which has beneficial effect, it makes the slot electrically larger so physically smaller slot would do.And as a side note, it is this slot antenna which is inadvertently formed when people route traces over a slot in a PCB ground planeRegards,Janne

author: nctnico
author_rank: Super Contributor
timestamp: March 20, 2013, 07:02:27 pm
content: Quote from: KJDS on March 20, 2013, 08:22:28 amActually they are all very wrong if you get far from 50 Ohm. Including Wcalc and the one you can download from Rogers (a RF PCB laminate manufacturor). A couple of weeks ago I wrote a program to calculate a particular type of RF filter and I had to implement my own transmission line calculator based on formulas from a book to get correct results.

author: KJDS
author_rank: Super Contributor
timestamp: March 20, 2013, 08:09:33 pm
content: Quote from: nctnico on March 20, 2013, 07:02:27 pmWhich formula did you end up using?

author: nctnico
author_rank: Super Contributor
timestamp: March 20, 2013, 08:23:48 pm
content: The ones described here:http://ntuemc.tw/index.php?node=microwave-filterIn lecture 3:http://ntuemc.tw/upload/file/2011021716275842131.pdfThey are actually from a book called:Jia-Sheng Hong, M. J. Lancaster, “Microstrip Filters for RF/Microwave Applications,” John Wiley & Sons, Ltd., 2001

author: KJDS
author_rank: Super Contributor
timestamp: March 20, 2013, 09:03:42 pm
content: Quote from: nctnico on March 20, 2013, 08:23:48 pmNow I'm cinfused, as those are Hammerstad and Jensens equations which are the ones used by TXline and the AWR equivalent.

author: nctnico
author_rank: Super Contributor
timestamp: March 21, 2013, 12:19:21 am
content: Quote from: KJDS on March 20, 2013, 09:03:42 pmI guess I didn't try that one

author: andyturk
author_rank: Frequent Contributor
timestamp: March 21, 2013, 02:41:21 am
content: Quote from: Kremmen on March 20, 2013, 06:27:43 amRF is way over my head, but I'm curious about the advantages of rolling one's own antenna for 2.4GHz designs. I've used a Panasonic 132X Bluetooth module (with a built-in antenna) in project and it seemed to work fine. Panasonic also has a 131X variety (used in the new Pebble smart watch) without the antenna.The cost of the two is within a few percent in quantity, so what's the decision criteria for designing a custom antenna or using the OEM?

author: Kremmen
author_rank: Super Contributor
timestamp: March 21, 2013, 05:21:36 am
content: Quote from: andyturk on March 21, 2013, 02:41:21 amSpace. Or the lack of it, actually. This application is embedded in a cnc touch probe where the available volume is a cylindrical cavity of 42 mm diameter and roughly 15 mm high. In that volume you need to pack everything, including a 110 mAh 3.7V LiPo battery. There just isn't room for a regular rod antenna, and the antenna would be in a more or less hopeless orientation as well. The PCB antenna radiates in the correct direction and the slot it needs is easy to protect against entry of dirt and swarf. So it isn't a cost question at all.

author: westfw
author_rank: Super Contributor
timestamp: March 21, 2013, 08:29:22 am
content: QuoteIt looks to me (as another non-RF person) that even if you use an external or packaged antenna, you wind up having to do RF black magic on the traces TO the antenna anyway :-((I don't suppose that there is a rule of thumb that says if the trace length TO the antenna is sufficiently short, then I don't have to worry about it?)

author: SeanB
author_rank: Super Contributor
timestamp: March 21, 2013, 08:48:22 am
content: At 2.4GHz the longest length of trace that is sufficiently short is no trace.

author: nctnico
author_rank: Super Contributor
timestamp: March 21, 2013, 10:38:40 am
content: As long as the traces have the proper impedance the length doesn't matter. The only thing to consider is that a long trace has more loss than a short one. If you have the right tools (RF generator and a spectrum analyser) it is not really difficult to get into RF designs. Nowadays you can simulate a lot (for example with Sonnet Lite which is free) so you can actually see where the current is flowing at certain frequencies.

author: Kremmen
author_rank: Super Contributor
timestamp: March 21, 2013, 11:53:36 am
content: Quote from: nctnico on March 21, 2013, 10:38:40 amYep, that was my question earlier and now i am in the process of creating a Sonnet model to get the feeder traces right. The antenna itself is a reference design so it should be right except for the thickness of the PCB. The effect there should be minor as far as i understand from jahonen's comments.

