Why do I need a VNA rather than a SA with directional coupler to tune antenna? - Page 1
Forum section: electronics / rf-microwave

author: trevwhite
author_rank: Frequent Contributor
timestamp: September 30, 2017, 07:51:40 pm
content: I am wanting to do a bit of work tuning antenna. Be that PCB, helical or other. I have an SA with TG and a directional coupler on the way but someone told me I really need a VNA for this kind of work. Anyone explain why to me or point me in the direction of some useful article(s) that explain why?ThanksTrev

author: xaxaxa
author_rank: Regular Contributor
timestamp: September 30, 2017, 08:38:39 pm
content: There are two main reasons, the most common being that you only get magnitude and not phase information with a SA + directional coupler; with phase information you can make better guesses as to what changes to make to your antenna to tune it.The less known reason is that directional couplers have limited directivity, and a VNA allows you to calibrate away the imperfections in your coupler (and you can easily measure down to -50dB S11) whereas a SNA could not.

author: tautech
author_rank: Super Contributor
timestamp: September 30, 2017, 08:48:53 pm
content: Quote from: trevwhite on September 30, 2017, 07:51:40 pmHope these help:http://www.eham.net/ehamforum/smf/index.php?topic=91151.0https://mediacdn.eu/m/media/downloads/Siglent-RBSSA3X20-Reflection-Bridge-Manual.pdf

author: trevwhite
author_rank: Frequent Contributor
timestamp: September 30, 2017, 09:05:45 pm
content: Quote from: xaxaxa on September 30, 2017, 08:38:39 pmHi, xaxaxa. Will your vna design enable antenna tweaking/tuning? How long do you feel it will be before they are available?Sent from my SM-G950F using Tapatalk

author: trevwhite
author_rank: Frequent Contributor
timestamp: September 30, 2017, 09:44:05 pm
content: Quote from: tautech on September 30, 2017, 08:48:53 pmThanksSent from my SM-G950F using Tapatalk

author: tautech
author_rank: Super Contributor
timestamp: September 30, 2017, 09:49:22 pm
content: Quote from: trevwhite on September 30, 2017, 09:44:05 pmCheck the usage docs on the SSA3kx Siglent FAQ page on the websites.I haven't got it open on this laptop but there should be some more guidance there.There's nothing you want there it seems.http://www.siglentamerica.com/prodcut-pxzl.aspx?id=5113&tid=227&T=2

author: xaxaxa
author_rank: Regular Contributor
timestamp: September 30, 2017, 09:52:11 pm
content: Quote from: trevwhite on September 30, 2017, 09:05:45 pmYes it is definitely sufficient for antenna design; I have used it to tune up several antennas already. If the kickstarter is successful I can start shipping by march of next year; I have already gotten quotes for pcba and components, and will put up the kickstarter soon.If you need it sooner you can always build my design or hforsten's design; both are fully open sourced and have all design files online.

author: trevwhite
author_rank: Frequent Contributor
timestamp: September 30, 2017, 09:56:28 pm
content: Do you have any spare boards that you want to sell? Better to give you some money than build from scratch myself. I would like to reward your hard work if possible.Thanks for making it open source, that really is awesome of youTrev

author: trevwhite
author_rank: Frequent Contributor
timestamp: September 30, 2017, 09:58:59 pm
content: tautech, thanks for your suggestions. I will check the FAQ out. You have a lot of energy for answering in the forum. I know you are a distributor in NZ but still, you seem to work hard and it is very much appreciated. The SSA3021X appears to be a beast. I dont really know what I am doing at the moment but I really like the unit so far.Trev

author: xaxaxa
author_rank: Regular Contributor
timestamp: September 30, 2017, 10:33:27 pm
content: Quote from: trevwhite on September 30, 2017, 09:56:28 pmI have just the one prototype shown in the pictures, and I generally don't sell hand-assembled boards because of reliability concerns (solder joints are not perfect);If your directional coupler has >20dB directivity, it is good enough for antenna tuning and should do for now.It mainly depends whether you're designing antennas or just fine tuning antennas. If you are building an existing antenna design that has already been characterized, a SA+coupler is more than sufficient since you only need to fine tune the resonance to the right frequency. A vna is only needed for designing new antennas when simulations are either unavailable or not accurate enough.

