#112

[@hankhamner3671]

Hi Alan, I used your approach and it works so long as the measured impedance is close to 50 ohms. However, I tried something a little simpler and compared my results to both two separate calibrated nanoVNAs and my calibrated RigExpert AA55 and my results came out within 5% of these analyzers. The two nanoVNA's were within 2% of each other and the RigExpert was a little further off. My circuit is simply a 1% 220 ohm 1/4 watt carbon resistor connected in series to a function generator and an HF antenna. A 200 MHz digital scope with phase measuring capability set up for 10X probing on channel 1 and 2 was used. No 50 ohm terminations were used or needed because all I was interested in was relative RMS voltage at the top and bottom of the 220 ohm resistor referenced to ground and the phase difference. I plugged the two measured voltages at 7.213MHz and 14.250MHz along with the phase into an Excel spreadsheet to calculate the load impedance, return loss, VSWR, both rectangular and polar coordinates and reflection coefficient. I used the scope's averaging to get the voltage to settle on the sine waves. The results I got were within 2% or better of the nanoVNAs and within 5% of the AA55 RigExpert antenna analyzer. I have not tested large impedances relative to the 220 ohm resistor but I suspect if the impedance is between .1 to 10 times the reference resistor (22 ohms to 2200 ohms) I should be reasonably accurate. I would appreciate your opinion on my setup and any issues you see. I may just have been lucky to get close measurements. I got the idea to try this simpler approach from Kenneth Kuhn's article titled "A Simple Circuit for Measuring Complex Impedance" "www.kennethkuhn.com/electronics/impedance_measurement.pdf" Thanks and I really appreciate you taking your valuable time to teach all of us electronics. Hank Hamner K5HHQ

[@achernarscardozo69]

Gracias por responder. Creo que los valores que encuentras son más que suficientes para una estación de aficionado sobretodo cuando estos se encuentran en las proximidades de los standard y como tú dices además y es que finalmente el transmisor “ve” todo lo que haya más allá del conector. Siempre me ha interesado la medición real de la impedancia de una antena de HF y esto no es muy práctico por la dificultad a un acceso directo a la misma o al diseño de un analizador de características muy especiales. Abrazo.

[@OctavMandru]

Dear Alan, 9y later your lectures still provide a masterclass on RF and EE. It is great to see you juggling with numbers like that. Thank you Sir Regards from yo4ocm

[@bain5872]

You have an amazing ability to make one of the most complicated subjects into a very straight forward and most understandable presentation! I wish you had been my lab instructor back in the day! I sit in amazment at your soft and to the point teaching style with a hint of a very understanding attitude. Absolutely fabulous knowledge and teaching style! Kudos to you Sir....! I admire your vids.

[@w2aew]

I'm glad you liked it!

[@wa9kzy326]

100% agree with your views.

[@rusty1187]

I agree! I have had maybe two good teachers in my life, and you, sir, would be the third.Easy to understand, and you don't talk down to us all like a certain lab channel that i watch.

[@jgwimmer]

I'm a software engineer, but started out working on my EE while I was in the Air Force, 21 years in the Comm business...you've given an absolutely fantastic instructional video. Easy to follow along, easy to understand, and outside of the math for some folks, a very easy to understand way to apply theory to real life. I applaud your efforts and hope you do many more instructional videos to help those who don't quite understand.

[@w2aew]

Thank you! I've got 187 videos and counting! I hope you enjoy the rest as much as this one.

[@tomsmith3045]

Just finding this today. This is such a handy thing! Just a simple bridge circuit, but never thought to do it to tune a circuit. I'm going to have to build one of these. For weird bands where you don't have an analyzer, or for working with 75 ohm systems, it's a great trick to have up your sleeve.

[@wa9kzy326]

For a few scraps of PCB, some connectors, and resistors, this is a brilliant method to "see" the workings of an antenna/transmission-line system. The advantage of the 'scope is it's very high voltage sensitivity-10 mV/div. It will work with a favorite of amateur operators--MFJ259B as a signal source. In spite of its limitations, it's worth a try. And voila, resonance and resistive match in one fell swoop. This is an excellent candidate to sell as a kit. Your level of practicality is amazing and a badly-needed adjunct to the amateur "rags" of today. Words of thanks are barely adequate.

[@AppliedScience]

That's pretty cool! I've never seen a visual antenna tuner, but that makes a lot of sense. Good idea.

[@MochaboyRC]

That was fantastic...Very concise and easy to follow. I make my own high frequency antennas (433, 1.2ghz, 2.4 and 5.8) and I use a DIY SWR meter, but this fills all the holes for the other measurements I was looking for. Very handy technique - thanks for documenting.

