RSGB: Useful practical skills - tuning a dipole with a NanoVNA

when building antennas it's important to ensure that the antenna is resonant on the frequency you wish to use it on one way we can do this is with an antenna analyzer the analyzer we'll be learning about today is called a nano vna it is available quite cheaply and gives you a handy visualization of the resonance curve of your antenna each time you adjust the antenna length right haley so last time we built a balum to go at the center of a dipole and you quickly soldered a couple of bits of wire onto uh onto the ends of the dipole center and i think you cut those to sort of roughly the right length for a 20 meter dipole i think yeah that's right cool so so how long is each leg at the moment uh i think it's each leg is five meters at the moment approximately great so that's sort of a quarter of a wavelength on 20 meters uh for each half um now we've not played this rate uh this area up with a radio yet but what we're going to do is to try and cut it down to exactly the right length should we head inside and connect up that area as it is at the moment to a vna which is a vector network analyzer and that's going to let us see exactly where this aerial is resonant okay so you've got a nano vna these come in around about the 50 pound mark uh and connected before we connect the aerial we're going to calibrate this uh this vma and that's basically telling it yeah what a 50 ohm uh impedance aerial should look like um and what a short circuit looks like and what an open circuit looks like okay we're going to use the nano vma saver windows application you can do this if you're using a nano vma you can do it all on the device itself um but actually it's easier to connect it to a pc with a micro usb cable um and then just drive it with a keyboard and mouse and it it makes it a little bit easier to uh to use one thing that's worth looking at by the way on this software is if you press the display button down in the bottom left you can use all sorts of different graphs uh with the vma down at the bottom there's the displayed charts option and we've gone for s11 vswr that's simply the the standing wave ratio on that first channel the other one that's useful to know about by the way is up in the options you can enable that show lines tick box if you don't have that on you'll just get a lot of very tiny dots if you enable that show lines then it makes it a lot easier to read the graph great so the first thing we're going to need to do is to set the frequencies that we're interested in and this is a 20 meter dipole so ideally we want it to be resonant about sort of 14.2 megahertz which is in the middle of the 20 meter band so you'll see on there you've got uh the center frequency in that box up top left so if you put 14.2 in there and then we're going to decide how far either side that we're going to look at the aerial and for a sort of starting point we'll go quite wide because we're not really sure where it is so maybe if we go for a span of uh perhaps five megahertz that's that's way wider than we'll uh we'll end up with but it'll it'll give us a good visualization to start with now we're ready to calibrate uh you can select the serial port that your nano vna is connected to that's down in the bottom left so we need to tell the tell the application where the vna is so if you click the little um rescan button that's great so you can see now down at the bottom left it's got the serial port listed as com3 and in brackets nano vma so if you press the calibration button down in the bottom left i'd use the calibration assistant wizard here this takes you through all of the steps one at a time and tells you which connectors you need to uh to use you'll find when you buy one of these dnas it generally comes with a calibration kit and that's a selection of different connectors with uh with different types so in the this case it's going to first of all ask you to connect a short circuit so this is literally the inside of that connector connected to the outside onto port zero that's the uh the top port on the vna we're now going to disconnect it and leave it open circuit so this is an infinite impedance that we're measuring now and again press ok and finally we're going to connect a 50 ohm load so this is the uh the sort of target that we're aiming for it's pretty much a 50 ohm resistor built into a little sma connector so you screw that into that connector and press ok once more that's great now that's all we need to do to look at an antenna if you were going to do something like a filter where you want to go from input to output that's when you'd use that second socket and there are a couple more steps that we'd need to do in the calibration if you were doing that but for an antenna what's called a one port calibration isn't off so you can just press the apply button here and then we'll be uh ready to go we're now ready to connect the aerial so you can unscrew that 50 ohm load and instead connect your coax coming in from your dipole that's the one yep you'll probably need an adapter because most hf antennas will tend to use an s0239 or pl259 connector or perhaps a bnc but you can buy those adapters pretty cheaply cool so if you now press the sweep button what that's going to do is to measure that swr across that frequency range course you can see now the lane rather than being along the bottom of the graph is now a