author: andyturk
author_rank: Frequent Contributor
timestamp: March 21, 2013, 11:44:36 pm
content: Quote from: nctnico on March 21, 2013, 10:38:40 amWhat if youdon'thave the right tools?I just stumbled across theTiWi-uB2 Bluetooth modulethat's half the price of the TI part I've used before. The TiWi doesn't have a built-in antenna, however. Is there a half-decent IFA footprint somewhere that I can slap onto a $10 OSHPARK 2-layer board and achieve RF bliss?I don't need a production quality design, just something that'll have enough oomph to transmit from one end of my desk to the other.

author: westfw
author_rank: Super Contributor
timestamp: March 22, 2013, 03:55:57 am
content: QuoteDo you care about legality?This was posted recently:http://dangerousprototypes.com/2013/02/24/texas-instruments-antenna-selection-quick-guide/It bothers me considerably that I can't get the online stripline/coplanar calculators to come up with the same numbers that TI comes up with in their App Notes, though :-(

author: andyturk
author_rank: Frequent Contributor
timestamp: March 22, 2013, 06:25:15 am
content: Quote from: westfw on March 22, 2013, 03:55:57 amAs a general principle, no.But I'm not sure what the issue is with the link you posted. Some sort of FCC thing?Those designs are very interesting. Thanks for posting the link!

author: freker
author_rank: Newbie
timestamp: March 22, 2013, 06:38:04 am
content: The antenna reference designs from TI LPRF are free to use for anybody.The meandered inverted F antenna (the one used by the OP) can often be tuned by simply reducing the antennna length. Putting in footprints for a series and a shunt component close to the antenna feed point is anyway smart. For proper tuning access to a network analyzer is of course required.You should also check out the TI E2E forums for tips and advice:http://www.ti.com/e2e-low-power-rf

author: tylerl
author_rank: Contributor
timestamp: March 22, 2013, 07:35:12 am
content: There's a great walk-through of designing a PCB trace antenna that is certainly well-worth the read:http://colinkarpfinger.com/blog/2010/the-dropouts-guide-to-antenna-design/QuoteHis approach is a "brute force" method which largely ignores the math and theory, and just gets down to results -- after all, even if you start with the math, you'll end up here anyway; you might as well start here.It's pretty much like this: you design your antenna with what you have available; datasheets, calculations, whatever. Obviously you want to get the shape and grounding and board layout right; but that's typically the easy part. But you make your antenna trace a bit longer than necessary.You print a few test board and solder on a SMA (or other) connector socket right where you antenna feed line begins, and connect it up to a spectrum analyzer (make friends with someone who has one). You compare the return loss against the desired frequency range.You can probably guess the next part; you start trimming down your antenna trace in small increments by scratching off the end bits of the trace with a very sharp blade. Once you find the spot where it peaks in your desired frequency range, you've found the optimal size and shape for your antenna. Measure and adjust your pcb layout accordingly.And done. And no intimidating math either.

author: westfw
author_rank: Super Contributor
timestamp: March 22, 2013, 07:40:30 am
content: Do the current crop of RF transmitter chips have any antenna diagnostics capabilities built in?The "legality" comment was because you can't just copy a design from an App Note and get a legal implementation, without doing a bunch of certification testing. (OTOH, they say "used with the same design rules and chip antenna, the design is pre-certified" on some designs...)

author: freker
author_rank: Newbie
timestamp: March 22, 2013, 02:27:36 pm
content: Quote from: westfw on March 22, 2013, 07:40:30 amI see. I would like to see the hobbyist who takes his one-off design through FCC and/or CE (ETSI) certification.

author: nctnico
author_rank: Super Contributor
timestamp: March 23, 2013, 01:31:41 pm
content: Quote from: westfw on March 22, 2013, 03:55:57 amHow far are the numbers off? The Er of FR4 can roughly vary between 4.3 and 4.8 which gives quite some spread in the width of a trace. The thickness of the PCB also determines the impedance of a trace. Did you try this one:http://wcalc.sourceforge.net/cgi-bin/microstrip.cgiIf works OK for 50 Ohm traces.

author: westfw
author_rank: Super Contributor
timestamp: March 23, 2013, 08:11:22 pm
content: Usinghttp://wcalc.sourceforge.net/cgi-bin/coplanar.cgito analyze the trace described in TI publication SWRU331 (CC3000 boosterPack eval board), I'm getting 61ohms instead of 50ohms for the same inputs.

author: jahonen
author_rank: Super Contributor
timestamp: March 24, 2013, 09:44:04 am
content: Using sonnet thick metal model (unfortunately not in lite version) I get about 51 ohms:Normal metal mode gives 58 ohms but that is to be expected when gap is small between the conductor and ground, so that coupling along sides become significant. It looks like that original calculations have been done using Polar Instruments Si8000 or something like that.Regards,Janne