Antenna tuning question - Page 1
Forum section: electronics / projects

author: uski
author_rank: Frequent Contributor
timestamp: January 24, 2018, 07:53:19 pm
content: Hi !I'm working on a small project with a 2.4GHz radio IC from Nordic Semiconductors.I have made prototypes and they generally work well. I'm now in the process of tuning the antenna to enhance the performance (and to learn how to do it ! because it's fun)The design uses a chip antenna, part number AMCA31-2R450G-S1G-T3 (DigiKey 535-14095-1-ND)This chip antenna is connected directly to the radio IC output, through a pi filter network (3 footprints, two shunt which are open, and one series which is a solder bridge).I acquired aminiVNA tiny from AliExpressto do this task, and I calibrated it with the provided loads/shunts/shorts.Initially (before tuning) here is my antenna smith chart plot from 2.35GHz to 2.45GHz (main freq : 2.425GHz) :As you can see the antenna is behaving like a short circuit, because it's totally on the left. Which is very surprising to me.In order to move the point toward the center (the tuning itself), I added a 47ohms series resistor (leaving the two shunt elements open on the tuning circuit).Which gave me this result :To move toward the center even more, I'd need a 2.7-3.3pF capacitor in series.My concern is that I have never seen any antenna tuning done with a series resistor, let alone such a large value. I am concerned the power will be radiated inside the resistor instead of being radiated by the antenna. Also, the antenna datasheet says it's "matched to 50ohms"... which looks like it's obviously not !This also does not match (pun not intended...) what is described in themanufacturer application notefor this antenna.Is my miniVNA f*cked up, is my process wrong, or am I misunderstanding something ? This is the first time I am doing this myself.Here is my measurement setup (with a 0201 47ohms resistor in the antenna path) :Thanks for any pointer.-- uski

author: metrologist
author_rank: Super Contributor
timestamp: January 24, 2018, 08:29:07 pm
content: what is the DC resistance of the antenna? it looks shorted. also, what does the load look like on the vna? you can also build a simple dipole to test your measurement and play with tuning to gain confidence in your technique and equipment.i cannot see enough detail in the photo

author: dmills
author_rank: Super Contributor
timestamp: January 24, 2018, 08:42:30 pm
content: Something very wrong there somewhere....How did you design the trace feeding the antenna? It looks very thick for 50 ohms on a reasonable board, and if done over a ground plane then the surrounding copper looks very close unless you were trying for a CPWG, which I am guessing is not the case.A 50 ohm dummy load makes a wonderfully broadband match, but radiates basically no power, so you usually do not perpetrate a resistive match in these situations.Have a look at thishttp://www.abracon.com/chip-antenna/AMCA-Series-Multilayer-Chip-Antenna-Application-Note.pdfRegards, Dan.

author: uski
author_rank: Frequent Contributor
timestamp: January 24, 2018, 09:00:11 pm
content: Here is my PCB layour for the antenna :Quote from: metrologist on January 24, 2018, 08:29:07 pmUsing an ohmmeter, reading the DC resistance between the coax ground and the coax line, I get an open circuit.I also get a DC open circuit when reading the resistance between the antenna feedpoint and ground.Using my "matched" antenna with the 47 ohms resistor, I get this SWR :Which actually looks good and matched... except that I guess everything is going into the resistor instead of the antenna ?!Quote from: dmills on January 24, 2018, 08:42:30 pmTotallyQuote from: dmills on January 24, 2018, 08:42:30 pmIt is a CoPlanar WaveGuide, 2 layers in a 62mil board.Maybe I didn't do it properly ?Trace width is 32mils, spacing to surrounding copper on top layer is 6mils (this is my design spec - and I doubt the board house changed it much).Quote from: dmills on January 24, 2018, 08:42:30 pmWhat I don't understand is the picture above (VNA output SWR) ==> the antenna looks matched to 2.4GHz. So it's like the antenna is a short at 2.4GHz only and the 50-ohms series tuning makes it "matched".Quote from: dmills on January 24, 2018, 08:42:30 pmThanks, I used that document. I actually linked it in my first post. I'm very surprised.

author: uski
author_rank: Frequent Contributor
timestamp: January 24, 2018, 09:09:28 pm
content: I have some official GPS antenna evaluation kits here.So in order to validate my setup I have measured one of these.See the attached result for the VSWR and the smith chart around 1.575GHz (WARNING : this is not my design - it's a 1.575GHz GPS antenna test kit, this test is meant to test my miniVNA)The frequency looks a bit off but I do not know if this devkit has been tuned. I will calibrate the miniVNA against my GPSDO in a few minutes.So I believe we can assume that the miniVNA is working properly.

author: uski
author_rank: Frequent Contributor
timestamp: January 24, 2018, 09:16:15 pm
content: Here is another test : testing a SAW filter in transmission mode. See the attached file.(WARNING : this is not my design - it's a 1.575GHz GPS SAW filter test kit, this test is meant to test my miniVNA)Again nothing seems awfully wrong with the miniVNA. Also, it doesn't look like the frequency is terribly off.So... something is wrong somewhere in my design.