[@w2aew]

to John Brown - Yes, this is not uncommon - and is one of the things that makes analyzer like the MFJ-259B difficult to use on low-band antennas sometimes. A bandpass filter can often help, but it all depends on the frequencies of the incoming (interfering) signals.

[@gelubotezan3917]

+w2aew I couldn't find John Brown's message so I just suppose he had problems trying to measure with his analyser into the lower HF bands. I use a VK5JST analyser and had problems with a MW Broadcast station about 25 km away. I just connected an 1:1 choke balun between the line and the analyser and I am able to steady measure in all bands. Measurements confirmed by trcvr working with the antenna as set up with the VK5 JST analyser

[@skyking6989]

Ham radio operator and currently studying Electrical Engineering. Ham Radio and electronics has always been a hobby. Now a career

[@MarceloMoraes]

An excellent explanation on how to tune an HF antenna that I ever have seen. Congratulations! Great!

[@davidlong2691]

So cool! I just learned today that these bridges are the foundation for how the NanoVNA version 01's work - though the oscope is replaced by low sideband mixers and a 3-channel audio ADC.

[@JimSky]

W2AEW you are beginning to seriously eat into my free time with these great videos. Tonight I was working on a bridge by k6bez for an arduino based antenna analyzer. I just felt like I could count on you to have a related topic. This was perfect. Thanks -kh6sky

[@w2aew]

Very good! I'll have to check out the Arduino project...

[@6345788]

Very nice demonstration with the visuals of impedance. I really think the use of the scope and tuner gives a great picture of antenna matching. Great teaching skills, my friend!

[@dimitriapproved]

Just built it and it works!. It was faster to build it than finding an antenna analyzer on a Sunday night !!!

[@suriyontm]

Thank you for all the awesome tutorials on transistors. You are the only one that really explains things very well and easy to understand. Keep up the great work!

[@kendeken2868]

Much more information than a standard SWR meter -- good explanation.

[@nicholasbantell2520]

I just finished building mine. It works rather well. Thanks for the all of the videos. I really appreciate all of the free education you pass along.

[@w2aew]

Nicholas Bantell Excellent! This is what I always hope to hear, that these videos have inspired people to build, test, experiment, learn, think, etc.

[@robertw1871]

Little bit out of date finding this.... I can see a couple ways to make it more complex, but eliminate some of the other hard to control bits. A couple of opamp instrumentation buffers could drive the scope with low Z to eliminate all of the termination and coax impedances, you’d most likely have to layout a PCB to pull that off though. Using some board edge BNC and SMD components to further reduce parasitics, would be worth while I think. Might take advantage of the opamps to invert one of the outputs and shoot for a null rather than trying to match waves. Also a great time to use that old analog scope with its infinite bit technology as 8 bits of vertical resolution is a rather limiting factor for most ‘affordable’ DSOs. I’ve been kicking around ideas to do something like this and you provided me a starting point, or rather, near end point. Thanks for the great presentation.

[@Digital-Sparks]

This is my kinda ham radio stuff, great video, and thanks for making it, I work on radios of all types, also a General Class operator and a industrial electrician, self taught electronics tech, and I LOVE this aspect of our hobby. There is nothing more satisfying than making your own antenna's and gadgets and gizmos and being able to say when asked, were did you get "XYZ" from..... I made it! Although I am a "maker" I love these type videos, and I hope that it inspires others to experiment in making things for their stations.

[@alfbaz5248]

Brilliant and thank you! Being a self taught hack you have given me one of those moments were several loose pieces of knowledge all come together in a wonderful symphony with regards to polar, rectangular and complex notation. If you do make a video about that subject, I for one, will be very greatful

[@BalticLab]

As usual, pretty darn good. Even though I am familiar with most concepts of the electronics you are explaining, the way you present information gives me nice ideas for teaching others. And on top of that I love your little creations with big effect, like your probe holder for instance.

[@Afrotechmods]

Brilliant!

[@DAVET38]

Another very good video Alan. As usual I enjoyed your circuit analysis. It is nice to see all the complex theory I learned in my electronics courses used in a practical context.

