Excellent teaching i am an old electronics engineer but not a RF expert i was searching for videos about coax cable impedance i did not get any thing until i found this video .Thanks sir you are a brilliant teacher better then other English teachers.
Thank you so much for making this video and simply explaining this subject. I knew about what you explaining but it didn't cross my mind how to be able to simply work out the impedance.
I never even gave a though ever for testing an unknown coaxes impedance. Great demonstration on how to do so! I think I'll try it here dor the heck of it using my DE 5000. Also. Thanks for showing us the receive and transmit impedance chart for coax. Now it easily understood by me why the compromise for 50ohm coax for ham, cb, etc. usage is used. Happy New Year Peter! 73 Tom
I've heard about this topic many times, but only now I'm sure I'll remember how it actually works. Thank you very much for this great explanation and demonstration.
It's always useful to have some tips about coax cable. Wit coax being very expensive, it's also ways necessary to re-use it again if it is still in good condition.
Very interesting and informative. A technique that needs no new or specialised equipment which is a good thing at this time of the year. Thank you for the videos from 2016 and I hope you feel motivated to produce more in 2017, they are appreciated.
Good Morning Peter, this is a real eye opener for 50 OHMS. Thank you for your video on the thus subject and your explanation on L &C. Happy New Year best to your family.
Great stuff! And you got me to buy the same calculator you have! Haven't bought a calculator since 1986, when I bought my HP 16C calculator for programmers. Still works today, but I remember laughing back then when I read the instruction manual that it could do calculations on 64-bit numbers. I never imagined that I would have access to that kind of power. Now you have it in your phone. :)
Hi Jack, yeah when I saw the calculator 2 year ago or so I couldn't resist and bought it LOL. Technology is really improving fast..Thanks for watching 73
Very useful demonstration how to measure and how to use the coaxial cables : 75and 50ohms. I did not know about this difference of impedance , for receiving and transmitting purposes thank you for easy and practical video.
Hi Peter, Great video. Earlier this year I bought an MFJ-269 off Ebay to determine how much 50 ohm coax I had left on a 250 ft spool that was 30 years old. It showed I had 135 ft and when I measured the length it was 126 ft. Close enough for government work. HiHi 73 and Happy New Year WB3BJU
Nice one Peter! It should be pointed out that you can always use coax with larger diameter to handle more power but then the cost is higher. Impedance is related to the conductor spacing, the dielectric constant of the insulating material and also the center conductor diameter. Lowest loss coax will have a mostly air dielectric with large center conductor diameter and correspondingly large outer diameter with correct ratios to make the impedance come out to what you want. 73's and Happy New Year!
Peter, Thank you for this great video! You are teaching an old Dog new Tricks all the time! Please keep the Class room videos coming! 73's de K8KEM David from Ohio
2:40 comment. It's not always easy to get old cable data by looking up whats printed, e.g. try to search some odd Chinese brand. These are often sold without any real product codes and thus lack proper identification for the cable itself. In addition you may find very limited technical information on the web site (if any information exist at all). Your tip is thus very useful. Thank you Peter.
Great video. This formula is working for any transmission line, not only coax cables. Its general form is Z0=sqrt(Zs.c. x Zo.c.), where Zsc and Zoc are complex impedances so the imaginary part gives an information about the losses. So this measurement can also be done with a miniVNA :)
Very interesting - thanks. It would also be fun to perform the measurement by putting a signal generator and an oscilloscope at one end, and a variable resistor at the other. Then tweak the variable resistor to minimise the reflection and read off the cable impedance by measuring the resistance. I always wondered why 50ohm was used in some applications and 75ohm in others: it's interesting to learn that there really is a very good reason !
When I was in school for Physics we tested the velocity factor and impedance. I don't have a specific pulse generator to use at home and I'm wondering if there's a circuit I can build to convert my simple frequency generator to do so. Maybe a a square wave with duty cycle set short and an active RC filter ???
