Build an ideal Directional Coupler for VNA front end - Page 1
Forum section: electronics / rf-microwave

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 20, 2025, 09:00:44 am
content: Back from a Two-Week Trip (Academic Exchange)After being away for about two weeks on an academic exchange, I'm finally back and getting back into my projects.During the trip and especially after returning, I spent a lot of time thinking about the VNA project I’ve been working on. To be honest, trying to build a full VNA as a one-person team is probably too ambitious right now. There are too many small details that could be missed. So I’ve decided to narrow the focus for May and June — my goal is to design and build a reliable directional coupler.Coaxial Directional CouplerI tested the coaxial directional coupler I designed, and unfortunately the directivity was quite poor. Around 20 GHz, it couldn’t even clearly distinguish between a load and a short. After some measurements, I found that the issue is most likely due to impedance mismatch.I checked the S11 of the RF input, and saw large deviations at high frequencies. The real part of the impedance dropped to about 10 ohms at around 8 GHz. This suggests there may be a problem with either the coaxial line or the PCB trace impedance.Here are a couple of issues I ran into:1. I contacted JLCPCB, and their support told me that choosing “20% impedance control”basically means no impedance control at all— that was very disappointing.2. I measured my coax cables with a VNA and found significant deviations at high frequencies. The supplier claimed their cables had ±2 Ω tolerance at 20 GHz, but in reality, it wasn’t even close. Unfortunately, there are a lot of misleading sellers here.Because of these problems, especially the limited directivity of the coaxial design.I’ve decided to abandon the coaxial approach.The Microstrip Directional CouplerFor a long time, I thought it wasn’t possible to get good low-frequency coupling performance using microstrip couplers, and that’s why so many designs use alternative structures. But recently, I came across a very helpful paper: IET-MAP.2011.0471, which explains how to design a “wiggly-line” microstrip directional coupler in detail.This gave me some new ideas, and I’ve decided to try replicating that design.My Plan (To-Do List)1. Develop a mathematical model – this part I’m quite comfortable with.2. Simulate and fine-tune the design – this may take time, as the layout is sensitive.3. Fabricate the coupler – this is the most difficult part. The design requires high precision, so I either need to figure out how to process alumina boards myself, or I’ll have to pay nearly $1,000 per prototype run, which is not feasible for me. (If anyone knows a more affordable option or is willing to help, I’d really appreciate it.)4. Once the coupler is working, I’ll go back to working on the VNA.If anyone has experience with alumina PCB fabrication, or has worked on wideband microstrip couplers before, I’d love to hear your insights or suggestions.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 20, 2025, 03:59:27 pm
content: You can buy these:https://www.knowlescapacitors.com/getattachment/Products/Microwave-Products/Couplers/FPC07180_DATASHEET.pdf?lang=en-USAlso can you measure your setup with the TDR on a VNA? I suspect the microstrip to coaxial transitions are causing a lot of mismatch (which is what I saw on the simulator)Might also be worth learning OpenEMS (or using a professional simulation tool if you have access as a student)I find it a bit hard to believe that the cables/microstrip are causing the deviations in impedance, especially a very large one like going from 50 ohms to 10 ohms. It's probably a bad transition on your board between the microstrip and coaxial cable.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 20, 2025, 07:09:27 pm
content: Quote from: EggertEnjoyer123 on May 20, 2025, 03:59:27 pm10dB directivity is good enough for your homemade VNA?

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 21, 2025, 01:14:55 am
content: I have this kind of directional couples, but none of them can maintain the same coupling Degree at low frequency.And I also want to build a vector network analyzer using all the components I made myself.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 21, 2025, 01:26:18 am
content: thanks for ur advice EggertEnjoyer123. I use keysight ads to simulate the whole plane design, but I didn't do the 3d simulation due to the lack of magnetic ring data. And also I have a thru cal wire, I measured it, It also have really big impedance drift. Also I measured the sma to sma sample provided by the coaxial wire seller. they also have really serious impedance mismatch. I used another VNA and got the same result, so it's not that there's a problem with my VNA.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 21, 2025, 02:03:43 am
content: Are you using professional calibration standards?Quote from: joeqsmith on May 20, 2025, 07:09:27 pmYou can also just buy a good directional coupler from ebay (or get this one, which has 14 dB directivity:https://www.knowlescapacitors.com/getattachment/Products/Microwave-Products/Couplers/FPC07181_Datasheet.pdf.aspx?lang=en-US). Also Keysight has 15dB uncalibrated directivity in their VNA. The coupler is for 20-40 GHz but if you get the S parameter model you can see it has good directivity at low frequency too.Quote from: JoeyDeng on May 21, 2025, 01:14:55 amYou can't use this method at all for low frequency. The coupler can't be electrically small (which it will be at 10 MHz) otherwise your coupling would be horrible.I'd look into using a 4 layer board and using suspended stripline if I were you (most wideband couplers are built in a similar fashion)