[@markbell9742]

WOW; Knock My Socks Off. I am not a Ham guy but, I have built a couple antennas for radio astronomy (Jupiter lightening and Crab Nebula). Great information and presentation. Cheers, Mark * * *

[@g0fvt]

I just stumbled across this video, I probably have already watched it. I understand the methodology etc but nevertheless it was fascinating and very well presented. Well done once again

[@patrickbouldinkm5l143]

What an awesome video, content and generosity in sharing knowledge. This is the essence of amateur radio. Good job! - Patrick KM5L

[@RichardKCollins]

If you take the digital data from the scope curves, you can find the best match statistically. I do not know if there are digitally controlled tuners, but that matching can be done to arbitrary precision by using all the data in the signal and letting it run for a while. The statistical fluctuations will be local changes in temperature, electric and magnetic fields, internal noise in the power. Then fields from power systems, electromagnetic manmade sources, and natural sources. I am trying to get down to levels where the gravitational field can be measured, and that means identifying and adjusting to all the noise sources mentioned above - in real time, or by recording everything and doing it after the fact. Gravitational detector arrays are growing rapidly, but every instrument needs to be checked carefully for electromagnetic noise of all kinds. Since electrons have mass that "noise" will include seismic and vibrational noise, and movements. However tiny the signals, they will usually be there. Being thorough helps. Recognizing that every "noise" is a signal of something happening somewhere helps. And every signal has a group that is working on it. Nice video. Thank you for explaining carefully and precisely. If you record the data and apply some basic regression you learned in school it will help make sense of what is happening in gravitational engineering these days. Richard Collins, The Internet Foundation

[@dalesmith8666]

If memory serves me right, the AD8302 would be a perfect candidate to measure the phases of the voltages. Nice video. I use the HP4815a, and the HP4193a, but unfortunately affected by strong RF signals, from signals received on the antenna. I should try your concept, of course using none reactive glow bar resistor, I hope! LOL!. For those interested in more info, about the ad8302, see, Analog Dialogue 35-05 (2001) 73's, Dale.

[@PeterWalkerHP16c]

Great video. One of the best for this subject. This is my meat! You could use this idea not just for antennas but for for visualising filters etc. One could also use the inv and/or add functions on the scope to to aim for a max sine-wave or flat line. XY might also give some interesting results. With persistence, and a good CRO (scope), HP calculator, variable network filter and LTSpice et al and you can immerse yourself at a practical level till you really, really *_know_* this stuff.

[@ronaldlijs]

Excellently explained as always Alan, thanks for sharing your knowledge in a way we can all understand clearly and easily! Regards, Ronald

[@sasines]

You do an outstanding job with your videos! Very clear and easy to understand. I have subscribed and look forward to seeing your future videos as well as going back and viewing your earlier ones. Thanks very much for taking the time to do this. 73, Alan W3AL

[@MichaelLloyd]

This was exactly what I was looking for. Thanks a bunch

[@ernestb.2377]

Interesting exercises to follow along and to play with. I am not used to the XY-mode of a scope, but could that help some to read the phase shift more accurate on the scopes that do not have automated measurements? But very good to use these measurements and calculations to exercise the complex number calculations, and the compare with different measurement devices. But as the frequency goes up we have to take more and more care about the used components and the setup in general. Thanks !

[@skitliv]

I would love to see more about how to apply complex math and do calculations effectively in actual situations. Some tips/tricks maybe? I've studied theory at uni but it was presented in a very non-practical way, not easily applicable to actual circuit design. Seems to me that 99% approximations and 1% ltspice gives good enough results in less time, Maybe audio isn't considered real circuit design? :) Thank you very much for doing these videos! Highly appreciated! Greetings from Sweden

[@jonathanblack1416]