variable swr from 12 to 1 that's very very high at the left down to about five to one on the right now that's that's still very high and also it's looking like that dipped happening at quite a high frequency which suggests that the aerialism isn't long enough now we can't see a minimum at the moment so what i suggest is if you open out that sweep range a bit if you change it to maybe a 10 megahertz span and then run a sweep again let's see what that does cool so you can now see where it's resonant now it's not particularly low swr resonance but it's not bad if you drag one of those markers onto it so grab one of the the red or the green or the blue down there that's sort of sitting at around 18 megahertz by the looks of things um now that's a long way a lot higher than the 14.2 that we want which suggests that our aerial is quite a bit too short for 20 meters now where is it exactly 17 17.6 megs so i tell you what we can do is rather than soldering a bit more onto this area let's make this a 17 meter aerial instead which means we're going to aim for a resonance about 18.1 megahertz so let's get a closer look at this now if you change your center frequency to 18 megahertz and change the span to one we're just going to to narrow in on that area of interest and do another sweep cool so you now see a little bit more detail of where we are and you can put a marker there and find out what frequency it's at so it's currently about 17.6 megahertz now if we wanted this to be resonant on 18.1 megahertz instead of currently 17.6 megahertz that means that we need to make the antenna a little bit shorter higher frequency is shorter lower frequency is longer and to do that we need to work it out we're about half a megahertz 500 kilohertz to lower to low in frequency at the moment so we take 0.5 megahertz divide that by the frequency we're going for 18.1 and that will work out as 0.0276 so roughly three percent so we want to take three percent of the length off of each half of the uh antenna at the moment each half is about a quarter wave so a quarter a quarter of 17 meters about four and a quarter meters uh so if you multiply that up uh 4.25 times 0.0276 that'll come out as around about 12 centimeters off of each leg of the dipole because it's easier to cut stuff off than it is to solder it back on uh we'll go a little bit less than that to start with so maybe 10 centimeters off of each side to start with make sure you cut that off of one side and then the same off of the other side keep that same uh on both legs don't kind of trim it off both one side twice um but otherwise you should be good to go so let's grab some wire cutters and head outside i'm going to take about 10 centimeters off of each end i've got some wire cutters i'm going to measure that with a ruler all right about 10 centimeters will be about there so that's now 10 centimeters shorter i'm going to do that on the other end as well perfect now you might find that the reason that it's not at the frequency we expected it to be to start with is just the height above ground of the aerial if it's if it's lower to the ground it will be resonant on a different frequency than if it's a lot higher up normally in a perfect world you'd want to get the sort of the the middle of your dipole at least a quarter of a wavelength above ground if you can so for a 20 meter dipole you'd want that center up at about five meters above ground or 15 foot sometimes that's not possible but it may just mean you need to adjust that length so now that you've done that you can see where our resonant frequency was before it was about 17.6 if you now press the sweep button again hopefully we'll see that move across to the right to a higher frequency and it has it's moved up a bit although i think that was actually just the the graph rescaling slightly but now that minimum is on the green marker which is 18.0 megahertz that's much higher that's not quite as high as the center of the 17 meter band but you can see you've got it into a much better position ideally for a dipole you'd want to get that swr down to about one and a half to one or less but the reason that it's up at two and a half is probably just the low height of the aerial i suspect if you could just get that aerial a little bit higher off the ground then that swr would all come down quite a bit maybe also get it away from any other adjacent metal bits you can see it's it's still just a fraction too long um but you can see the effect that taking that 10 centimeters off has had if you took another maybe two centimeters off it would just refrain at that little tiny bit higher up uh to kind of 18.1 megs but you know what this is probably close enough yeah this aerial is now resonant roughly where we want it's going to change a little bit anyway as the wind blows it around as it gets wet as the ground gets wet or dry um it will move so you don't need to get it absolutely spot on but you can see now that the swr is better than three to one across the whole of the 17 meter band that's 18.068 up to 18.168 and you can plug this in and make some contacts on 17 meters now if you wanted you

Channel: Radio Society of Great Britain

Views: 3,874

Rating: 4.949367 out of 5

Keywords: Radio Society of Great Britain, RSGB, amateur radio, radio amateur, NanoVNA, dipole, antenna, aerial, amateur radio construction

Id: YTzGjLOV1mk

Length: 12min 28sec (748 seconds)

Published: Wed Mar 10 2021