Finding Loop antenna resonant frequency

hey folks Paul w1 VLF here tonight we're gonna be testing some loop antennas that I made receiving loops somewhere for the broadcast band somewhere for a left somewhere aircore some were ferrite and I'm just gonna show how you can tell if your loop is resonant whether you're in the ballpark of what you thought you were winding and ways to fix them if it's too high in frequency or too low stay tuned hey everybody my name is Paul welcome to the W one VLF lab got an interesting video for you tonight over the years I've made a lot of loops here's a couple enameled wire loops let's wire ferrite core loops for long wave frequencies oops air core just literally 50 maybe more loops of different sizes and dimensions so here's the question there's plenty of videos on YouTube how to build these loops and people say to me quite often geez I'd like to build a loop like I see on videos on YouTube but I don't have exactly the same materials and I don't know exactly how many turns to put on it and so tonight we're gonna take a look at one of these loops actually we're gonna take a look at a whole bunch of them different kinds and we're gonna find out what the resonant frequency of this loop is and this loop and this this is a coil but we're still gonna find out what the resonant frequency is it's very simple you just need two things so we'll sculpt in an RF generator I'm gonna set those up here on the bench and show you how this can be done oh and a clip lead clip leads very important so we're gonna set this up on the bench I'll be back in a few minutes for you it'll only be about maybe a couple of seconds and we'll find out what the resonant frequency of this loop is if it's too high then we can take a turn we can add a couple of turns if it's too low we can take turns off so you don't have to be afraid that jeez I don't know where this loop is will it tone what frequency is it on build the loop with the materials that you have you'll see how non-critical this is build it with the materials that you have and then go from there to tune it properly to get it into the frequency range that you want this might end up being two parts but anyway I'm going to set up and I'll be back okay I'm back well at least pardon me is back so what do we have here you have a RF generator here in this case it's an old Heath excuse me HP function generator I think it's good 250 megahertz yeah 50 megahertz in an old oscilloscope now if you're an experimenter there's no reason in the world not to have at least these two items on your test bench or your bench 25 bucks 25 bucks at flea markets so it's not a bunch of exotic test equipment it's just run-of-the-mill stuff that anybody anybody can acquire rather cheaply okay so right now I just have the RF generator connected directly to the scope probe and you can see there's a display on the screen okay all right so there's a nice nice RF display I think we're at two and a half or two point seven eight megahertz right now so the first coil that we're going to test or not coil but antenna this is a roughly tuned somewhere in the middle of the broadcast band this was something that I had built an awful long time ago it's litz wire it's mounted on an eighth inch piece of PVC cut it off with a with a table saw and then smoothed it out so that you know so it look decent not like the rest of the stuff around the lab here and then of course they just poked a couple little holes right here right there that one Marilyn and that's where I bring the wires out so I don't remember the inductance of this I don't even remember where he tuned it where I made it for but it's because it was probably 10 years ago so first thing we're going to do is we're going to connect this to the cell phone okay it's not critical which side is which so I'm going to put the ground lead here and the actual scope input side there and adjust the game nothing so now we take the RF generator let's see can you see the loop can you move it around so you can see the loop better you can see both of those connections yeah so the connections no you can't connections are here and here is the RF generator test the output from the RF generator right here so here's where the quick lead comes in simply take the output this is the ground weed this is the hot lead and connect it to the cookie okay there now with this generator running driving this we're going to be coupling some energy into this loop but it's obvious that we're not at exactly the right frequency so now's now comes a time to sweep the generator to move it physically in frequency up or down chances are with that many turns that closely-spaced it's going to be around the megahertz or so something like that so let's do that let's let's let's move the frequency down we're at two megahertz now 2.78 let's go down to one point seven eight and when we hit resonance you're gonna see this amplitude jump off so now that we're at one point seven eight we're gonna go down in smaller steps you could already see again getting bigger okay and go down down down okay so we're down to nine hundred kilohertz I guess I'm a little off with what I remember about this coil and now at 800 kilohertz I have to turn the gain of the scope down one step now this is the fine tuning knob you could just as easily do this with a generator with a with a physical knob in fact it's actually even easier so all by itself this coil is already resonant and we'll find out exactly where okay so what I'm looking