This is awesome, o only know very limited knowledge of electronics from my dad growing up, he was in the military for electronics and worked on radios and the original huge night vision scopes. He was stationed in Taiwan. I built quite a few things growing up like the lie detector and an guitar distortion box I still have today somewhere that I loved the best. I found you here looking how to get to find the exact frequency of antennas or more accurately for what in doing exactly is making copper coils as a necklace pendant that I’ve been working on for 7 years now daily and I’ve always been driven crazy trying to find certain frequencies that do certain things for healing and I’ve see miracles with them but I’m in the dark what frequency I’m making because the wavelength changes when I coil the wire into a custom coil I designed of my own that’s really amazing but I’m goin insane because I do t know what length to make them to compensate for the winding so it’s pretty maddening for me, then I heard someone out here talk about finding the frequency of an antenna with the oscilloscope and I didn’t think it was possible really but I’m seeing in my head it can be. I found a cheap used one near to me to get once I get my tires put on I can go get it. Is there an easy way to see the exact frequency of a copper coil that you know so I can see what in ending up with so I can find a compensation to get what I’m wanting to make in the end after winding it up? I need some sanity lol. I’ve been working bday and night on this for 7 yrs trying to solve this mystery without the tools or skills to see what I’m getting in the end. Just want I help people feel better easily and I’m getting Somwhere good with it finally just recently the hard way but I’d like to refine it to perfection this way. I’m good with doing the electronic work and can solder like a robot since I was 4 or 5. But my technical knowledge of the finer parts of it all is blah 😂. Was never good in school or math. That’s why I can’t believe I’m doing this for 7 yrs mile a mad man. It’s my wanting to help people not suffer that drives me to do what I’m the worst candidate to do it. I care probably way too much and want to find easy and safe ways to stop suffering for us all and not hide it. I’m doing a good job of that already now for 35 years and it all led me to invisible light colors bam dive got down how to make any thread of any color of invisible light for some years now by El wavelength and all I need to finish this off is to find what the coil is after winding it and then I can help people way better. I’ve seen literal miracles with this I created. If I can know exactly a frequency when it done I can cure anything by people just wearing a necklace tuned to cure just about anything. I do it underground to keep my life of you understand. M Any help will be helping not so much just me but mostly others really. I do it all for free as I believe it should be. It’s cost me at least a quarter million over the last 33 years curing people of just about anything you can imagine at my own cost. So now I’m broke but I keep going. I’m a bit crazy as you can tell 😂 m It takes crazy to do stuff like this.

[@g0fvt]

Another excellent video, great to see it done from fundamentals and actually it seems to pretty well vindicate the MFJ as being a useful piece of test equipment, certainly at HF.

[@kasperandersen7396]

First of all. Great tutorial and really usefull. In the demonstration the freqency is about 18 MHz, but is it possible to do the same analysis with a higher freqency band, lets say 2.4 GHz? I've imagine that the paracitic effects will have a higher influence with a higher freqency?

[@fir3w4lk3r]

You used an instrument to tune the antenna. After that you use the number on the dials to design a equivalent circuit to hook before the antenna?

[@flyingfrancisco]

Hi Alan I knew I would enjoy this. Thanks for the wonderful explanation. Regards

[@cny02253]

Super Alan! Great ball park double check to the Antenna Analyzer. Since some cheaper DSO's now have tracking cursors, would any improvement that may give to the delay measurement (to get phase angle) be significant? Also, if one used construction like your attenuator (symmetrical, compact, SMD, SMC jacks, etc) -do you think that would add any significant improvement, especially at higher Hz? Or is that not worth the effort due to scope error limitations.

[@volkerblock]

I like this Video. Because I am lazy, I used Freemat (it is free) for calculation. --> Vin=64; R=50; Vx=27.8*exp(i*13/180*pi); --> Zx=R*Vx/(Vin-Vx) Zx = 34.2726 + 14.2773i -->

[@eugengrzondziel1706]

A great video, congratulations for the density of information! Very interesting and usefull for every ham. My very 73 and 55! P.S. How about UHF-antenna impedance measurings?

[@Jjacobs1986]

You're a mad scientist. I love it.

[@KD0RC]

Excellent video once again Alan! Is this essentially a return loss bridge with the detector split into the two scope channels? WOW, You really made me learn something here... I just put my scope across the two detector inputs (top end of the RF xformer...), one channel on each side of my homebrew return loss bridge, and can see visually the peak on one channel and the null on the other... Had to put the spectrum analyzer on zero span to make the measurement. So with the xformer in the circuit, I don't see the same thing as in your video, it looks more like an swr bridge - peak on one side, null on the other.

[@w2aew]

It's really not a return loss bridge, but simply an impedance bridge which is used to compare the impedance at the DUT port to a fixed 50ohm resistor.

[@buffplums]

This is such a great video. I love your demonstrations they are really helpful and inspiring

[@acestudioscouk-Ace-G0ACE]

Very interesting, I followed the theory and demo OK but my maths is not up to this level, so I need to put some work in! Thank for sharing this.

[@ve6wo]

Thank you for making these videos. They are greatly appreciated :-)

[@StanleySeow]

Hi, firstly, I would like to mentioned that you have lots of very good video with very simple and easy to understand explanations... Keep up the good work!!!! I've made SWR bridge and was feeding it with a AD9850 modules that can output 1Mhz to 30Mhz m just nice for the HF bands... I just follow all the circuit that you did and I was able to tune HF antenna using an antenna tuner. I find the AD9850 voltage is rather low at 20Mhz and above ... Can you suggest a method to amplify the output voltage of the AD9850 ?? Does the 50R/50R voltage divider causes the readings on the oscilloscope to read a low voltage ? Thanks ,,,

[@w2aew]

Often times a drop off in amplitude at higher frequencies is a circuit layout problem - so it is difficult to comment why you have a drop off. You may be able to improve the layout to bring the amplitude up. A simple common collector amp can be used as a buffer (if the issue is simply loading), or a common-emitter amp would give you some gain. You'd have to follow the common-emitter amp with a common-collector amp to regain the low output impedance.