author: tautech
author_rank: Super Contributor
timestamp: September 30, 2017, 10:47:49 pm
content: Quote from: trevwhite on September 30, 2017, 09:58:59 pmThanks Trev my energies are juggled between Dist, hobbies, other business commitments, family and EEVblog.I too don't have much experience with analysers but I know there's a lot you can find online and in the forum here.I'll do some checks with my SSA to see if Phase measurement is possible....member hendorog gave me a tiny SMA directional coupler and along with a WiFi SMA antenna it shouldn't be hard to cobble something together to get some SWL info and maybe phase info.Why ? Because I'm learning too and the some/little knowledge gained helps me support customers better and you guys too.As Arnie says; I'll be back, hopefully with something that's useful.Give me a bit, it's lunchtime here and I have to hunt all the bits out.

author: trevwhite
author_rank: Frequent Contributor
timestamp: September 30, 2017, 11:06:52 pm
content: Thanks for the replies.I have an rf module that comes in a few different frequencies. 433, 868 and 915Mhz. I have the module tracked to a board with a smallish ground plane of around 6cm x 6cm. I am not sure if a 17cm wire will be okay with this ground plane for the 433Mhz version. My thoughts were to solder a wire to a bare board. Then connect the output of the coupler to the ground plane and the antenna and trim the wire accordingly and see what I get. I was then thinking of doing something similar for the other frequencies.After that I wanted to experiment with some helical designs to see if they work as well as they improve on space.Am not sure how I will be able to compare designs for gain values at the moment. It is all a bit new and experimental at this stage.

author: PA4TIM
author_rank: Super Contributor
timestamp: September 30, 2017, 11:25:07 pm
content: http://www.pa4tim.nl/?p=1594I made a series vna tutorials.The difference is that a SA+TG only gives you the return loss but no phase info. So you do not know if there is a phase jump. Besides that you can not calibrate a SA the same way as a VNA. And that is the most important part of a measurement. The result can never be better as the calibration. Without that you only have an indication and the result can be so way off that it is kinda useless. The higher the frequency the more important calibration is for the end result.For measurements on a pcb or other cases you can not calibrate and measure direct at the vna port you need to do more as a normal calibration and modern VNAs have functions for that, they can shift the reference level. A SNA is useless for those kind of measurements.

author: tautech
author_rank: Super Contributor
timestamp: October 01, 2017, 12:01:59 am
content: Quote from: PA4TIM on September 30, 2017, 11:25:07 pmEducate us please.Is the calibration you mention not the same as Normalise ?After Normalise I can set the displayed Reference level and numerous other parameters to obtain detailed info on a spectrum of interest.Quote from: tautech on September 30, 2017, 10:47:49 pmGrrrr.....I need an adaptor that I looked at online recently and wondered what on earth would I use that for. Male SMA male to Male SMA female....no not the outer thread but the inner pin.Two Male SMA females for the WiFi antenna won't get a connection, will they ?More RF bits needed. <sigh>

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 01, 2017, 12:12:45 am
content: Quote from: tautech on October 01, 2017, 12:01:59 amI think the issue with the SA + coupler is that we will get a frequency response indicating the resonant frequency but what we do not get is the phase angle of the impedance? The VNA shows this phase angle and thus indicates if a circuit needs more L or C to bring the antenna in phase with the transmitter.Sent from my SM-G950F using Tapatalk

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 01, 2017, 12:14:03 am
content: Looks like I need to build a VNA!Sent from my SM-G950F using Tapatalk

author: tautech
author_rank: Super Contributor
timestamp: October 01, 2017, 12:16:31 am
content: Quote from: trevwhite on October 01, 2017, 12:12:45 amYep I get that, it was PA4TIM's comments on calibration.What expressly does that mean ?

