158 Directional Coupler Basics how to sweep SWR of an antenna Return Loss VSWR




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in today's video we're going to introduce you to a directional coupler we're not really going to talk about the theory of how they work but more so you know what their characteristics are how you might use them and how to interpret the specifications unit I have here is from many circuits it's a model ZF DC - 20 - 5 Plus this is the device right here so let's take a look at really what a directional coupler is and look at some of the specifications and what they really mean you know we go and hook it up to a circuit what does it actually do a directional coupler kind of an interesting RF device really internally what it consists of is kind of a main transmission line path okay called the main line and it's intended to have very low loss typically less than a DB or so but depending the depends on frequency of course and then there's also a coupled line okay now the idea is that when you have power traveling from the input to the output a portion of that is coupled to the coupled line but it's only for a signal that are traveling in that direction for signals traveling in the other direction there's very little coupling into that coupled line now really directional couplers are kind of bi-directional devices and oftentimes if you have a directional coupler that just has a single coupled output it means that the other coupled output which is coupling from the other direction is internally terminated and that's kind of represented here in the schematic so for this device we've got a main line path that goes in this direction and our couple paths that goes down that way and then any return or reverse power coming this way will be coupled very little to the end now all directional couplers whether they're made from coupled strip lines or RF transformers will always have a usable frequency range and that will usually be printed prominently on the device or certainly listed in the datasheet this particular device is rated from 100 kilohertz out to 2 gig earths so it's a pretty nice broadband directional coupler okay so one of the prominent specs is the main line loss and this is how much power is law from the input to the output and here's some typical data for this device over its operating frequency range and we can see it's typically you know a DB or less it goes up a little bit closer to 2 dB at 2 gigahertz but it's under a DB down at lower frequencies so all that means is that we're going to have essentially very little loss in that forward direction or anywhere along the main line from input to output the next important spec is called the coupling spec now the coupling spec basically says for that coupled output how many DB down is it compared to the signal on the main line and we can see for this one that is typically about 20 DB it's typically actually just a little bit under 20 DB but we can call this a 20 DB coupler so what that means is that the signal coming out of this port here will be 20 DB down from the main signal going here in terms of voltage that's a factor of 10x so let's take a look at that on the scope okay I've got an input signal here 20 megahertz coming from the function generator into the input port of the coupler and that is going out the output port and that's going into channel 1 of the scope and that's the yellow trace here it might be able to read down here the RMS value about 225 about 225 million volts and the coupled port right here is going into channel 2 of the scope and I've purposely put a channel 1 and channel 2 on the same scale 100 millivolts of division so that we can just visually see that it's about a factor of 10 and amplitude and you kind of see it's almost exactly a factor of 10 I'm looking at twenty three point seven million so just a little bit less than a factor of ten and that kind of makes sense we looked at this the typical performance specs here we're typically just under 20 DB down at the lower frequency range so that's what we mean by you know the coupled port ratio or the couple port spec but the signal coming out of the coupled port is about one tenth of the power in this case 20 DB down of the power in the main line in the forward direction from the in to the out so the next spec to talk about is directivity directivity is this is how many DB down the couple port output will be compared to this value when the signal is going in the opposite direction so it so you really almost want to add up these two values the coupling and directivity if you want to figure out what to expect out of the couple port when you send power in the reverse direction so we can see here I've got numbers 25 33 34 you know in the 30 35 DB range of directivity so that for the reverse power we'd have that 30 35 DB plus the 20 so we're going to be down about 55 DB in the reverse direction so that's a lot to the attenuation so let's take a look at that on the scope okay so what I'll do is I'll just reverse the input and output so I'll take my signal generator off here take the output test signal going to channel 1 of the scope and go to the input and go to the output here so now if we take a look I'm still getting my full signal through the main line path the main line loss is basically the same in either direction but now that couple port is barely visible and it's only reading in the hundreds of micro volts now probably just looking more at noise but it's going to be you know 55 DB down or more in that reverse direction we've been looking at this currently with both the main line properly terminated into 50 ohms as well as the coupled line properly terminated into 50 ohms at the scope inputs and of course with the main line path being properly terminated there's no reflected energy coming back so there's that no energy coming back through the coupler in this direction and that's one of the reasons why we're seeing very little signal here now an interesting use for a directional coupler is to measure how much reflected energy we've got coming back from a load and might be for tuning antenna or a diplexer or something like that so let's take a look at what happens if I miss terminate that load so if I set the scope channel termination instead of being in 50 ohms I set it to 1 mega ohms what that means is that effectively all of the energy that is being that is coming to this end of the coax here is being reflected back down the line back towards the generator so that means that I've got signal now flowing through the input of this coupler through the output port back to the generator and that's essentially the forward direction for the coupler so now I'm going to see a signal that's 20 DB down from that reflected signal coming out to couple port and there I am they're back again with the 24 millivolt or so reflected signal coming