[@StanleySeow]

The drop in amplitude as freq increases was stated in the AD9850 datasheets. Can you give some part number examples and sample circuits on the common collector/emitter amp as a buffer ? Could I suggest you do a video on the topic to how to increase the gains on RF signals ?

[@w2aew]

Stanley Seow That's a good idea for a video, but it will take me quite a while before I get to it. Each video is often weeks or more in the planning and preparation stages... You might get more prompt answers by consulting the applications engineering department at Analog Devices - the maker of the AD9850 - they may have some reference designs already prepared.

[@StanleySeow]

Ok thanks... let me do more readings and lookup their application notes...

[@einball]

Hey! Nice video! A quick question: Why do you need to measure with 2 channels terminated to 50 Ohms? It would be easier to use one channel over the bridge: If there's a missmatch you will see a (distorted) sinus. You can then tune to 0V on the scope. That would work too, if I'm not mistaken?

[@nbesnerson]

Nice. Very much appreciate this video.

[@einball]

What a bummer! Didn't think about the ground reference. I seem to use too much C code and too little actual hardware. Thanks!

[@afterthought138]

Hi Alan. Great video as usual! What material is the board used to mount your bridge circuit made from? It looks like copper-coated perf board of some sort. It looks nice for shielding purposes while still being easy to drill holes through.

[@pu2clr]

Very nice video. This idea is very useful to build a auto-tuner. Thanks. PU2CLR, Ricardo.

[@mastertheinternetlearningn5266]

Great Video. Can I use this in one of my classes? To fully master the Internet you need to master the skills you talk about, particularly the math skills. Thank you for this effort.

[@syarifhidayatofficial1088]

really a great teaching video,.. easily understood , even it is not an easy subject. thanks for sharing

[@hubercats]

Another great video. Thanks!

[@starlino]

I am curious if this method can be adapted to NanoVNA, in other words use port 1 as a signal generator, and port 2 instead of osciloscope. Certainly you can measure an antenna just with port 1 using reflected voltage, still I think a comparison of various methods to measure antenna impedance with NanoVNA would make a very interesting video.

[@w2aew]

This resistive bridge is exactly what is already inside of the NanoVNA to sense the forward and reflected signal.

[@M6MRP]

Once again very informative and well presented tutorial, I have also seen this method used for tuning HF dipole traps. 73 Phil M6MRP

[@rupertrooksby]

Enjoyed that a lot. Very nice.

[@3cu14rs]

This was a great vid, thanks for taking the time to explain it to the rest of us :) After measuring the impedance, how would you go about designing a dedicated matching network?

[@peterporkolab2800]

Watch his video about using Smith's chart, and You will know.

[@scottrharris]

Good stuff! It would have been fun to place the scope in XY mode too.

[@tonybell1597]

Great video.... Probably need to hit the text books now and work out what all the rectangular math is all about!

[@wk2w]

This is an extremely interesting video (both parts). Thank you very much. The 50 ohm thru-terminators are interesting. Would it also be possible to use BNC Tee connectors with the input signal running through to a 50 ohm terminator on the female connectors, with the "male" connector into the scope (similar to an end-of-the-run on a coax network)? - wk2w

[@w2aew]

At HF frequencies, a BNC tee with a terminator can work acceptably when located at the scope input.

[@MarkBaldridge]

So how does the MFJ "directly" measure complex impedance? Would it have a differential amplifier for for measuring the amplitude, and an electrical phase detector of some sort for measuring the time shift?

[@manideepdunna827]

Thank you for the great videos. I am wondering on what kind of boards do you make solder your circuits? Can you provide a link to them.

[@EvandroBubiak]

Thanks a lot! Very nice, very usefull. I want to measure the impedance of a wire antenna (helical connected only on one end on my PCB), can I use this method? need to change something?

[@wilvanlierop]

Very nice and instructive. Thank you.

[@anissastafford8088]

Very nice thanks. I noticed that the antenna port trace (blue) actually "leads" the reference trace. If the blue trace "lagged" the reference would the angle be considered negative?

[@w2aew]

Yes.

[@gixerags750]

Outstanding,.!!

