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

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

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!

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

What a masterful explanation! Great video.

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

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 :-)

Excellent demonstration, as always. Thank you!

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

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)

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

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

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

Superb explanation - Many thanks!

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

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

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.

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

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

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

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

Thanks Alan. Excellent lecture with practical demonstration.

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!

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

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

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

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

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

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....

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

Thank you. Excellent presentation as always. Really appreciated.

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

Excellent presentation Alan - in all respects! Chris VK3CJK

Excellent presentation, as always, thank you.

Thanks! Excellent explanation and demonstration, very educative.

Thanks, just what I needed to grasp that topic!

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

Thanks Alan,very helpful tutorial!

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

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

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

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

Thanks a lot for these explanations, great job !

Great tutorial as always!

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

This is delightful. Thank you.

Wow that was a great video! Thanks for making this

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

Thanks Allen a great tutorial

Thanks, very clear explanation and demonstration

Awesome as always!

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.

You re a good teacher.

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

great explanation, thanks

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

very high quality explanation, thx

thanks for uploading this vid..

Thanks a lot for this excellent explanation

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.

Excellent video.

That's the best explanation ever!

Nice video. Thanks for sharing

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.

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

Great video! Thank you!

thanks Alen. it helps a lot

Great clip, thanks.

Fantastic ! The math really helps in understanding this.

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?

Thank you very much this gold inform.

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.

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!

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.)

Brilliant tutorial

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

Nicely done.

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

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?

Very didatic. Thanks for this lesson.

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

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

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

Great tutorial, thank you! I have one question: What is the type of material you use for the transformer core and what are the frequency limits in which the directional coupler is expected to work?

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.

Excellent, thank you.

Hi. Thank you for the video. What´s the range frecuency that it works? and how can i design the transformer for a range of 50 to 100 Mhz. I need a directional coupler in that range. Can you give some tips?

Thank you, Alan.

Thanks, I learned something.

Do you think it would be possible to obtain a similar circuit, replacing the transformers with a gyrator? Then measuring voltage across a gyrator load-resistor on the other end?

Thanks for the clear explanation. Would the lower core saturate before the upper since the turns ratio is inverted, hence imbalance as the power increases?

Thank you for sharing.

Excellent presentation... as always! :) 73 de k1hop

73 thank you for very good explanation..

thank you, one of the best!

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 ?

Lovely presentation that made me subscribe for the first time. Thinking of buying or building a antenna analyser. Have you a presentation of the Math for calculating the impedance and inductance of an antenna. Is it done by comparing vswr for two given known frequencies? Tried to find the priciple used by something cheap as the sark 100. 73 LA6FK

Thank you very much for teach me. How many turn are in transformers for 10 Mhz and 80Mhz signals?

A very informative lesson and explanation...Thank You! So, If you construct an antenna system correctly, there will be NO Reflected Power which means ALL of the power from a Transmitter will make it to the antenna? Is it critical,when constructing the transformers, that both have equal values?

Hi w2aew, your video is quite helpful, like usual! I made a copy of your design myself and it works. But I have a question in the construction: we connected the coax shell to the ground, will this blocking the signal going through the core?