for is a peak on the oscilloscope about 870 kilohertz very very simple and now you're gonna see this change because I'm coupling more or less energy you want to be able to couple some energy into that loop but not so not so little that you can't tune it so watch I'll move up a mega cycle and it'll trigger 100 kilohertz and it'll drop another 100 kilohertz and it's gone alright so this one's about 870 Kelvin so let's grab another one block the show so here's loop so you can't use this to receive anything above 870 kilohertz so if I wanted to make this into a broadcast band loop I would take a couple of turns off it would move the resonant frequency up and then we retune it with a capacitor I'll show you how we retune it with the capacitor here in a second so we're disconnecting this this one and here's another another loop this one was designed to be like this to stand up on end you still see the scope let's see this out of the way regardless whoops there's still I'm gonna just lay it flat we're gonna connect this loop up to the center lead on the scope pardon my old eyes and stuff gets smaller every year and to the ground and there's a little bit of coupling there my guess is that this has a lower resonant frequency because it's got more turns so let's drop down and see what happens as we go down and going down in small steps you're seeing it grow well past it okay so this one is tuned to 620 kilohertz now what do you mean it's kind of resonant frequency okay you're seeing that the coupling difference changed between the two wires between this this is essentially a transmitter this is a receiver this coil has inductance but it also has distributed capacitance between all these turns so you have inductance with parallel capacitance equals a parallel resonant circuit and at the pitlane parallel resonant circuit is resonant at the specific frequency energy will couple from this one turn coupling loop here that I made into this loop let's move on let's go to something else a little bit bigger I mean a little bit more inductance so this is a sweet yoke off of some old ISO scope I think I'm not gonna change anything except for connected up to the connected to the oscilloscope okay now you don't see any you don't see anything there because this is going to be resonant much much much lower than any of the other coils because this has a million turns if you turned took the tape off you can see there's a million turns on there so we're gonna drop down 100 kilohertz at a time we're done at 420 kilohertz 322 2120 now we're down at 92 kilohertz so I'm gonna start going down in smaller increments and at some point sure make a light on it you turn the gain back up a little bit at some point you're going to see that fill the screen again where it comes very close here comes whoo this coil has a very bad cue that's why it's a doesn't fill the screen quite as well but you can see that there's a definitely a frequency where this is resonating sulfurous eighty seven and a half killers that's not really good coil it's not designed to be an antenna it was designed as a deflection coil let's move on to something else this was an antenna that I used to manufacture for the 24 kilohertz band for people who wanted to do sudden ionosphere disturbances this was with the right capacitor this will resonate at 24 kilohertz but as I said it has to be higher in frequency so the capacitance can pull it down in frequency let's hook it up and see where we're at obviously this is a ferrite core there's two large ferrite cores so this is not a purely air core inductor or loop antenna all right so at 87 kilohertz there's not much going on there so let's go up in frequency look there we go okay so which yeah stand by back it up okay where is this one wasn't it right there okay okay so this one is resonant again it's not a really great coil let me let me get a little more coupling in there let's put the coupling a little closer now that when I say coil I mean anytime it's not okay so now we have this loop from the RF generator coupling more energy and so we can turn to sculpt gain down now we can move up and down okay so this Peaks out at 137 kilohertz when you put capacitance in parallel with it then we'll bring the resonant frequency down okay so let's go back to one of the first loops that we made that we were looking at because it's easier to do this test with see if our RF coupling loop here go back to this one this is that litz wire loop I'll connect that back to the scope and the other side to the scope all right and I think this was like 900 kilohertz so let's go up in big steps okay using the vernier peaking it out 970 kilohertz so that's like a little boy of the middle of the broadcast man so let me bring in another tool that's really cool for doing this this is a little jig that I dealt as you can see it has little one of those little Russian capacitors you can buy - a little nice vernier and everything and I can put I can connect this to the loop and show you what happens as you tune it by putting the cassadines in parallel so here's the output of this little variable capacitor and I'm gonna put this right in parallel with the loop you probably can't see this very well okay see if you can if you can see that better can you see that better yeah that's not I'm looking at a little screen that's about this big so it's difficult okay so you notice there's no no signal there or very little and geez when I open it all the way up so if I put capacitance across that loop