author: uski
author_rank: Frequent Contributor
timestamp: January 24, 2018, 09:48:19 pm
content: I questionned my setup and especially the calibration procedure.Previously, I was calibrating by the miniVNA by itself, placing the loads (when directed to) directly on the miniVNA terminals.Then, when doing my measurement, I added some coax cable.I figured this calibration procedure didn't take into account the possible impact of the coax cables and connectors and even the PCB.So this time, I did the calibration directly on the PCB itself : I left the PCB connected all the time, but I did the open/short/load directly on the PCB using the pi matching components footprints. This way, I'm sure the calibration takes into account 100% of the setup, and the miniVNA really sees what the RF transmitter will see (except that I used a 51ohms standard resistor instead of a true 50ohms RF resistor).Here is a picture of this calibration and measurement setup :So after calibrating this way, directly on the PCB, it gives me this measurement result (measurement done with no tuning : open shunt components and a shorted series component)The antenna still looks way too much like a short circuit. The phase is different, which is probably because of the coax cable length I removed because of the new calibration setup.I'm totally lost now.

author: The Soulman
author_rank: Super Contributor
timestamp: January 24, 2018, 10:45:10 pm
content: Ground plane or other plane underneath the antenna?

author: ConKbot
author_rank: Super Contributor
timestamp: January 24, 2018, 11:01:43 pm
content: Best thing for tuning is your finger, or a metallic pokey bit. Since you have a provision for shunt components, get a small screwdriver, put the end vertically on your ground pad, and rock it over towards the RF pad. The capacitance will either help or hurt, and you, can get an idea if you want shunt capacitance (if screwdriver helped) or series capacitance (if it hurts)Also, just to be sure, as I missed it if you explicitly stated it earlier, you have no ground plane under the antenna, correct?Your cals to the other examples look reasonable, as does the GCPW dimensions.

author: dmills
author_rank: Super Contributor
timestamp: January 24, 2018, 11:16:29 pm
content: GCPW looks ok bu those numbers, 53 ohms at 4.6Er by my calcs, more then close enough.Seems to me that the antenna is resonant at about the right place, just with a very low radiation resistance, hence why the resistor 'matches' it.Can you get a measure calibrated at the OUTPUT side of the matching network, you want to know how the antenna and its feedline appear to the matching network so that you can then calculate the series and shunt elements.This is going to be a fairly high Q network I suspect, which gets interesting at those frequencies.Regards, Dan.

author: uski
author_rank: Frequent Contributor
timestamp: January 25, 2018, 12:12:07 am
content: After playing around with the setup, I was able to get a result that makes more sense.To confirm : there is no ground plane below the antenna, as per the antenna datasheet/instructions.I used a 3.9nH shunt at the antenna, followed by a 6.8nH series inductor to the radio feedpoint, and it's now properly tuned in free air.6.8nH is Murata LQP15MN6N8B02, 3.9nH is Murata LQP15MN3N9B02 (not necessarily the best parts - may be too good or not cost effective - just a kit I had laying around for a quick test)I say in free air, because if I put plastic around the antenna, it totally detunes it. So I will have to redo the tuning once I get my plastic case done, but that's OK.Honestly I am not sure what changed in the setup. In my various attempts to tune it, I may have fixed a hardware issue.It's also possible the antenna was too close to my wood table and it was too severely shifting the frequency in a way that prevented me from tuning it.(edit: the tuning described above is okayish with the board on the table - see below for the most likely explanation)I also noticed adding some tuning parts greatly reduced the sensitivity of the antenna to de-tuning. So I added some parts from the first values I had, which stabilised everything. Then I worked from here, ajusting the values, until I got something that made sense. Doesn't look very scientific, but it worked in 3-4 tries (of course ajusting the components as per the resulting smith chart, not by random attempts)Here's the tuned result now (using the calibration I have done with the loads directly on the PCB) :If I load the calibration data that I have done with the loads on the VNA itself, it gives this result :So the difference is not huge which makes me believe I probably have a reasonnably good tuning.If someone is considering this "miniVNA Tiny" thing, I think I can recommend it - pretty easy to use and if the results are good, it's a steal at this price.I was also able to compare the performance of a tuned board with a non-tuned board (no shunts and shorted series tuning element) and the tuned board performs better, from crude range testing. So while the tuning may not be perfect, it definitely got closer to what it should be.Thanks all for your help and meaningful suggestions and tips, it definitely helped !

author: xaxaxa
author_rank: Regular Contributor
timestamp: January 25, 2018, 04:45:13 am
content: The Q value of the inductors don't look too good, and since you have a huge series inductor I wonder if most of that power is dissipated there; I would try putting 1W into the antenna and see whether the inductors get hot.