If 75 ohm cable has a lower loss but we don't we use it for transmitters because of the power handling, what about low power transmitters? Would not 75 ohm be a better choice because of the lower loss if only a small amount of power needs to be handled?
Except he pronounces some words wrong. Coax is pronounced co-axe not coax. He does similar with the word balun which is pronounced bal-un not balloon. Cable too which is kay-bull not cobble.
Thanks for sharing your knowledge and measurements. One issue in your first calculation (12:50 in video) is misnaming/labeling inductance as Farad rather than Henry. Simply an oversight and the concept and procedure are clearly communicated.
Perhaps the symmetric cable is meant to operate at a frequency that is different than the test frequency of the meter. What I'm saying is that the cable might be 450 ohms at MHz frequencies rather than kilohertz.
Hi Peter. Really enjoyed your video on determining the impedance of feed lines, and especially about how the use of 75 and 50 ohm coax came about. I was also wondering if you determined the number of significant digits or figures you had in your measurements of capacitance and inductance. I could not see all of the measurements on your meter so I am not sure how many significant digits or figures you were able to read. If you only had one significant digit or figure for example in one of your measurements then only the 5 in the 50 ohm measurement, the 7 in the 75 ohm measurement and the 4 in the 400 ohm measurement would be significant.after the calculations, If this case then measuring longer lengths of cable may give you more significant digits or figures and a greater degree of precision if your meter is precise ,or more accuracy if your meter has been calibrated to a standard. Please give me your thoughts, and please keep making the videos I have learned so much from you. Thanks again John C
Hi John, yes the challenge for the meter is higher with a short test lengths so it is always good to improve the general conditions. Thanks for watching 73
well done my question is usually we match 50 ohm radio with 50 ohm impedence of antenna but still how RF is transmitted in air when impedence of air is around 300 ohm?
Hi Adnan, thank you! Sorry I'm not able to follow up all e-mails....simply to many.... Z(W) free space vacuum = 120x pi x ohm=376 Ohm. That is a nature given constance and describes the condition in the far-field of an antenna. The condition in the close-field is very much more complex and different to the far-field condition... 73
Could the impedance of the ladder line be changing due to proximity of conductors near it? I know that's a big problem when using ladder line since it is unshielded. Would it read differently out in the open away from objects on your bench? Thanks for a very informative video.
Please help one problem: I bought a 50 ohm cable with velocity factor 82 and it does not work on the cb antenna. the capacity is the same 80pf / m but measured capacity at one end with the length of 10m it has double capacity compared to rg58
danke fuer die super erklaerung. wie erklaert man das power handing von 450 Ohm ladder line und high power application? kannst Du bitte eine video machen mit dem nanoVna als test geraet? merci
I bought one of those Ebay LC meters on the strength of what you show here but I can't achieve anything like the kind of accuracy that yours does. I've even tried measuring long pieces of 50ohm coax (M&P airborne 5) and the closest I could get was 60 ohm. I was as far off as 250ohms when measuring a short piece of RG58. I'd love to try this with something that might be a little more accurate and reliable one day.
TRX Bench yeah I'm going to try and do a few other tests over the weekend. I should be able to measure an inductor with my scope and signal generator and compare the impedance with a known resistor to get a ball park figure to compare it with.
great video, is it possible to work out what mix ferrite rings are, I have a few and not sure what type they are...I did see a video once on how to check ferrite rings for certain applications..but that was long ago...thanks ! 73s
Thank you Cliff. Well the best is to measure the core material out. You get an idea on which frequency the material work and can decide if it fits into your project. In video #101 I show how an unknown ferrite can be tested... Hope that helps 73
Thank you. Very useful and educational information. I have a request - please show how it is possible to test the effective shielding on coaxial cable. I have some new stuff with more dense braid and some with less dense braid and I would like to know the difference regarding RFI (shield effectiveness) etc. Also, I have some old and some used coax - is it still good regarding loss and RFI. 73 joe n8ea
Thanks for feed back Joe. Well a real shielding factor measurement is a bit complex to perform in my lab. Bur I remember a kind of predicting approach may be that could be done with the equipment I have.. Let's see. 73
On twin antena CB set up, we use a RG-59 75ohm "Y" harness Is it because we need to rise the impedance because we run parallel antenas? (Parallel mean half impedance of a single antena)
I believe you both are correct in the answers. Yes it is because the impedance is a slight bit different but also this acts like a transformer to correct the impedance difference. That is what I understand about the "Y" harness setup. So you both answered the question but it also depends on the frequency and coax length. Coax is not 75 ohm the whole length. At a given frequency it is at certain lengths but that is a more complex way to look at it.
