#196

[@charlesrichards1673]

This is a really superb explanation. The operation of the directional coupler is hard to decipher, and you made it easy. Thank you!!!

[@davidlisney2059]

This is probably the best explanation that I have ever seen for the workings of a VSWR meter. Well done once again!

[@akm3726]

That's an excellent clear description of how these circuits work. I always had trouble getting my mind around how it 'knew' which part of the signal was forward and which was reflected and why. Thanks!

[@mortenlund1418]

Absolutely perfect timing, voice, drawings, logic, testing etc. Thank you.

[@noakeswalker]

Have only just discovered your channel... I've been a radio amateur since 1972, this is the first time I've properly understood what's going on in the VSWR meters we all use - thanks Alan :o)

[@sconzey]

I’m studying for my U.K. full license, and this video was incredibly useful!

[@KCLIBURN-mj9qx]

Thanks for the lesson and theory! Your tutoring is excellent and I like it as well as many others in the comments. Experimentation for me is a little difficult because generators and test equipment are expensive. But I sat transfixed as I've used directional couplers before and really had no idea how they worked until now. Thanks again W2EAW!

[@Derundurel]

Thanks for this. I am mainly a digital guy, but am trying to get my head around RF things a bit more. This sort of guide with practical demonstrations is really helpful.

[@StefanGeets]

Thanks Alan. As always, clear theoretical explanation added with a pratical demonstration !

[@queuerious]

Clarity, brevity, inspiring. Alan, you remain Top Dog in the sharing and 'you can do it too' school of such topics. Thank you for making your channel and contents! I'm off to build your simple Schmitt Trigger TDR to a) measure my 1/4 wave transformers and b) to remind/convince myself that voltages actually DO bounce off impedance mismatches and reflect! I'll have a go at making this coupler, embedded into a QRP AMU that I have plans for....

[@jeromewhelan6723]

Very instructive to see the forward voltage port remain unchanged, while seeing the total voltage trace respond dramatically as the changing termination impedance changes! Having only seen the changing "total" voltage displayed on a scope before, it was enlightening to see the separated "forward" and "reverse" voltage simultaneously displayed. Nice job making the math entertaining! AB3SX

[@hubercats]

A wonderfully clear presentation on directional couplers. I’ve long wondered how such devices, which heretofore seemed almost magical, work. Thanks to your efforts I think I now understand the basic idea. Thanks much! -Jim

[@yoanndelacalle3304]

Thanks!! Great explanation and easy to understand. Thank you very much

[@marcusjenkins]

Alan, I wish just some of my lecturers at Electronic Engineering school were half as good as you at explaining things. Good job. 73, EA5IGC

[@simoncroft9792]

What a masterful explanation! Great video.

[@BaldurNorddahl]

Wow took me 30 years to understand that circuit. I tried to build it as a boy but I couldn't figure out how it would work. Now I know :-)

[@octavmandru9219]

Your videos are like a knowledge milestone for me. First time, I understand very little, but at a later moment in time I come back and watch again, to see how much I understand. When I can basically recite from my mind, I know I reached that milestone. Thank you Alan, your way of teaching is absolutely top notch I have a hard time to understand why you used the coaxial line and not just a bare conductor. Is is that you want to stop electrical field to pass to the toroids, and retaining only magnetic field? If so, I was not aware it is possible to sample magnetic field like that. If I understand correctly, it would not have worked if you connected the braid of the coax in both sides

[@billycullen2779]

Well explained, you have the perfect voice for lecturing and very professionally done.Thank you for sharing your knowledge with us! 73 Billy EI9KB ,

[@godarklight]

Built one today and tested for a home brew radio, although not as pretty it gets the job done!

[@TheHubblethebubble]

Thanks Alan. Very clear explanation of something i couldn't get my head around. 73's PA3DSB

[@basspig]

Good explanation and presentation. The nuances are the core material (permeability) and the number of turns on the core.

[@OctavMandru]

It really is RF magic. That definitely proves Alan is a magician.