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 21, 2025, 02:44:17 am
content: Quote from: EggertEnjoyer123 on May 21, 2025, 02:03:43 amI have experience in photolithography. So it's possible for me to build it, and also I just want to try make some cool microstrip stuffQuote from: EggertEnjoyer123 on May 21, 2025, 02:03:43 amI saw the directional coupler in anritsu innstrument used suspended stripline. but they still cant maintain good coupling degree and coupling flatness due to the thickness of the dielectric material.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 21, 2025, 02:53:49 am
content: Quote from: JoeyDeng on May 21, 2025, 02:44:17 amThe flatness doesn't matter. The coupling just needs to be high enough that your receiver can detect it. Some of the directional couplers used in old HP VNAs have 30dB coupling. Flatness can always be calibrated out, unlike directivity (which can only be calibrated to a certain extent).Your choices if you want your VNA to work to DC are to either use an RF switch to switch between a high frequency and low frequency path (using different couplers) or using a resistive bridge circuit.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 21, 2025, 03:32:32 am
content: I mean, the thickness drift of the plate is very serious. If you use a plate with vertical thickness to make a directional coupler, it will become very unstable. so the coupling degree will change every time.And the directional coupler mentioned in the paper can work at very low frequencies and matain good coupling, so it is worth a try. I think.The switching method you mentioned is not realistic. A new RF switch with good parameters and a frequency of 20Ghz is very expensive, which will increase the cost a lot. In addition, each switch will bring about a change in the sparam of the switch, which will have a impact on the overall measurement accuracy.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 21, 2025, 03:37:06 am
content: Also, if the coupling degree is reduced, it means that the ADC usage range will be reduced, which will directly determine the accuracy of the measurement, so I think a stable coupling degree is still needed.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 21, 2025, 06:08:12 am
content: Quote from: JoeyDeng on May 21, 2025, 03:32:32 am2 GHz is quite far off from 10 MHz though (or 45 MHz, which is the lower limit of the TI chip). Consider that a quarter wavelength is over 1.5 meters at 45 MHz.Quote from: JoeyDeng on May 21, 2025, 03:37:06 amIt is not critical. You can always just increase the gain at the ADC end anyways. (Also keep in mind that too much coupled power will cause nonlinearities in your mixers, which will destroy your performance).

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 21, 2025, 08:19:42 am
content: OHHHHH NO, sry bud, I just miscalculate the coupling degree, after building the math module and calculate the shape,length,coupling factor,the best matchingof the wiggly-line. I simulated and found at low freq the coupling degree is only -58db at 10mhz, but as the simulation this directional coupler really works good. I will update the ads simulation data tomorrowso as u say adding the amp to the afe of the adc seems the most possible way to solve this.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 22, 2025, 12:37:02 am
content: Quote from: JoeyDeng on May 21, 2025, 08:19:42 am-60dB is probably too much. Keep in mind that your noise floor doesn't change. Some signal loss is fine but not that much.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 28, 2025, 10:21:40 am
content: It works well, but the vna in my home can only measure 8ghzplz wait for the final result from the lab

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 29, 2025, 07:41:34 am
content: I suspect that if you overlay the S11 chart on the S31 chart you'll see that the directivity is impacted by the matching. If your microstrip to coax connection has reflections it will ruin your directivity.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 29, 2025, 01:12:42 pm
content: Sry I cant export the data cuz I don't have the GPIB communication protocol for this instrument. "ko4bb is dead.... Dang".However There is a significant impedance drift at 8 GHz, which caused a major drop in directivity. This observation aligns pretty well with the simulation.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 29, 2025, 02:33:03 pm
content: I got a hold of a faulty 26.5GHz HP8510 S-parameter test set. I'll crack it open and take a look when I have time.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: June 04, 2025, 04:52:22 pm
content: 15db directivity across the range from 10 MHz to 20 GHz.not bad, with the strip line topology. I’ll make a shell for it first and I will try to use the wiggy wire to decrease the phase speed.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 05, 2025, 01:07:55 am
content: Quote from: JoeyDeng on June 04, 2025, 04:52:22 pmI think you sent the 8 GHz measurements. I'm interested in how good the matching is with these connectors.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: June 05, 2025, 03:33:46 am
content: I didn’t take a photo of the measured vswr results, but the s12 is measured. And the matching of the port is really worse before I modified this kind of connector into flat pins. also the copper added to the design is just used to get all the port matched. If u don’t want to waste a lot of time and also get really unstable result I suggests that u should use the proper rf connectors designed for high frequency usage.And actually the proper rf connectors they’re really cheap if u got them from the factory, only abt 50cents per each.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: June 05, 2025, 01:30:19 pm
content: Quote from: JoeyDeng on May 29, 2025, 02:33:03 pmLet me know what you seeBTW, the patent on the triaxial coupler is helpful: US4588970