[@fishyrider3528]

Hy, first of all thank you for the knoledge. You are a great techer. Second i have a request. I want to align an fm module for my radio. I do not know the impedance at the dipol antenna imput and how to connect the sig gen to it. I have a rigol sig gen with fm modulation and a rigol scope. I can not find a detailed enough video on how to determine the impedance of the input dipol antenna and to connect the sig gen to it to match the impedance. Pls help.

[@w2aew]

The easiest thing to do is to connect the dipole antenna, then attach a small wire (non critical) to the siggen output, and let the dipole on the radio pick up the signal. It will be much stronger than anything else around.

[@StiegeNZ]

Heya, I'm just at the end of my electronic degree (but need more RF prac!). Just a few conceptual questions I was hoping you could answer. Could you have had the lower resistor of the bridge as 100 ohm and straight paralleled it with the coax heading into the oscilloscope? This may be a bit more complicated when it comes to solving for phase? Also with the 1K res,having larger just means a smaller the signal going into the oscilloscope? Thanks

[@karmyncraig7220]

Can you use this technique to measure the input/output impedance on a transistor amplifier or a filter?

[@SlowburnFoos]

Impressive video! Where did you get the antenna tuner from?

[@w2aew]

The tuner is the ZM-2 Z-Match Transmatch kit by Emtech. Here is the link: emtech.steadynet.com/zm2.shtml

[@ibsrensen1911]

you have a great way of explaining even the most stupid people living - walking or whatever on earth they dó, actually get- it! 😎 I used your video to explain how an CB antenna tuner is working. Trying to figure out a way of making non-technical friends understand things like this is not always a easy task.😂 Purely 👍👍👍 from me, keep it coming 😁

[@researchandbuild1751]

Doesn't this also sort of depend on the antenna type as well? I know certain antennas dont show up as 50ohm impedance naturally. Is that what the tuner does basically is match say, a 75ohm antenna to 50 ohms?

[@w2aew]

yes

[@ornithopterindia]

👍Thank you sir.

[@Fireman9143]

Is there a way to sample actual radio Tx for real world monitoring (without blowing up scope), or is this only effective as antenna analyzer? Thanks for all your videos!

[@w2aew]

Yes, look for my video on RF samplers.

[@achernarscardozo69]

Con este procedimiento no se determina la impedancia de la antena sino la impedancia del cable y la antena. Con diferentes longitudes de cable pueden obtenerse resultados diferentes.

[@w2aew]

Yes, this is true - you're really looking at the antenna *system* impedance, including the impedance transformation/rotation that is added by the transmission line. At the end, this is OK, because this is the impedance that will be seen at the transmitter.

[@glennwoodward77]

Great video! Love all of your circuit build videos. Would this method work when trying to test and tune 2.4 or 5.8ghz antennas?

[@glennwoodward77]

w2aew figured as much, thanks!

[@leobotler]

Very good video! Could you please explain why do the oscilloscope needs 50 ohms terminations?

[@w2aew]

Leo Botler Yes, the terminations are added to the ends of the cable to prevent reflections of the RF signals that are coming down the line. If your scope has 50 ohm terminations built in, you can use them instead of the through terminations.

[@leobotler]

***** First I would like to thank you for your answer, but I was not clear enough in my question. I didn't understand why you placed a big resistor (1Kohm) in series with the oscilloscope and also a 50 ohm termination, once the input resistance of most oscilloscopes are already very big (>1Mohm). How does it avoid reflections? If you have a reference that can help me understand it I would appreciate!

[@w2aew]

Leo Botler The big resistor in series with the coax is placed there so that the 50 ohm resistors in the bridge dominate the impedance seen at the bridge nodes. Once RF signals are in a transmission line, the current/voltage waves assume a relationship of Z0, the transmission line impedance. If the transmission line is not properly terminated, then there will be a reflection will will affect the amplitude of the signal which will be frequency dependent. So, by properly terminating the lines, no reflections occur. You might find this video helpful: kzbin.info/www/bejne/nZDNqZtmhsqSfLs

[@leobotler]

***** Thanks! Also a great video. Isn't this reflection already avoided by the 50ohms resistor connected to the ground? I mean... The reflection you avoid in the oscilloscope input is happening on the 1kohm input... Am I wrong? The input impedance in the path going to the oscilloscope is much bigger than 50ohms anyway, so in my understanding you could place the oscilloscope directly on the antenna.

[@w2aew]

Leo Botler There are RF signals injected *into* the pair of coax cables that lead to the scope through the 1K resistors, and they travel down to the scope. When they reach the scope end of the coax, they need to see a 50 ohm termination to prevent a reflection at that point.