[@ledu306]

Thank you very much for great explanation. I have learned a lot of things watching your videos. It would be great if you could make video dedicated to SWR measurements using resistive bridge (without inductors). That can be useful if there is only basic equipment available like multimeters, and also inductors are harder to make, especially for us that do not have much experience. That instrument could be great fot quck antenna performance checking. Thanks again for knowledge that you share!

[@KD0RC]

Thanks again Alan, I watched this and your Basics of Directional Couplers and am finally getting a handle on all of this... I built a return loss bridge and now get the difference between it and a DC. Might round out your series nicely to do a session on RLBs. 73, Len

[@jdmeaux]

Although I thought this was going to be another construction video, you did a great job at explaining what was happening and how it effects other values. Thanks for a great video. Moe K2JDM

[@FirstWizardZorander]

Excellent demonstration, as always. Thank you!

[@Sjampster]

Thanks for the explanation about couplers. The best I have ever seen!

[@МиткоДимитров-о9б]

Thanks Alan. Excellent lecture with practical demonstration.

[@carmelpule6954]

My congratulations on your presentation. Excellent. Would I be correct in saying that since you are sensing both the current and the voltage, and combining them together as you did, there is a relation to the watt meter of yesteryear where two coils were used to combine the current and the voltage hence the power and in this case the voltages are common while the currents (fwd and reflected) are in opposite directions hence the " fwd power and the reflected power are in the opposite direction" Really an electrical miracle to separate the two, as you well describe. May I also congratulate you on your, neatness, your workmanship and also your diction and language , and your handwriting and mathematical layout. You have a very organised mind. I wish I had that equipment as I tend to build all mine and I am not as neat as you are, you certainly taught me many lessons. Prosit.

[@w2aew]

Carmel Pule' Thank you for the nice comments. I will say that it takes me many hours or preparation, research, setup, experimentation, fiddling, and re-writing of notes to *appear* to be organized!

[@crazyham]

Excellent Explanation ! I love this 🙏 Thank You 🙏

[@BalticLab]

Good job, as usual! Thanks for leaving stripline couplers for me ;)

[@w2aew]

Thanks! I'm looking forward to your treatment of coupled transmission line couplers, and circulatory, and all of the other microwave magic bits.

[@HansVanIngelgom]

+KF5OBS I'd love to see you make a video about that subject. I just built a stripline coupler for testing purposes, and although it works great for a first prototype, I'd love to get some more insight in how it works exactly.

[@WarrenPostma]

Fantastic! So well explained. I usually find people skip steps when they find the math simple, and because I saw all the intermediate steps in your math I could follow along clearly. If a viewer can remember ohm's law, the rest should make sense. Visually the way it works is quite beautiful, the symmetry of the design, and the interesting phase reversal of the two on the scope makes the whole thing clear to me.

[@rickwise9910]

I watched this before, and it's wonderful! Coming back to it again, I do have a question about the math: In the first diagram and equations for how it works, it uses -Vr in the current equation, but Vr in the Ir equation [ Ir = -Vr / Z0 ] and in the line current equation [ I = (Vf - Vr) / Z0 ]. This creates an inconsistency, because if I take the line equation [ I = (Vf - Vr) / Z0 ], I should be able to derive V from that. I * Z0 should equal V, and from that last equation, that yields V = Vf - Vr, whereas the first equation says V = Vf + Vr.

[@KX4UL]

Very clear explanation. Thanks for the demo. I really appreciate your channel.

[@Gabbe0x58]

I really like your videos Alan, they are all very pedagogical. I have learnt a lot from you! Thank you very much for sharing your knowledge!

[@richardphillips2405]

I had trouble understanding the reflected voltage part (the seesaw at approximately time 12:30). Thank you for this video.

[@njnear]

You've peeled back a layer of the onion for me. For some reason I had assumed that directional couplers used wave guides, which I've never had time to try and understand. This makes perfect sense. Also, this would be fun to model in a spice program.