author: hendorog
author_rank: Super Contributor
timestamp: October 01, 2017, 01:41:10 am
content: Quote from: tautech on October 01, 2017, 12:16:31 amI don't quite understand what PA4TIM meant either. A SNA can be normalised (calibrated) so that the measurements have meaning.Maybe he meant that the VNA can compensate for the coupler directivity:With an SNA measurements of return loss of an antenna do become less and less accurate as the return loss approaches the directivity of the coupler - i.e. you need a high quality coupler to measure a high quality antenna. Good summary with pictures here (Figure 5):http://www.wetterlin.org/sam/Reflection/3BeadBalunBridge.pdfA VNA can compensate for this effect. It can also compensate for other limitations in the equipment like the return loss of the test ports. The cost of this is more calibration time and the need for precisely characterised calibration standards.I read somewhere about a retiring HP directional coupler guru saying that he wasn't needed anymore as powerful CPU's made calibration cheap so now any old coupler would do the job.Edit: I realised that PA4TIM meant reference plane extension - where a VNA can user clever maths to remove the cabling and adapters, and make measurements at the PCB itself as if the cables and adapters were never there. An SNA can compensate for the loss in the cables and adapters, but not for other effects like return loss of the adapters.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 01, 2017, 02:54:26 pm
content: Quote from: xaxaxa on September 30, 2017, 09:52:11 pmHi xaxaxa, I could not find the gerber files for your design? I found the cad files I think. Do I need to install the cad software and generate the build files to make my own? Also I know you said you only had the one prototype but what about bare PCBs unpopulated? Any of those kicking around for sale?TrevTrev

author: KJDS
author_rank: Super Contributor
timestamp: October 01, 2017, 03:56:28 pm
content: I've just done a quick spreadsheet comparing the measurement error using a 30dB directivity coupler (which is as good as they get) and a calibrated VNA that assumed 47dB effective directivty, which is about what I would expect with a freshly calibrated unit. The errors are +/- and all figures are in dBRL error, coupler error VNA10 0.872 0.12312 1.099 0.15414 1.388 0.19416 1.757 0.24518 2.230 0.30820 2.844 0.38822 3.660 0.48924 4.785 0.61626 6.454 0.776So for a 12dB return loss antenna then there is 1.1dB of measurement uncertainty using a good coupler, compared with 0.15dB with a VNA.For a 20dB return loss antenna then it is 2.8dB compared with 0.4dB.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 01, 2017, 05:20:37 pm
content: Wow, that's great. Thanks, Kev. Are those measurements what I could expect with one of the VNA you have for sale?Trev

author: KJDS
author_rank: Super Contributor
timestamp: October 01, 2017, 06:27:47 pm
content: Providing it's been calibrated, for that you'd need a cal kit, otherwise it's only as good as the coupler approach.There are usually reasonable cal kits on ebay, Those from Dr Kirby are probably the most reliable route to a good one at a reasonable price.

author: KE5FX
author_rank: Super Contributor
timestamp: October 01, 2017, 07:57:30 pm
content: Quote from: KJDS on October 01, 2017, 03:56:28 pmKeep in mind, though, that this is the difference between an SWR of 1.16:1 for 22.8 dB RL and 1.22:1 for 20 dB. So if you are the least bit budget-constrained, it's better to cheap out on the test gear and put the savings into the antenna system itself.

author: KJDS
author_rank: Super Contributor
timestamp: October 01, 2017, 08:18:55 pm
content: Quote from: KE5FX on October 01, 2017, 07:57:30 pmIt's plus or minus 2.8, so between 17.2 and 22.8dB for a measured 20dB.

author: KE5FX
author_rank: Super Contributor
timestamp: October 01, 2017, 08:53:37 pm
content: True enough, but the difference still amounts to only 0.06 dB of mismatch loss.That's about what you might save by putting another $25 into your coax fittings, especially after they've been exposed to weather for a while.

