#318: NanoVNA comparison measuring a duplexer - NanoVNA-H4 and SAA-2N

one of the questions i get asked a lot about these nano vnas is are they good enough to measure and adjust an antenna duplexer like that might be used in a vhf or uhf repeater system so it's a good question and my initial gut feeling says probably not and mainly because of the limitations of dynamic range in these little nano vnas compared to a professional unit so let's go take a quick look at a comparison between two different nano vnas and a professional vna here measuring this duplexer for those who might not be familiar a duplexer is really a set of filters that allows a repeater to simultaneously receive a signal on one frequency and transmit it out on another frequency and the filters ensure that the receiver doesn't get overloaded by the high power transmitter the most repeater systems use duplexers that are composed of band pass band reject filters where each path from the antenna to say the transmit and receive has a band pass characteristic to give low loss for the desired frequency in that direction and a band reject filter to reject the frequency that you don't want to go in that direction and then the frequency assignments are reversed for the other side i didn't have access to a bandpass band reject filter set so this duplexer is just a set of band reject filters you essentially set a rejection frequency on one side and the other they don't have quite as good of uh rejection of adjacent signals and things like that but it'll certainly work well enough for this demonstration all right the common terminal for the antenna is here and we've got essentially our low side and high side a little over 464 megahertz and a little over 469 megahertz so what that means is on the low side we'll have a high rejection of 469 and on this side we'll have a high rejection of 464. so we have the vna hooked up between the antenna and the low side so if we look at s21 or the insertion loss or the transmission coefficient we should see a deep notch right at this around that 469 megahertz range okay so here's the s11 and s22 curves for the low side path s11 is just the reflection coefficient and we're seeing about minus 10 or 12 db at around the pass frequency of 464 so that's not too bad but the curve we're really interested in is s21 which is the transmission coefficient and we can see that we've got a good transmission for our low pass or low frequency 464 if we look down around the 469 we're down around minus 100 or minus 110 db of attenuation so that means that the transmit signal is going to be attenuated by 100 db or more on the receive uh port of this duplexer and that's really what you want i mean most repeaters will have an attenuation goal of 90 db or better so this one is a 100 110 you know sometimes eeking down to 120 i think we're just looking right down into the noise floor of this particular vna but you can see why you really need a large dynamic range to measure something 100 db down or more in order to see this rejection so let's take a look at what we see when we look at this on the nano vnas one of the things i should have mentioned at the outset is that for each of the measurements i have shown you and the ones i'm going to show you i've already taken the time to carefully calibrate each of the vnas over this 460 to 475 megahertz frequency range okay so here's the nanovna h4 that you've seen in some of my other videos over the same 460 to 475 megahertz frequency range the same 10db per division that i'm showing here on the professional vna i set it for 12 divisions because there was enough dynamic range to see down nearly 120 db so i had to do that here i don't have i've got only eight divisions not 12. so we can see that the input return loss or input reflection coefficient i should say is about the same but i'm really more concerned with the band reject characteristic because that's the thing we'd be tuning and we can see that if i adjust the marker to about that same frequency range we're sitting down oh about minus 60 minus 60 63 db and we were seeing close to 100 110 db down on the professional vna so that means there's another you know 40 db or so of rejection that this duplexer is offering that can't be seen on the nano vna or at least the nano vna h4 and also you might be might think that the actual notch is down here because that's a little bit lower but i'm not sure exactly why that's happening because we know that from the professional unit that the the notch is actually very close to where i've got the marker here i think what's happening is we're hitting the noise floor here off a fairly steep edge of the filter and then as we're coming up here the filter isn't quite as steep so it's incorrectly showing you that the notch is actually down in this area when it's really over here so you know i think you could do a basic check of the duplexer uh with the nano vna h4 but if you want to measure and adjust to get a rejection that's more than 60 db down you'd probably be hard-pressed to do it with this unit now some of you sharp eyed viewers might say hey that display looks a little bit different from the other nano nano vna h4 videos that i've done now i'm actually running a different firmware on here this one is uh offers a number of enhancements including some enhanced dynamic range this is even better than i was getting before updated the firmware this one is by a developer called dislord and offers actually 401 points in the sweep instead of 101 that we've talked about in previous videos and also offers me the ability to adjust the measurement bandwidth so instead of being several kilohertz that's by default with the nano v8 nano vna h4 i'm using a measurement bandwidth here of one kilohertz note that i was using a five kilohertz uh measurement bandwidth on the professional unit so in theory if everything was equal this would actually show me better results but i think again just the the lower noise floor lower dynamic range of this unit just means that again it's going to have limited use for really adjusting and validating the settings on a duplexer now a few weeks ago i decided to pull the trigger on this unit you may have heard me talk about the saa 2 which is a another nano vna but with a little bit different hardware design and designed to go up to three gigahertz this is a variant of that that has a about a four inch screen and end connectors and a nice metal case so i saw it became available and figured you know again it's pretty inexpensive so let's go take a look at how this one performs now as with the two previous vnas i've also carefully calibrated this over the same 460 to 475 megahertz frequency range and we can see that if we look at the return loss here it's actually hitting the bottom of the screen so i'm going to change the scale for this display to give me let's say 12 db per division instead of 10 so you can kind of see how far down the s21 curve goes now if we move the marker down there we can see that this guy is getting down around minus 70 minus 75 minus 80 db so a lot better than the nano vna h4 you know at least 10 db better maybe even a little bit more than that so i think we're getting closer with the saa 2 in this case the ssa saa 2n but again i think uh it would be good to use this vna to do a check on a duplexer just to be sure things aren't grossly off but in terms of fine tuning that notch i still don't think that these nano vnas are going to have enough dynamic range to really let you optimize the maximum amount of of loss in each of those paths now overall i say that the the saa2 is a really nice improvement in performance over the the original nano vna h and h4 and i like the metal case i like the end connectors available in this version but neither neither of them are really going to have the necessary dynamic range of say a professional vna like this this tech ttr 500 series in order to do a proper job on calibrating or measuring and adjusting uh high rejection duplexers but that being said i think the the vnas are a tremendous value for the average electronic hobbyist that dabbles in rf amateur radio operators and things like that for doing things like that we typically do designing filters maybe looking at the insertion loss of various devices as a really nice antenna analyzer for analyzing the input impedance of antennas and helping to design matching networks and things like that these nano vnas are really an incredible value for well under 100 which is uh orders of magnitude less expensive than a professional unit so other than this one particular job you know i'm still a big fan of the nano vnas unfortunately i just don't think they're up to the job for looking at duplexers so i hope you enjoyed the video learned a little something if you liked it uh please give me a thumbs up if you haven't subscribed to the channel already please do so and thanks again as always for watching take care

Channel: w2aew

Views: 37,938

Rating: 4.974359 out of 5

Keywords: W2AEW, Tek, Tektronix, NanoVNA, NanoVNA-H4, SAA-2N, SAA2, duplexer, measure, comparison, demonstration

Id: GipCVEsiqXc

Length: 9min 52sec (592 seconds)

Published: Fri Oct 02 2020