[@w2aew]

Some do. Some use coupled transmission lines. There are a lot of ways that directional couplers can be built.

[@WPF465B]

Excellent presentation, as always, thank you.

[@jonahansen]

This is such a great explication. Thank you so much - you are one in a million!

[@우승웅]

Great video for the principles and experiment of SWR. Perfact !!

[@teuluPaul]

Superb explanation - Many thanks!

[@drsaleem1]

Excellent explanation. Physically what is happening is that when there are no reflections the current and voltage waveforms on the through line remain in phase but when there are reflections their phase differ. This coupler is measuring current and voltage wave instantaneous and show any difference in phase as reflection voltage.(in sensing line voltage section Vfwd"=(Vf-Vr)/2N and Vref"= -(Vf-Vr)/2N. No effect on result though!

[@quantenschaum]

Thanks! Excellent explanation and demonstration, very educative.

[@cyborg251]

Great Video, and a VERY good explanation for directional couplers!

[@chrismorley9339]

Excellent presentation Alan - in all respects! Chris VK3CJK

[@gregcook9915]

This video is really well done! Thanks a lot...this has always puzzled me, but you made it perfectly clear.

[@whippoorwill1124]

Great video as always - thanks Alan. Around 11:20, the forward and reflected current samples aren't in phase with the mainline current - they're out of phase with it. (They're in phase with each other of course.)

[@w2aew]

It's been over four years since I did this video, but I believe I had the phase of windings reversed on that one.

[@dave-d]

Thank you. Excellent presentation as always. Really appreciated.

[@the_ALchannel]

Thanks, just what I needed to grasp that topic!

[@gooball2005]

Thank you so much! I've wondered for a while what the theory behind coupling a signal one-way was.

[@jamesmcintyre9985]

Nice explanation. The missing bits are how to determine the core material and cross sectional area of the core, which I assume have to do with the frequency and power level of the main line signal.

[@eljarrito8181]

A minor inaccuracy: when you disconnect the secondary, the primary impedance goes way up because you have a toroid around the line, adding a series inductor. Better to short the secondary to reflect a short to the primary. The effect will of course be more obvious at the high end.. David W Harris

[@Cyrob-org]

Very Simple and clear explanation,perfect....Thanks

[@NatureAndTech]

Thank you. This video and the one about IQ modulation have proved really useful to me. 73 from OZ7JBH.

[@NeedleBender785]

Great video, I am attempting to build this device to be used as a SWR protection device for an AMP. Question that I have is: does the polarity of the windings matter. I noticed that the diagram is a little different then the prototype that you built since both windings were grounded in the middle. Thanks KE8SPI

[@inthenameofjustice8811]

The way I have visualised forward and reflected current between a transmitter and antenna in the past is to think of a bath full of water. If one was to place ones hands in the water at one end of the bath and then push them rapidly forwards, the pushing action would represent the burst of power from the transmitter when the PTT button was pressed on the Mic. The wave generated by this action down the bath towards the other end would represent the power being transmitted through the coaxial cable towards the antenna. The wave will eventually hit the end of the bath and bounce off it and begin to travel backwards towards your hands. In an antenna, no matter how inefficient, some of that energy would travel up to the antenna and be dissipated in air as a radio wave. The wave in the bath travelling back towards your hands would represent the reflected wave. Assuming that this analogy is good enough and I have understood correctly, what an SWR meter is doing, is measuring both the power contained in the initial pushed wave and then the power contained in the reflected wave. These readings may be displayed on two separate meters or, perhaps more usually, on a single meter but with different scales depending upon a switch position set between fwd and ref. Do I have that right and if so, does the analogy hold as a good one?

[@w2aew]

The analogy is pretty good. If the antenna system is matched to the line impedance, then all of the energy will be absorbed at the "load". SWR meters take many forms, some with dual meters, some with dual cross needle meters where the SWR is measured by the intersection of the two needles. Others use a single meter where the forward energy is used to calibrate the meter, then reverse Energy is used to measure SWR.