author: PA4TIM
author_rank: Super Contributor
timestamp: October 01, 2017, 09:46:22 pm
content: Network analysis is very complex. Not something that can be explained in a few posts. My tutorials are just the basics and written for the beginner in this field. If you want to know more, buy the book about this subject from R&S. Be prepared for an awful lot of very complex math. Keysight has a free impedance handbook as pdf. That is more practical.Buying a VNA is easy, making measurements too, making good measurements is harder, reading the data correct even more.You need:- a VNA- if not build intern you need directional couplers and/or bridges depending the frequency. They are not universal usable from DC to daylight. I have a bridge that goes upto 100MHz, a bridge upto 500MHz and a coupler that goes from 2GHz to 4GHz.- a calkit, do not under estimate that, they are expensive upto extreme expensive and only useful if they come with data. Dave Kirby is a good source for N and Rosenberger has very affordable sma kits. See my tutorial about calibration.- For some older analog VNA's you need things like powersplitters and adjustable extenders (telescopic rigid coax tot match phaselength of both paths). A vna has two receivers and one transmitter. Both receivers must get exact the same signal magnitude and phase or know the difference. That is the way they measure. They measure the difference between both receivers. And that is why you calibrate.-decent cables and connectors, not the chinacrap. I got some new and calibrated N cables from a friend. The difference in measurements above 1 GHz is significant (my VNWA goes to 1300MHz but this cables and a good calkit make it usable to 1500). Those cables are 260 dollars a piece !!! You do not need them but just to let you know what is used in the VNA world. Calkits over 10k dollar are not extreme.-fixtures for things that do not have a connector. And if your VNA does not do port extention then you need a special calkit you can connect instead of the DUT. But you can make those and calibrate them with your good cal kit (I often use a working kit that I check against my good kit because SMA connectors wear and I do not have a moment wrench for them so they wear maybe more)I do not know how the normalisation of the modern cheap SA's are but my old ones only do the amplitude for S21. A VNA calibrates at the point you connect the DUT, so on the output of the coupler. My SA's can not calibrate that.A SA gives you the impedance, not the phase, 50 ohm without phase does not tell you if there is resonance (so the reactance/imaginairy part/jX is zero)For adjusting a commercial 2 meter antenna no problem, but for building one from scratch at the frequency you mentioned and then also embedded I do not think a SA will be the right tool. It can be usable if you are lucky. There is a reason VNAs are so expensive

author: tautech
author_rank: Super Contributor
timestamp: October 01, 2017, 10:04:32 pm
content: Quote from: PA4TIM on October 01, 2017, 09:46:22 pmNormaliseon the Siglent SA accounts for coupler and cabling right to the DUT.See the linked pdf that explains this process.https://mediacdn.eu/m/media/downloads/Siglent-RBSSA3X20-Reflection-Bridge-Manual.pdfThanks for your reply.

author: G0HZU
author_rank: Super Contributor
timestamp: October 01, 2017, 10:39:41 pm
content: The 20dB spec for directivity isn't that great for the Siglent bridge. Is it better than this in reality? At the other extreme you could consider something like the Anritsu/Wiltron 6N50 return loss bridge if you can find a used one at a decent price. This offers about 40dB directivity up to about 2GHz and you could make fairly good scalar measurements with this and the Siglent analyser perhaps?I think this is the external RLB that was originally shipped with my old Advantest TR4172 spectrum analyser. This special spectrum analyser had a factory option fitted that meant it could be used with the tracking gen and the external bridge and it could measure impedance like a VNA. But it was very fiddly to set up and calibrate. However, I wouldn't recommend using the TR4172 as a VNA, it was much better as a spectrum analyser. Sadly, the 6N50 bridge was long lost before I bought the analyser. But it still has the impedance option fitted internally.

author: tautech
author_rank: Super Contributor
timestamp: October 01, 2017, 11:25:42 pm
content: Quote from: G0HZU on October 01, 2017, 10:39:41 pmThe info in the link is all that we have for it AFAIK, whereas it would be better to have a proper datasheet or better still a proper characterization chart like is provided with the Siglent SA cables.I haven't got the Siglent bridge in stock to check one and probably don't have the skills and knowhow to do so.I'd like to have shown the tiny SMA coupler performance hendorog gave me in a screenshot but alas I was missing an adapter. Tempted to use a snippet of soft copper wire to get connection between the SMA females.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 01, 2017, 11:27:54 pm
content: Tautech, what is the worst that can happen?