author: uski
author_rank: Frequent Contributor
timestamp: January 25, 2018, 06:36:42 pm
content: I don't have the equipment to do that, but it's getting interesting.I'm working on another project where we experience extremely poor antenna efficiency at higher frequencies and it's possible the power gets "eaten" in the tuning circuit (our radio manufacturer is doing TRP testing and finds out that our design is not radiating enough power at higher freqs).I'm looking for a way to test for this but so far I've been unsuccessful in finding an easy way to do it which doesn't require a chamber and exotic RF equipment.I do have a Agilent 8648B signal generator but the output is limited to 2000MHz (short of 2.4GHz) and 20mW... I do have a thermal camera (FLIR E4) but I doubt I will be able to see let alone quantify the energy loss in the inductors at that kind of power.Any idea ?

author: Howardlong
author_rank: Super Contributor
timestamp: January 25, 2018, 06:54:39 pm
content: I would say it looks reasonable now.There is zero point calibrating at the VNA, you -must- calibrate at the DUT. FWIW, I build Murata MM8030-2610RKO PCB connectors into my boards, replacing them with an 0402 cap or zero ohm link in production to minimise cost. I also have my own open/short/load calibrator terminations built with them. These have the massive benefit of acting as a switch to isolate from the tx/rx when you plug in your test lead. The downside is that the test probes aren’t exactly cheap (MXHQ87WA3000) but can of course be used hundreds of times.

author: uski
author_rank: Frequent Contributor
timestamp: January 26, 2018, 01:11:45 am
content: It's funny... I have these exact test connectors on my other product, but I never thought to use them for antenna tuning (they were meant for production testing of the radio output power and sensitivity). Good idea! Thanks

author: Howardlong
author_rank: Super Contributor
timestamp: January 26, 2018, 03:46:14 pm
content: Quote from: uski on January 26, 2018, 01:11:45 amDepends which way around you put them!

author: uski
author_rank: Frequent Contributor
timestamp: February 06, 2018, 07:48:48 pm
content: I just made a panel of calibrators. It's a perfect calibrator to calibrate a VNA up to the tip of the Murata connectors.Then, by integrating the Murata connectors to your designs, you can easily calibrate the antennas. Then as suggested above, for production runs, remove the connector or bridge it.There are 4 calibrators (open circuit with ground plane, open circuit without ground plane, 50ohms precision resistor (49.9ohms 0.1% non-inductive), short circuit) + 2 bonus antennas to play a little bit. One is pre-tuned (approximately !) and the other is not.Connectors are Murata MM8030-2610.If someone wants one, I'm selling them, I don't need all of them.Assembly was a bit expensive and parts are not the cheapest so the cost is $65 + shipping.Or, you can make your own too !

author: hcglitte
author_rank: Regular Contributor
timestamp: February 07, 2018, 12:10:02 pm
content: Hi, There is actually another solution which I saw specified somewhere. But it involves a little math.First calibrate as normal directly on your VNA.You use your Murata connector or pigtail approach. Then at the PI (matching section) you place no components (open).Then measure your s11 result and store the s11 file.Then connect the antenna and place a 0 ohm / "high" cap at the series PI section.Measure s11.Divide the two files (subtract for MAG and phase) (write a script) and you get your antenna s11 as seen from the PI section.Calculate matching components, fit them and do a new s11 and divide with the "open" file to verify that your antenna is matched.I have done it, the normal way and this way, and I got the same result. I also tested the approach with RFsim99 to dummy test the approach.https://www.johansontechnology.com/tuning.html

author: uski
author_rank: Frequent Contributor
timestamp: February 07, 2018, 04:56:29 pm
content: If I understand properly, this method is equivalent to calibrating at the pi filter directly. Basically, the measurement "open" is the new calibration, and this new calibration is used to measure the proper values (because the measurements are divided by it).

author: hcglitte
author_rank: Regular Contributor
timestamp: February 07, 2018, 06:28:48 pm
content: The calibration is actually something that is required due to VNA temperature and other stuff (including setting the reference plane to the VNA port).Doing the "open" measurement calculates the phase change of the cable and loss - I guess this is what VNA's that has "port extension" actually is doing.The "open" measurement and no_antenna dividing just moves the reference place to the PI section.I think this solution is quicker. Has anyone else also tried this before, experience?

author: uski
author_rank: Frequent Contributor
timestamp: February 13, 2018, 07:08:43 pm
content: Quote from: xaxaxa on January 25, 2018, 04:45:13 amI was wondering... what kind of Q value should I aim for and why ? I am trying to select inductors part numbers now and I'm a bit lost.Thanks !

author: Howardlong
author_rank: Super Contributor
timestamp: February 14, 2018, 09:43:29 pm
content: I’m assuming this is for the matching and/or fliter balun network. In general, I start of with wirewounds which give much better Q than multilayer for 2.4GHz in 0402 or 0201 and are lower loss. You are welcome to try multilayer, they are certainly cheaper, but losses will be higher. Dependent on your circumstances and logistics, even PCB fab’d parts might work for you.In short, R&D with wirewounds at 2.4GHz, then if you need to cut costs, look at multilayer and see if the losses are acceptable for your application.