which way do you think it's the resonant frequency is gonna go up or down it's going down so let's move the generator down from 870 okay there we're just a bare minimum of capacitance we're not the bare minimum there's the bare minimum of capacitance get this a little bit more gain this is what you're doing when you tune that loop you're moving when you tune your am broadcast loop or your your loop that you're trying to use for receiving signals you are moving the resonant frequency so I'm going to add some capacitance over here you see it falls off now I'm going to move down in frequency so now the loop is resume at seven at seven sixty I'm going to put a little more capacitance in now the loop is resonant at 680 put in a whole bunch more now it's resident 570 kilohertz it's dumped in a bunch more okay you notice it always Peaks up sometimes a little higher than others but now we're resident 475 kilohertz so again I'm going to take this off the capacitor and here the freespace resonant frequency of this loop is what what do we say eight something eight seventy so it's too low already for the top end of the broadcast being so if I was gonna sacrifice this loop I would disconnect these wires and I would take two or three turns off and resonant frequency would be much higher and if my resonant frequency was too high I might add a turn or two and watch move my generator to discover where its resonant so you wind a loop basically like what you see on one of these other build videos test it using this method and you find out it's off so you know if it's too high in frequency you either need to add some capacitance or an additional turn or to add two turns come back test it again move your frequency generator up and down okay so now we're going to go to another coil just so we can keep going with this all right what I do with that know where they put it okay let's go on I'm gonna have to stop for a second all right I'm back or at least part of me is so here's another coil this is gonna just look at that small diameter closely wound this is going to be higher in frequency than those low there are other larger coils let's see if that holds true okay we're connected you still have our coupling a loop we're gettin we're going to put this loop in the middle of the coupling to get a bunch of energy into it and now we're going to move the resonant for you let's go up and see what happens okay so that tiny little loop right there or coil has a self resonant frequency of 2.4 mega Hertz now we just go over here and grab a ferrite rod all right we're back let's put this coil a little closer there okay so this coil is resonant now what do we say yeah something like 2.3 mega Hertz right so I'm gonna put a piece of ferrite in it then it drops why did it drop thought the fairy antennas worked really well well it moved the frequency down so let's find that spot again there it is in fact I think it made the Q of that coil better take it back out so now it resonates at 1.1 measures by the way always always always mark your ferrite materials this is a number 61 rod this is the number 33 rod can you tell any difference this one has some effect this one has a different effect ferrets are designed for various frequencies always mark your favorite rods so you know six months from now you're gonna look at those ago the 31 which were 33 which one's a 69 so again not a build video or anything like that but just a way for average Joe with 50 bucks worth of equipment well wait a second I have an air core a larger air core over let's take a look at that one I was gonna end the video but I'm not going to now do I grab that other loop okay all right I showed you this loop before right this is bigger a big air Court you probably can't see it that well but it's a big air core loop so we're gonna lay it down here on the bench the group W bench and there's a lot of capacitance between these leads on the turns on this coil because they're tightly bound together and there's a lot of area so here we are with one megahertz the coupling loop is in there I don't see anything on the screen so let's go down in frequency 100 kilohertz at a time nope there it is so that loop by itself it's resident at 242 killers so I must have made this for the long way band and then by putting a capacitor in parallel with it lump constant parallel with it you're able to bring that resonant frequency down wherever it wherever you want so that's the scoop any questions let me know this is w1 v love signing off and remember I'm good in the call book okay so the shops all back together as much as you could tell you this shop is back together I just want to say if you're watching this video and you have a question leave it in the comment section I don't seem to get many comments and maybe it's because nobody's watching but if you do had some interest in this let me know up or down at least I know if I'm doing something right or wrong thanks again for watching and oh if you could subscribe and then hit the notification bell just to the right of the subscribe button that would be great thank you very much 73 w1 bill F

Channel: W1VLF

Views: 30,458

Keywords: AM, AM radio, AM Dxing, weak am radio, more am radio stations, Loop antenna, tuned loop antenna, ferrite loop antenna, build your own ferrite antenna for am, loopstick antenna, Medium wave dxing, medium wave antenna, AM radio antenna, Tuned loop antenna, Resonant frequency

Id: r6o0s_0fKVY

Length: 23min 22sec (1402 seconds)

Published: Thu Oct 31 2019