author: hendorog
author_rank: Super Contributor
timestamp: October 01, 2017, 11:48:14 pm
content: Quote from: tautech on October 01, 2017, 11:25:42 pmThat typical measurement quoted has to be at 2Ghz - i.e. a worst case. I'll bet its much better than that at the lower end.Sounds like you have RP-SMA adapters? Damn annoying, just bridge it with a small bit of wire if you haven't got anything else. That little Narda coupler is a 4-8 GHz one, so it will be out of range, and didn't have great directivity in the first place so don't expect too much

author: vk6zgo
author_rank: Super Contributor
timestamp: October 02, 2017, 04:24:50 am
content: Quote from: trevwhite on September 30, 2017, 07:51:40 pmYou don't!Until VNAs became relatively inexpensive, Spectrum Analysers with a Tracking Generator were widely used to tune antennas, as were RF sweepers used with a detector.Hell, Hams often tune antennas with just an SWR meter!Experience teaches various "work arounds" to get as close as possible.The advantage of a VNA is that your don't need work arounds---the thing tells you the whole story!In the past,such esoteric devices as vector impedance bridges were used if it was necessary to look at it the phase angle of the impedance.( a long drawn out experience, as a lot of spot frequencies needed checking.)If EEs & Techs could have had a VNA that was relatively cheap & a usable size back in the day,they would have jumped for joy.

author: tautech
author_rank: Super Contributor
timestamp: October 02, 2017, 05:38:24 am
content: Quote from: trevwhite on October 01, 2017, 11:27:54 pmWell I hummed and harred when I bought mine not knowing anything about SA's and asked here on the forum....I think I've pointed you to those posts, right ?In the end I decided that my early usage cases weren't likely to challenge datasheet spec or the 50v DC input protection so I just shelled out for the Siglent UKitSSA3X accessory kit......but I can buy these at cost.http://www.siglentamerica.com/prodcut-fjxx.aspx?fjid=1558&id=5113&tid=227&T=2Most of what I do is looking/identifying frequencies of interest and much can be done with just a coax sniffer loop where there's little chance of exceeding input spec. Even with the strongest of signals from a coax loop the internal attenuation available can easily suppress it.I've needed little more in the way of accessories to date, only adding some 6" SMA patch leads and straight SMA connectors for hendorog's little directional coupler.Direct connection need be thought of exposing you to two risks: overvoltage and/or signal strength.Internal attenuation can handle most needs and the overvoltage risk can easily be managed if we use that thing on top of our shoulders.....we should anyway.High power/voltage is where things get tricky and you must shell out some $ to get the job done. Period.Go looking for attenuators on eBay and you'll see there's all sizes and shapes that can handle/do what you need.Of course none of us want to be the first to bust our SAbut to start with simple stuff and work forward as you gain knowledge is the path I've chosen. Understanding dB and what that means to the input ratings is important.When in doubt, dial in a heap of attenuation and if the signal's not there, start reducing it until you see what you need. And keep reading whatever you can find.....btw, in the SSA3kX thread, rf-loop has posted many links to great SA documents....grab them all.

author: PA4TIM
author_rank: Super Contributor
timestamp: October 02, 2017, 07:23:23 am
content: Be careful if you need to measure big antennas, like a ham dipole for 80 meters or 25 meters endfed wire etc, there can be a lot of signal on there and static can cause very high voltage. (like kV's)

