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I want to see how many wavelengths are in my feedline - but

DougVL All Messages By This Member #20107 I want to see how many wavelengths are in my feedline - but I have not been able to figure out how. I've searched previous posts and checked many web pages that google found for me. Everything I've found so far seems to be only for measuring cable length or velocity factor. I've done that before. I've read pages about measuring antennas where they keep saying that measurements vary depending on whatever fraction of a wavelength there is. For example, if the feedline is 2.7 wavelengths, and I change it to 2.4 WL's, what effect should be seen? (No, I'm not in a position to try that right now!) I've used the TDR function and it works well, giving readings in meters. What I would like to see/have is a display that shows a sine wave (with number of cycles in proportion to the actual signal) at the CW test frequency and also shows the impedance bumps or discontinuities and joints and changes from co-ax beginning to the antenna connection. Maybe even to the end of the antenna wire! I'm not saying that this might be a 'useful' feature, I'd just like to see what it might look like with my two HF antennas (a random wire and a GAP Challenger vertical). -- Doug, K8RFT
Bob Albert #20109 Display Smith chart.? With nothing connected to the nano you should see the trace on the right edge of the chart, meaning open circuit. With Start frequency at 50 kHz and Stop at 1 MHz, feedline connected, you will see a part of a circle starting at the right edge and going clockwise. Keep increasing the Stop frequency until the trace makes exactly one full circle, 360 degrees.? That is the frequency at which the feedline is one full wavelength.? If you like, move the marker around and read the display numbers. If you increase the frequency beyond that, a new circle will begin but it will be slightly smaller diameter due to losses. Obviously the termination (antenna) will not be 50 Ohms except at its resonant frequency so the circle will have a bump toward the center near that (if the line is long enough) but you can ignore that. Bob toggle quoted message Show quoted text On Wednesday, January 20, 2021, 03:14:38 PM PST, DougVL <k8rftradio@...> wrote: I want to see how many wavelengths are in my feedline - but I have not been able to figure out how.? I've searched previous posts and checked many web pages that google found for me.? Everything I've found so far seems to be only for measuring cable length or velocity factor.? I've done that before. I've read pages about measuring antennas where they keep saying that measurements vary depending on whatever fraction of a wavelength there is.? For example, if the feedline is 2.7 wavelengths, and I change it to 2.4 WL's, what effect should be seen?? (No, I'm not in a position to try that right now!) I've used the TDR function and it works well, giving readings in meters. What I would like to see/have is a display that shows a sine wave (with number of cycles in proportion to the actual signal) at the CW test frequency and also shows the impedance bumps or discontinuities and joints and changes from co-ax beginning to the antenna connection.? Maybe even to the end of the antenna wire! I'm not saying that this might be a 'useful' feature, I'd just like to see what it might look like with my two HF antennas (a random wire and a GAP Challenger vertical). -- Doug, K8RFT
DougVL All Messages By This Member #20112 Thanks, Bob! I had thought there should/might be a way by using the Smith chart, but didn't think of your way. Thanks for the info, and for making it so clear and simple. -- Doug, K8RFT
Bob Albert #20118 I think I may have slipped up.? One revolution of the circle is one half wave, not a full wave. Bob toggle quoted message Show quoted text On Thursday, January 21, 2021, 05:02:00 AM PST, DougVL <k8rftradio@...> wrote: Thanks, Bob! I had thought there should/might be a way by using the Smith chart, but didn't think of your way.? Thanks for the info, and for making it so clear and simple. -- Doug, K8RFT
DougVL All Messages By This Member #20123 No problem. I think about all of the Smith Chart "How To" training articles I've read describe the half wave/full rotation characteristic. And it would be fairly obvious the first time I tried, too! It's good to have the note about it in here, though, for other readers future reference. And thanks again for pointing it out. It's going to be interesting and helpful. -- Doug, K8RFT

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