[@Testequip]

Excellent. I'm curious, I have two HP VVM's viz 3575A and 8405A. Will the vector voltmeter be far more accurate than an oscilloscope. I used the VVM's a few times to measure resonance but notice there were differences between the scope, spectrum analyzer and vvm. Which is better?

[@bassdowg]

Hlo excellent presentation i really enjoyed this, can i do the same with my Nanovna-f ??? Tyvm 73s

[@w2aew]

A bridge like this is exactly what is used inside the NanoVNA. See my video kzbin.info/www/bejne/rpKZlaSubtqXmck to see how to use it to help with tuning an antenna/tuner.

[@cescofran6165]

Bravissimo.

[@fmbroadcast]

Hey, I love your video, amazing !! At 10.53 you speak about non linear curve about the voltage divider. How did you got this curve ? I am not sure to understand ? Because when I plot Vx=(Vin*Zx)/(Zx+50) I don't get this curve. Can you guive me more details ? Kind regards

[@w2aew]

fmbroadcast Are you sure? Did you change the x-axis to Log? If you spot check some numbers, they call on this curve. For example, for Zx=10, Vx calculates to 0.333, which is on the curve. For Zx=100, Vx comes out to 1.333, which also lands on the curve...

[@fmbroadcast]

***** Hello, thank you a lot for your reply. Ohhh yes okey you plot the x axis to Log Okey !! Yes i got the same value, Zx=10=> 0.333 and Zx=100 => 1.333. Frantastic !!! I love your video !!! Feel free to suscribe to my channel ;) Kind regards

[@fmbroadcast]

***** It's me again, something that tilt me. On the scope, the yellow trace represent the signal from the reference voltage divider is that correct ? we can see at 7.20 that when you match the antenna load, the yellow trace make some tilt in amplitude and phase, I guess because the signal generator impedance is of course not null but in the ideal case, is the yellow trace should have to change in amplitude and phase ? May be i miss understood something. But this is facinating me.

[@w2aew]

fmbroadcast Yes, the generator's output impedance is 50 ohms, so the output will vary as the applied load varies.

[@DeeegerD]

Another question if you don't mind? Do impedance matching circuits (ie: antenna tuners) create loss in power?

[@w2aew]

+Digger D Generally, very little, if they are made from capacitors and inductors. Since the voltage and current in a capacitor/inductor is 90 out of phase, they do not dissipate power. They store energy (in electric field or magnetic field), then return it to the system/circuit. The main losses are generally due to resistance in the wiring, inductor, switches, etc.

[@DeeegerD]

***** I was thinking that but some hams on a video were debating it the other day. If there is no impedance... (except for normal losses as you say). Thanks again.

[@w2aew]

+Digger D RF currents can be high in high-Q tuned circuits which can lead to higher resistive losses in the tuner. Also, if the tuner is located in the shack, then the feedline from the tuner to the antenna will be operating with a high SWR, which can also lead to losses.

[@DeeegerD]

***** Ok, thus the invention of the remote antenna tuner. Good to know and thanks again.

[@DJ369-Miami]

Instead of building this yourself, could I get an off the shelf bi-directional coupler? Any model you would recommend for HF?

[@mick2458]

is there a simple method for checking ceramic filters....the 3 and 5pin types?

[@XQ2CGSergio]

Excelent!!!

[@Wireless433]

very good work de su3mb

[@raydall3734]

Great Video

[@bundylovess]

hi alan loved the vid i just noticed that were the 10x probes clip in to, is hooked up befor the 1k resistors can you hook up the probes after the 1k resistors will this still work sorry im still learning

[@w2aew]

Yes, it will work that way too.

[@caravanken1]

Thank you very much. Love the videos!!

[@caravanken1]

Thank you for your videos. I am trying to setup this process but cannot find an inexpensive 50ohm through terminator. Will a BNC "T" fitting with a 50ohm end cap work the same as the device you have in the video? I am new and just need a little hand holding.

[@w2aew]

It will work reasonably well for lower HF frequencies, and may introduce some reflection artifacts at higher HF frequencies, but they should be fairly minor.

[@yoniziskind2115]

Great vid! one question though, can I implement this concept in evry input impedance measurement?(such as amplifiers, mixers, etc)

[@w2aew]

Yes, it should work for other devices - but layout limitations means that it will likely only be accurate for HF and below.

[@boonyang88]

can u do a video on smith chart and impedance matching . thanks!