author: PA4TIM
author_rank: Super Contributor
timestamp: October 02, 2017, 01:08:09 pm
content: Just found some time to test normalisation om my Marconi 2380/2383 with factory standard TG. Goes from 100Hz to 4,2GHzhttp://www.pa4tim.nl/?p=5587The TG output is already flat without normalisation but normalisation is the cherry on the pieMy HP coupler is a -30dB version made for 1,9GHz to 4GHz. It is a 4 port version so it can measure forward too. A thing to keep in mind, this is not the directivity but the attenuation so you can measure on higher power DUT without cooking your SA frontend. So be careful if you buy a DC. Be sure it is not fried and has a high attenuation. My Marconi was a high end SA and has a high enough dynamic range to still be usable with a 30 dB attenuation and the TG at -10dBm but I have seen modern Chinesiums that probably have a problem with that. If the normalisation corrects for dips of 20dB or more and the TG is -10dBm and your bridge takes another 30dB I do not think it will be very usable.I had a Tek 2710 and that TG was far from flat by itself (10's of dBs)My Marconi asks to insert a fixture/devise before normalizationIf I normalize at the DUT port of the HP DC with a short and with an open. The calkit, cables etc I used is all GR and have the same ref-plane. There was no real difference between open and short. That is a good test, they must be the same. So if you cal with nothing on the DC the refplane is at that point. If you use a homemade short that shorts at the and off the barrel that reflplane is much further to the DUT. If I used very bad opens/shorts the formalization was affected a lot. So a good calkit is also here important. In the siglent case an open but testing after normalization with a short and load is a good test. all 3 of the 50 ohm loads I used gave a return loss over 20 dB at the worst frequency. The best return loss peaks where over 40dB. At that point the TG signal is -10dBm, the DC attenuates the respons another 30dB, the RL was 40dB so the RX sees a -80dBm signal. If the noise floor of your SA is at -60 your RL dissapears in the noise floor.You have no correction options for the cal plane with SA's so try to do normalisation at the DUT. Refplane changes are "phase things". Problem there is that the phase transforming will mess up things.If you adjust your antenna to a |Z| of 50 ohms you want resonance too. In that case the antenna shows like a 50 ohm real resistance to the generator. There is a 180 degrees phasejump and the phase is 0 degrees at that frequency. At that point RL is better as 25dB. But a |Z| of 50 ohm does not mean there is always resonance. Often it is, and with a commercial antenna the magnitude alone will be usable.Return loss is usable because it will never be around 20-25 dB if there is no resonance. But things become complicated if you are measuring high frequency stuff and embedded. You need to be far enough from the antenna so you are no part off it. You need to be at least out the reactive field but better also out the near field. For high frequency stuff more easy.But in that case you need a cable and then you measure the impedance at the begin of the cable and that varies with length as long as the antenna is not 50 ohm and resonant. So try different lengths of coax. If the results on the SA changes the antenna itselve is not 50 ohm and resonant. A cable transforms impedance but never to 50 ohm if it is 50 ohm. If the RL is better as 20-25dB you should not see a change if you take a longer piece of coax.A VNA will show you the phase and modern VNA's have advanced forms of error correction, port extention/de-embedding, calibration methods. You need that to measure things like on pcb antennas or other stuff without a coax connector.I wonder what they normalise in SA+TG and VNA's , the output of the generator so it will give the same power over the whole sweep, or just do some math to compensate both receiver and TG aberrations. In old analog beasts they only can flatten the generator but with digital stuff a lot can be done in software but in that case the DUT sees a changing amplitude. For antennas not a big deal but if you measure amplifiers a bad thing. On the other hand, modern DSS generators can be made flat by software control very easyAbout antennas, the analyser (vna or SA) does not only see's the signal from its own source but also other signals and on bigger antennas (like a big shortwave HAM used dipole or longwire) that can give problems with falls readings.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 02, 2017, 04:40:34 pm
content: This is a fantastic thread, thanks for all the replies. Still re-reading and trying to understand half of it.

author: hendorog
author_rank: Super Contributor
timestamp: October 02, 2017, 08:27:28 pm
content: Quote from: PA4TIM on October 02, 2017, 01:08:09 pmThe SNA normalise function on my Signalhound is equivalent to the Response Cal on my 8753 VNA - which is itself the same as just using the Display function to save the trace and then display the measurement as a ratio to the saved trace. These are all done in maths without changing the source power across the sweep.Only one calibration standard is used.If you do that, it doesn't matter what you have connected to the ports (directional coupler or short), or what you are measuring (S11 or S21), the normalise function will give you a flat line at 0dB.Like you said though, the older gear is better at providing a flat source to start with, but as long as the DUT is kept in it's linear range it won't matter will it?