author: tszaboo
author_rank: Super Contributor
timestamp: June 05, 2025, 02:54:01 pm
content: Here is my home made coupler, that I made for the GPS frequency to test receivers with an SDR. I figured a coupler will provide DC isolation and attenuation both of which is something I wanted to have (in fact I needed like 80-100DB attenuation in the end).It is made out of some random copper clad PCB, and copper tape, and some random SMA connectors that were lying around.All the parts that were needed were less than 4 EUR in total. It's not broadband of course. I'm just showing it to encourage experimentation.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: June 06, 2025, 12:31:35 am
content: Quote from: jjoonathan on June 05, 2025, 01:30:19 pmThanks for ur information.I found that this bridge works completely fine so I won’t make further tear down. And the sampler and the comb generator inside the 8515a is just the same as things inside the 8720es. So actually the 8720 8722 8719 all of these are the son of the 8510. and the 87100 series scopes are using the same srd&divider.1st picture is 8515a2nd picture is 8720et3rd picture is the sampling module

author: JoeyDeng
author_rank: Regular Contributor
timestamp: June 06, 2025, 02:13:17 am
content: really cool stuffbut I advice u to use the bend to cut down the reflection

author: jjoonathan
author_rank: Frequent Contributor
timestamp: June 06, 2025, 02:02:08 pm
content: Quote from: JoeyDeng on June 06, 2025, 12:31:35 amNo worries, I gotchuhttps://photos.app.goo.gl/7Nc9CeS9XU2vt14i6This is before I got good at taking teardown galleries so it's a bit sparse, but hopefully it helps take the edge off the curiosity a bit, and the patent is the better engineering reference anyway.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: June 06, 2025, 02:43:24 pm
content: Everything works well with the proper rf materialand the directivity increased a lot(coupling degree is 16db and the worst isolation is 40db, so the directivity should be greater than 20db in 8ghz )I think it’s the time to make the shell and back to the vna project.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: June 06, 2025, 03:09:56 pm
content: Quote from: jjoonathan on June 06, 2025, 02:02:08 pmDid u measure the directivity of this bridge? Im really curious abt how well hp can do.So actually the coaxial directional coupler with really big size can actually reach much higher frequency than I think (there is someone sells the coaxial wire could run upto 110GHz)but I think the most reason why it is such big is that it needs to use the magnetic ring to modify the directivity (some one tells me that the old keysight pna with max frequency around 20GHz also use the coaxial directional coupler, they turnning the position of the magnetic ring to get better directivity) and improve the coupling degree at low frequency.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: June 06, 2025, 04:03:16 pm
content: No, at the time I didn't even have the equipment for a crude directivity measurement, let alone a good one that did not simply depend on having a better load. The datasheet says raw directivity is "24 24 28 27" dB and calibrated is "48 50 50 50" dB for 0.45-2G, 2-8G, 8-20G, 20-26.5G respectively.

author: tszaboo
author_rank: Super Contributor
timestamp: June 06, 2025, 04:13:09 pm
content: Quote from: JoeyDeng on June 06, 2025, 02:13:17 amJust wait, because that corner is not even soldered. It's just two pieces of self adhesive copper tape sticked on top of each other.I know the corner bend. I don't think it really matters at 1.5GHz. It measures good, so it should be good, right? And this is used for something very specific, so wideband performance is not needed.Quote from: JoeyDeng on June 06, 2025, 03:09:56 pmI wanted to build a coupler like this, even have the ferrite rings and some semi-rigid coax. I mean the balun. I just realized that I don't really have a very good way to measure even if I build one, or how to debug it.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: June 06, 2025, 04:25:15 pm
content: Quote from: jjoonathan on June 06, 2025, 04:03:16 pmI shall try that structure (not now, 20 dB is actually enough for now). 40 dB is fantastic. I think the most important thing is the dual coaxial structure.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 07, 2025, 07:03:31 am
content: The "40dB" is not something you can measure on a VNA. It's like how you might have a 1V reference that is +/- 0.01V, but it could be precise enough that after you measure it it will only deviate by +/- 0.0001V from that measured valueI wonder if ferrite can be used as the lossy dielectric. It would kill two birds with one stone.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: June 07, 2025, 10:08:53 am
content: This directional coupler uses dual coaxial wires to reduce parasitic capacitance.I believe that the "40 dB" figure refers to directivity, not stability. Also, the calibration he mentioned involves adjusting the screw on the shell.But you reminded me that I need to measure the stability of my coupler that's also really important.The directional coupler inside the low-frequency PNA can also achieve a directivity higher than 40 dB within the operating frequency range.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: June 08, 2025, 02:08:43 pm
content: Quote from: EggertEnjoyer123 on June 07, 2025, 07:03:31 amI know what EggertEnjoyer123 want to express is that the directivity of the bridge depends on the whole VNA system(like the source matching, the source PA matching), so directly measure the directivity of the directional coupler using VNA doesn't means the true directivity of the coupler in the whole system.And the raw directivity means the real performance of the bridge in the whole VNA system.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 19, 2025, 07:13:56 am
content: For the lossy material in the triaxial bridge design, it seems like ferrite can't be used. The ferrite stops being lossy past 7 GHz.I think I should try using conductive filament and ferromagnetic filament to try to introduce both magnetic and electric losses. Getting all the 3D prints to fit would be annoying.