[@inthenameofjustice8811]

***** Okay, Good. I have been using that analogy to try and show newbies what SWR is but I was not confident it was good enough. Thanks for affirming it and bolstering my confidence.

[@bashirshadi1773]

Thanks a lot for the explanation including the theoretical derivation, Excellent.

[@Brant_Channel]

That's the best explanation ever!

[@iu0itx-silveriosarra536]

Thanks, very clear explanation and demonstration

[@ozioldman4720]

Thanks Allen a great tutorial

[@georgempoteetsr3357]

I've always enjoyed your videos and found this one most informative, keep up the good work. AG5AJ George M Poteet Sr.

[@GodzillaGoesGaga]

Very cool. I've always wanted to inderstand this. Thanks for the great explanation.

[@uprogc]

Very nice! What is missing is the winding direction of the toroids compared to one turn winding direction. And what is the winding direction of one turn ?

[@andrewphillip8432]

Wow that was a great video! Thanks for making this

[@davidcureton9258]

I know this is a very old video but I could never understand how this device could measure power flowing in the forward or reverse direction.. now that I understand it I realise that it dosen't. It actually meausres the impeadance of the DUT and therefore implies the forward and reverese currents. Therefore it is not really a "directional coupler" per se but actually a impedance bridge... Thanks for your excellent videos.

[@siavasheskandary4101]

very high quality explanation, thx

[@krisjk999]

Hi, I have a doubt at 7:45 where you mention about terminating the transmission line only at one end. Does that mean, the field is propagating between the line and the copper ground plate in between the two halves? Another doubt I have is, what is the impact of having a grounded metal sheet (the outer shield of the coax in this case) in between the windings and the core of the transformer? Wouldn't there be eddy current losses in that shield due to the magnetic field coupling?

[@Arijit_VU3ICT]

As always you are great... Thank you..

[@SuburbanDon]

You re a good teacher.

[@urlkrueger]

Fantastic ! The math really helps in understanding this.

[@jvkew]

This is delightful. Thank you.

[@drstrangelove09]

great video, thank you!!! do you have a video on how you built this coupler?

[@aduedc]

Great clip, thanks.

[@stefanpynappels7959]

Thank you for finally explaining directional couplers, why they work and how they are used in SWR meters. I think I get the theory enough now to attempt a QRP SWR Meter using Arduino. One thing I'd like to check, the number of turns on the secondary winding governs the voltage on the test points for forward and reflected power, right? So putting diodes and capacitors on to give a DC reading will be simple, this DC value will be proportionate to the peak to peak voltage rather than the RMS voltage? Also, with an input power of 10W, this should give peak to peak of around 35Vac, so to maximise the use of the 5V ADC on an Arduino, would a turn ratio of 8:1 be sufficient, or have I missed something? Thanks again, your explanation is the first I've felt I've at least partially understood! Stefan

[@w2aew]

Stefan Pynappels The DC levels will be more or less proportional to the peak power. But, it doesn't really matter whether it was peak or average, you're only looking at the ratio between fwd and ref, which won't change re peak or average. You'll have to experiment with turns ratio, core selection, etc. because there are a lot variables that will affect the detected signal levels.

[@drstrangelove09]

I'm not clear, are those black sections sections of coax? is the inductor on the coax ground braid or is it above the center conductor? Do you have a drawing of the details?

[@w2aew]

It can be found on page 17 (27-16) in this document: www.qrz.ru/schemes/contribute/arrl/chap27.pdf

[@mjrtude]

Great job Alan, can you explain what limits the frequency range of the coupler with respect to the accuracy and now w nanoVNAs it should be fairly easy to demonstrate de K9IC

[@Sixta16]

Awesome as always!

[@michaellove7912]

Thank you, Alan.

[@BooktownBoy]

Excellent, thank you.

[@BarefootBeekeeper]

Nicely done.

[@rbtx99]

I'd love to see a video how a Return Loss Bridge works internally.