It might be nice to clarify even more that you measured absolute capacitance and inductance, and not per unit of length. Yet those would cancel out in the ratio between L and C. The formula for the transmission line impedance is indeed derived from specific not absolute values. this might ground better the intuition. What do you think?
hello thank you for this video I would like to know which cable is used for measuring devices (oscilloscope, counter) is it possible to make them yourself thank you
Lab & test equipment with coaxial terminations almost always expects 50 ohm cable, with narrow exceptiions, such as specialized gear meant field use in the CATV industry. In theory you could make your own coaxial cable, but would need precise and consistent control over its geometry (inside diameter of outer shield layer, outer diameter of center conductor, and dielectric constant of insulating material in between together determine impedance), also taking care to ensure good shield coverage to avoid leakage of RF, whether ingress or egress. Air-core coax can be assembled using two sizes of pipes nested within one another using spacers to keep them centered on the same axis, great for high power handling, but you then need a way to draw & keep moisture out.
Would a multimeter not work 🤔 measuring the legs of the cable and divide it to 1 cm and than multiply by 100 to get to the 1meter length and putting than the measured ohms in 🤔
Video yang sangat bagus...ini adalah perhitungan nilai z yang paling akurat menurut saya dari berbagai tutorial di youtube.... Ada pertanyaan sedikit .... Adakah cara yang paling sederhana untuk menemukan nilai z? Misalnya menggunakan multimeter. Terimakasih.......
A dated video but still great. How about giving us an updated video on how to do the same thing with a nanoVNA as they are now cheap enough that anyone can afford one? 73, KD4LLC
Thank you again for teaching me something I did not know Peter. So, if I understand it correctly, the values in the formula should be (L) in Henrys and (C) in Farads. Not in uH and pF?
Bora, you have to use the readings displayed by your meter to use it correctly in the formula. The link is helping to put the right exponents into the formula. 73
Always look forward to your instructional and informative videos Peter. I wonder if my MFJ 259b could make those L C measurements ? Happy New Year ! ! !
Awesome video! Learned a lot from you! Btw... Your voice totally reminds me of "Hans Oberlander" from the game Gran Theft Auto San Andreas as the radio DJ SF-UR. :)
I thought 300 ohm (ladder) twin lead type is better for receiving signals (lower loss), however interference is a problem with twin feed lines. Here's an interesting concept for HAM radio; Use different feed lines and antenna (30 ohm for TX and 75ohm for RX) Not practial but efficient. No body like standing waves that can cook you.
Erstmal ein gesundes erfolgreiches 2017, wieder mal ein tolles Video, ich muß jetzt auch mal meine Reste durchmessen. Ich hab nur eine verständniss Frage, wie kommt die Einheit Ohm zustande wenn ich nH durch pF teile? 73 DO1BSW
Hall Bernd, vielen Dank und ein Frohes Neues Jahr! Ja das ist eine sehr gute Frage. Formeln enstehen immer durch Herleitung von sehr komplexen Zusammenhängen, so auch hier. Die Herangehensweise an das Thema geht über das Ersatzschaltbild einer Koaxleitung und die sich daraus ergebenden Differenzialgleichungen für U und I. Durch einsetzen und wegkürzen kommt man am Ende auf die vereinfachte Formel wie im Video dargestellt. Du merkst aber sicher schon über diesen Ansatz das sich dhinter komplexe Mathematik über die imaginären Inhate ergibt. Hoffe das hilft erst einmal...73