author: szoftveres
author_rank: Regular Contributor
timestamp: July 14, 2025, 08:15:23 pm
content: Does the ferrite material matter at higher frequencies? I've built a coupler using type 61 material (ui=125); the directivity starts falling off after several hundred MHz, and above 1GHz it stays at around 10dB.I was assuming that above ~100MHz all known materials are lossy so different materials wouldn't make any difference above 1GHz but I could be wrong. Also there's plenty of room to improve my design (stop using 0603 resistors and switch to 0402, haha) but I was just curious whataterials others use.I couldn't find a datasheet for the ferrite bead materials mentioned here:http://www.ke5fx.com/Broadband_Coupler_Dunsmore.pdf

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 17, 2025, 11:13:04 am
content: I got some ferrite and measured their specifications then I put it into my coupler model, but the simulation result of my directional bridge can't match the measured results.So I gave up coaxial bridge.

author: tszaboo
author_rank: Super Contributor
timestamp: July 17, 2025, 11:19:23 am
content: Quote from: JoeyDeng on July 17, 2025, 11:13:04 amI see these bridges, they typically have some random parts soldered at the coax.It is for matching (I think), but I never really got a good understanding how this matching look like. Is it similar you do with an antenna? Are there any guides for it?Something else I saw was that at some Chinese PCB suppliers, Rogers PCB is quite affordable. I mean it's not 2 USD for 5 PCBs, but 100. Which is still a lot cheaper than buying a name brand directional bridge.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 17, 2025, 11:32:38 am
content: 100 bucks is insanely expensive I think.Please read the artical from Dr. Joel.http://www.ke5fx.com/Broadband_Coupler_Dunsmore.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: July 17, 2025, 12:11:11 pm
content: Quote from: JoeyDeng on July 17, 2025, 11:32:38 amThanks of the link.I mean 100 bucks is a lot. But if we put it in perspective, it's 20 per board, and you are going to spend a lot more on the 3.5mm connectors trying to get this to work at 26 GHz. Unless you have a secret source of cheap connectors. If you do, would you kindly please share

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 17, 2025, 02:50:16 pm
content: well I dont have any idea where to get the cheap 3.5mm rf connector.Also I showed the directional couplers I build using Rogers material.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 17, 2025, 03:12:32 pm
content: After spending a few days working on other stuff, I'm finally back on this project.I upgraded my measurement setup and realized the main issue affecting directionality is probably impedance mismatch. also the ferrites are not just absorbing at low freq to extend the low freq ability of the coupler. they're also helping absorb reflected signals at high freq. Since the reflection varies along different points of the coax, it seems like using longer coax cables and more ferrites to better match the impedance is actually a smart move.As for PCB impedance matching:So far, Iv tested the etched PCB test coupons I made—like CPW, MLIN, and CPWG—and they all performed pretty well, with very low VSWR. But things start to get unstable once I attach the coax cables. Eggertenjoyer123 mentioned in an earlier post that this instability is likely caused by the solder joint. I simulated it before and it works completely fine in the simulation. I used the test coupons to rule out the RF connector itself, so the issue must be the soldering of the coax.To double-check, I used a stencil to add solder paste so that the amount of solder wasexactly the sameeach time. I reflowed two test coupons that way and gotalmost identicalmismatched S-parameters. So that pretty much confirms what Eggertenjoyer123 was saying—the issue is with the impedance at the solder joint.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 17, 2025, 03:27:30 pm
content: QuoteI tried this in the stripline directional coupler design ,it can improve the vswr by absorbing the reflected wave but the directivity became worse bcuz the decrease of the coupling degree.btw im using the ferrite sheets that I took from the back of a wireless charging coil. unknow param and I dont want to measure it. so I wont make further simulation before I get some good Idea abt how to use it.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 18, 2025, 05:36:14 am
content: Consider where the E fields go through in the stripline design. The E fields go from each of the traces to the outer ground planes. If the E fields go through the lossy material, there will be loss, so with the stripline design there will be loss even when everything is balanced if you use lossy material.It does seem like soldering a coax is a bad idea if matching is important (even though R&S somehow got away with it on their VNA design). Making everything on a PCB is probably a better idea. You can probably use snap on ferrite cores.