back so this gives us a way of essentially looking at reflected energy coming back from a Miss terminated line or properly terminated there's nothing coming back and we're miss terminated and we've got a reflected signal coming back that's what we can see here is that the coupler can be used in either direction when used to tap off essentially a signal in the forward path and you could essentially get a sample of acidity being sent out to a load or an antenna or something else maybe a power amplifier and that couple port could be used to maybe go into a frequency counter or a spectrum analyzer to monitor the signal or maybe into some more sophisticated automatic power control loop or something like that when used to in essentially the reverse direction if you will measuring power coming back from a load that can be then useful like I showed here to monitor you know how much energy is reflected from a load and some kind of an RF system could be an antenna or something like that so let's actually go take a look at a practical way that we could use a coupler like this along with a spectrum analyzer and tracking generator to essentially sweep the range of frequencies into an antenna system and measure where that's resonant and essentially what the match is okay so we've got the directional coupler hooked up into the spectrum analyzer with the tracking generator and it's generally pretty good practice to put an inline attenuator like a 10 DB pad at the output of the tracking generator and the reason for that is if you're feeding into an antenna out of the out of the coupler who's that's not resident over a wide frequency range you don't want that varied impedance to be a direct load on the tracking generator so by putting a 10 DB pad in there it keeps the impedance that the load that the tracking generator is seeing relatively constant even if the signal at the output of the attenuator varies a bit over frequency with the antenna so we're coming out of the padded tracking generator into the output port of the coupler the reason we're going in the output and putting the antenna at the input is because we want the coupled signal coming out of the coupler to be representative of the power reflected back from the antenna okay we've got the analyzer set up to be center frequency of 20 megahertz a span to 10 megahertz the antenna that I have hooked up here is tuned roughly to the 18 megahertz band 17 meter amateur radio band might be a little bit off of where I had originally tuned it because it's here in the basement and not outside at a proper mounting position okay so we'll turn on the tracking generator and in doing so I can see right away that I have a bit of a dip here and with that dippers representing is essentially return loss return loss is a measure of how well matched the load is to the transmission line and the lower the value okay more DB down is better it means that you've got less and less energy reflecting back so what this is showing me is down at the 10 megahertz end and up here at the 30 megahertz end we've got a lot of energy being reflected back in fact almost all of it and then there's a particular frequency where the antennas resident where the antenna is absorbing and radiating all of the power and that's the point that we want to measure that's the resident location or resonant frequency of this antenna before we get a real accurate measurement what I want to do is normalize the trace so what we'll do is we'll set up the coupler so that all of the energy is being reflected back so I can use that as a reference so I'll simply disconnect the antenna and now I can see I get a flat line because that over this entire frequency range all of the energy is being reflected back and we're just essentially seeing you know our maximum coupled output so we'll use that as a reference whereas now every analyzer is going to be different than how you set this up in this case we go to trace math and go to normalize and I'll do an instant normalize and what that will do is measure that that value and give me a display line it might be able to see in red and then the measurements will all be with respect to that so now I've got things normalized I can go and hook my load back I can now go hook up the coax that's going off to the antenna and there we go so now my measurements will be a bit more calibrated and I can just throw a marker on here and if I move that marker around if I move it right down to that minimum point okay we can see that right about there and that's right at about 18 point zero nine eighteen point one megahertz or so we're down about - fifteen - fifteen point eight - fifteen point seven DB from our normalized value up here so that tells me that the return loss at resonance is about minus fifteen point seven DB or so okay of course knowing the return loss - about fifteen point seven DB or so we can calculate SWR or we could issue is a nice chart there's a really nice chart I picked up on line many circuits and if we scroll down here and look for that Oh fifteen point six fifteen point seven number we can see that refers to a VSWR of about - we knew one point four to one one point three nine one point four to one that's a pretty darn good match anything below about two to one is generally acceptable and and for all intensive purposes anything below about one point five to one you can consider you know pretty darn near perfect so that tells us that at that eighteen point one megahertz frequency we should have an SWR of about my about one point four to one so let's go verify that real quick there's one other tool we can use to verify the SWR versus frequency as an antenna analyzer like this MFJ 259 i've got the antenna hooked up to the test port up here and we can basically just adjust frequency watching SWR looking for that dip and once we find that dip we can fine tune it by watching the meter up top here and I can see that it's probably right about here and I'm right about that 18 point zero nine megabytes that we saw the spectrum analyzer SWR 1.4 that agrees to what we measured using the return loss method with the directional coupler on the spectrum analyzer and tracking generator so both of them can be used to make this measurement but the using the tracking generator on the spectrum analyzer it gives you a nice visual picture of what the return loss is versus frequency and that might be handy when looking at the rezident properties of a multiband antenna or if you're trying to tune like a resonant cavity for like a duplex or in a repeater system or something like that so I hope you learned a little something about directional couplers and what they do what the specs mean and a couple of ways they could be used in RF systems thanks again for watching comments are always welcome and if you're not a subscriber please subscribe to the channel thanks again for watching