[@ornithopterindia]

👍Thank you sir.

[@quantenschaum]

I just noticed, that the schematic shown is actually NOT symmetric. The secondary windings (the ones with higher number of turns) are both connected to ground one right side, which breaks the symmetry. The actual unit you built is symmetric, so there is an error in the schematic. The bigger winding of the transformer on the right side should be connected the other round. Right, or am I missing something?

[@clytle374]

Thanks, I learned something.

[@davidlisney2059]

Great tutorial as always!

[@ghlscitel6714]

How can the phase shift (approx. 5 deg) between forward and reverse be explained? How can the characteristis of this design be compared with the classical parallel strip line design in terms of impedance fidelity, and vorward/backward separation accuracy?

[@rs232boy]

Thanks a lot for these explanations, great job !

[@hobbified]

Pretty neat. How big would you typically make N? Maybe 30 turns for a 100W meter?

[@richardphillips2405]

I was wondering if you could apply the idea of a directional coupler to a smart wattmeter that measures power to and from a residence using solar panels where the solar panels is producing enough power to add to the grid. Thanks again for your video on directional couplers.

[@w2aew]

Note very familiar with what is available in the solar power world, but I would imagine that something like that exists already...

[@richardphillips2405]

@@w2aew I suppose that the utility has the smart meters. I would like to understand how they work. I thought that the operation of the smart meter would be similar to the directional coupler. My effort to try to understand how a wattmeter could differentiate power into and out of a device led to a somewhat complicated physics explanation of the electric and magnetic field around an ac circuit. So, I wonder how it is done. How would a wattmeter tell the direction of power flow in an ac circuit.

[@w2aew]

@@richardphillips2405 I think they simply use something like a hall effect current transformer to measure the current, then compare the magnitude of the current to the AC voltage to determine the direction of current flow.

[@buffplums]

Brilliant tutorial

[@bangkok4ucom]

Thank you for the knowledge of swr. from E21NZC QRU 73

[@homemdosaco2000]

Great video. What about the diodes' forward voltage drop? That introduces an error, right? Should be negligible in high power applications, but for small signals, wouldn't it completely squash the coupled signal?

[@WESDEANEMARTIN]

So, with the use of direction couplers, the positioning of the meter along the feedline would not matter given that it isn't measuring the instantaneous SUM of the for/rev amplitudes (standing waves), but instead measuring the RATIO of the for/rev amplitudes individually as they move along the cable? Thus, this meter can be slid along to any point in the feedline and indicate the same value?

[@w2aew]

The values read will vary with position if there is a standing wave, but the ratio of the measurements should stay constant.

[@SeAfasia]

Thanks Alan,very helpful tutorial!

[@Drforbin941]

Excellent video.

[@许官人]

thanks Alen. it helps a lot

[@072032786]

Thank you very much this gold inform.

[@Steven_Bennett_YT]

5 years old but still a classic of comprehensive and clear explanation. I would like to construct your directional coupler so may I ask what cores you used for the transformers? Many thanks.

[@w2aew]

It is modeled after the coupler in this article: www.arrl.org/files/file/Product%20Notes/2012%20Handbook/KAUNE.pdf

[@0MoTheG]

The key here is that with all values being at the matching point the voltage that is induced on the coupled side is the same in both inductors. That results to zero on the reflection side and whatever you chose on the reference side. To maintain matching even though there are inductors in the line they have to lower and raise the impedance to the same degree as seen from the input port. Seen from the output port the load is not equal to the line.

[@K7AGE]

I now understand how it works! It would have been nice to add the diodes and measure / calculate the SWR.

[@w2aew]

Thanks Randy. I thought about it, but the video was already over 20 minutes long, and I like to keep my videos shorter. It would be a good follow up video though.

[@K7AGE]

I have a QRP bridge kit that I bought from KitsAndParts that I will be building and playing with. I am thinking about interfacing it to an Arduino to measure and calculate SWR.