LiteVNA S11 or S11+S21 to tune antenna matching network - Page 1
Forum section: electronics / rf-microwave

author: okw
author_rank: Frequent Contributor
timestamp: April 04, 2025, 12:34:14 pm
content: I'm quite new to RF and the LiteVNA. I have a bluetooth project using ESP32-C3, pi-network and the antenna from AN043 (SWRA117D). At the moment no shunts, and 0R in series to begin with. In addition to assembled boards, I also ordered naked PCBs for matching, and I have mostly all standard inductor and capacitor values in 0402.I cut the IPEX tip off of two SMA cables and soldered the cables near the ESP RF out (S11 in VNA) and near the start of the antenna (S21). Should I solder in a 50 ohms resistor between ESP RF out and GND?Which LiteVNA options do I set to use it with both S11+S21 measurements? Am I just using two traces, and defining channel S11 and S21 and get two traces? They do not "work together" to give me some magical measurements?I see S11 and S21 are almost mirrored on the smith chart.I think I have understood how to do it with single S11 measurements (soldered the cable near RF output, then set S11 and smith chart and use "measurement" to get shunts and series components (though I didn't have much success, even with exact suggested passives)).With the current design (no shunts/0R series) I see a very nice dip at 1.9GHz with SWR near 1.5. I also see that if I cut the GND stub (near where it joins the copper pour line), I see much better results.Since I see such significant frequency shift (>500MHz less than desired), should I cut the antenna to move it upwards towards 2.45GHz or will I achieve sufficient results by just tuning the network (it would be more convenient, as next order is hopefully the final)?Also, the hardware design says "chip conjugate point (35+j0)". Is this something I can enter into the VNA that "simulates" the ESP, or is this number just for informative purposes?

author: Bud
author_rank: Super Contributor
timestamp: April 06, 2025, 10:12:09 pm
content: I do not use or have a LiteVNA so do not know what your screenshots show. But what you need to understand first is you have to calibrate the VNA at the far end of the cable where it is soldered to the PCB. Otherwise you will be measuring gibberish.This is what I would do: with matching circuit removed, cut the antenna trace at the feedpoint, solder S11 port cable to the left side of the cut trace, calibrate the VNA for Open, solder a short across the pads of the righthand matching component, calibrate the VNA for Short, replace the short with a 50 Ohm resisitor, calibrate the VNA for Load, remove the 50 Ohm resistor. You are now ready to measure the antenna S11. Solder the antenna to the feedpoint and run Return Loss measurement. Minimum of RL is at your antenna resonance frequency. Play with the antenna to tune the resonance frequency. When satisfied, note the final impedance R +/-jX. Unsolder the cable.Move the cable to the chip output pin pad. Use an RF matching tool to calculate initial values for the Pi matching circuit. Enter 35+j0 as input impedance, and your final measured antenna R+/-jX as output impedance. Prepare the components but do not solder yet. Repeat VNA S11 port calibration using same technique as above but usin the lefthand matching component pads for Open/ Short / Load standards. Now solder the matching circuit components and run S11 sweeps. Adjust the matching circuit to get Zin as close to 35+j0 as possible.

author: okw
author_rank: Frequent Contributor
timestamp: April 06, 2025, 11:25:15 pm
content: Quote from: Bud on April 06, 2025, 10:12:09 pmThanks. Very well explained. I'll do my best.The antenna is fed directly from the righthand matching component. In the future, would you recommend putting in a 0R (or similar) in series to the antennas feedpoint, so it's easy to separate the antenna feedpoint (instead of cutting the trace, scraping off the soldermask and soldering it back together) when calibrating? Or will even a simple 0402 0R mess/attenuate the signal?

author: Bud
author_rank: Super Contributor
timestamp: April 07, 2025, 04:27:55 am
content: A SMT jumper has inductance, and if its width is smaller than the trace width, you are going to have impedance discontinuity in the path. You said you have a bare PCB version, so use it to do VNA measurements, then use the measured component values on the production board, no need to cut trace there.BTW, industrial "adult" VNAs have Port Extension feature, which allows to dial the cable length out without running calibrating procedure on the board.

author: Kean
author_rank: Unknown
timestamp: April 07, 2025, 05:11:50 am
content: Yeah, you don't really want to add anything unnecessary in the RF path as it will very likely add a discontinuity. It may not have much impact depending on your RF band and power, but it isn't worth it when you should only need to modify a few PCBs for the matching process. Maybe you will do more mods on your early learning attempts, but hopefully less in future designs.I've seen quite a few PCBs with a 0R 0402 in the RF path that you can rotate 90° or 180° to select different antennas (e.g. on & off board), so it is doable and you may then want to adjust the matching components to compensate.

author: okw
author_rank: Frequent Contributor
timestamp: April 07, 2025, 10:55:32 am
content: Two questions. Should I try to cut as exact as possible at the feedpoint, or doesn't it matter as long as it's close to the righthand match component? You an see the feedpoint at the yellow cross.And second. I see that I have too wide thermal isolation for ground fill (6mil), so the matching shunts make breaks in the coplanar waveguide. I prefer a little (e.g. 2mil) for easier soldering, and at 2mil it doesn't break the waveguide. Would changing this for make a big difference in matching? Would it be recommended to order a 5 pcs batch of bare PCBs to match again, before ordering 50 pcs of the final product?

author: okw
author_rank: Frequent Contributor
timestamp: April 09, 2025, 07:54:09 pm
content: Quote from: Bud on April 06, 2025, 10:12:09 pmI did as you described, and I'm now playing with the antenna. It showed really good return loss (-40dB) without modifications, but at a resonant frequency of 1.725GHz. See attachment 1 and 2. I tried cutting the antenna in a few small iterations, which moved the frequency to about 2GHz, but with a lousy -15dB. See attachment 3 and 4. Is this normal when altering the length of a carefully designed antenna?I probably have to cut quite a lot to get 2.441GHz.Would cutting the GND stub be beneficial? Or do I leave it?

author: rteodor
author_rank: Frequent Contributor
timestamp: April 09, 2025, 09:08:07 pm
content: Pardon my ignorance but shouldn't the coax be soldered away from the antenna ? Soldering it in that position so close to the antenna for sure influences it.Also should it be soldered at the chip/matching net output pin (to increase the distance, reduce the coax influence and not have a lump of feed line in parallel with the antenna) ? After all, the feed line should be already matched, right ?(PS: don't know how to follow a thread without commenting and without email notification. Anybody has a hint ?)