[@SuperJetjockey]

I want to do this experiment, and when I checked my terminators with an ohm meter, they measured 72 ohms, even though they are marked as 50 ohm terminations. Is this a matter of math, and I can go ahead, or do I need to make up or purchase some 50 ohm terminators. Thanks for another great video. Bob

[@w2aew]

It would be best to have 50 ohm terminators. You can use the ones you have, and things will be close.

[@DJ369-Miami]

Not an expert here by a long shot....but a regular multimeter is DC and RF is AC so it all depends on the frequency, does it not? Not sure how the manufacturers like Mini Circuits measure their stuff...I have a couple N type attenuators and the measure 80 Ohms at 170mhz (100ohms at 3mhz) using the MFJ

[@SuperJetjockey]

What was torturing me, is that I have five of these that I purchased used from someone that was using them in an old 10T ethernet system, and all were marked as 50 ohms, and I thought the resistance should read 50 ohms. The only reason I even tested them all, was because I had one PL239 type terminater that wasn't marked that measured 72 ohm, and out of curiosity even though all the BNC terminators were marked as 50 ohm units, I thought I had better check them. I was very supprised to see that they were all 72 ohm and didn't think it was right. I had since found my old ARRL antenna booklet and discovered what you are confirming for me. I am just getting started again after many years away from the hobby, and it is amazing how much I have forgotten. Thanks again for all your wonderful tutorials. I just dug out my old 545b Tek scope so I can play with the differential amp. It has been sitting awhile, so I plan to use a Variac to slowly bring it up to the operating voltage. Bob

[@w2aew]

SuperJetjockey If they are attenuators (not terminators), they may only read 50 ohms when a 50ohm load is connected to the output.

[@SuperJetjockey]

***** I have solved the problem by ordering 10 of the 50 ohm terminators. I have some 72 ohm line that I am going to experiment with later, and decided that I didn't want to mess with the 72 ohm terminators, as they are high quality units. I am having so much fun with this stuff again, that I was even able to get my old 545b scope running again. I have the dual trace plugin, a differential amp, and a type S diode checker, I also have a 2246 and a 475b, but I thought the extra functionality of the plugins made it worth the time. I used the variac, and it seems to have done the job. It has been so long since I used the diode recovery plugin, that I will have to find some instructions. I really appreciate the way you make such complex topics so logical and understandable. Bob

[@raghutumati5178]

Hi Alen. I have a impedance matching question. I'm working on a hobby FM transmitter circuit on a 10/15/40m radio band. Finished designing the Clapp oscillator, LNA and output stage (PA). But Im not sure how I can connect an antenna to my output stage. Can I just connect a wire of a particular length to the output of my transistor (BJT). I would assume that there has to be some sort of matching needed here. If so, how do I go about this without an antenna tuner? P.S Hope your ankle is healing well!!!

[@w2aew]

It really depends on the output impedance of your transmitter. You'd ideally want to match that. Otherwise, just design the antenna system for resonance. However, this brings up the bigger issue of legality. In the US, these frequency bands are not license-free. Portions of them are reserved for amateur radio use, and FM is not permitted on the 15m and 40m bands, and is reserved for a portion of the 10m band.

[@raghutumati5178]

Thanks for the reply. You mean the small signal impedance of the transmitter right? How would I determine the impedance of a wire that I'm using as an antenna. Do you have any practical references that could help me to get more info? Thanks for bringing up the legal issue. Ill have to re-check the bands to see where it would be legal.

[@w2aew]

The most accurate way to predict the input impedance of the antenna would be to use one of the NEC-based modelling programs. Lots of good information here: www.arrl.org/antenna-modeling

[@raghutumati5178]

Thank you Alan!!!

[@coffeecuppepsi]

Just a query, the mismatched antenna is at the other end of a coax ... How does measuring the impedance this way affect the reading? Also after the matching network the radio might see it as 50 ohm.. but the true mismatch is again where the coax meets the antenna?

[@w2aew]

Correct, the impedance mismatch is occurring at the coax connection to the non-50 ohm antenna. The impedance measured at the transmitter end of the coax will not be the same as the antenna impedance (unless the coax is a multiple of half-wavelengths long). The impedance at the tx end of the coax can be predicted using the Smith Chart - see this video: kzbin.info/www/bejne/f56xg5aXasqWfJI You can place a matching circuit at the antenna (preferred), or at the tx end of the coax. The latter doesn't "fix" the impedance mismatch or the presence of standing waves on the coax.

[@DeeegerD]

If you do not have pass through terminators could you use T connectors with one end terminated with 50 ohms?

[@w2aew]

+Digger D Yes, for HF that will generally work well.

[@DeeegerD]

***** Great, thanks again.