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Bob Albert
I have a bit more information.? I have a cable going to a coaxial switch that shorts unused connectors.? So the cable, unless I select it, has a short at the end.
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The procedure, with slight modification, works as well for a short at the end. Instead of a circle starting at the right and sweeping down and around to the left edge, it starts at the left and sweeps up and around to the right edge.? When you take half the frequency of its intersection at the right edge and use that for CW stimulus, the screen shows inductance instead of capacitance.? So instead of the capacitive reactance calculation you do the inductance, where X = 2*pi*f*L and get the answer. I learn about electronics and, specifically, transmission lines every day. Bob K6DDX On Sunday, January 26, 2020, 04:37:51 PM PST, Kurt Poulsen <kurt@...> wrote:
Hi The nanoVNA partner does it semi automatic with a smart feature and a couple of mouse clicks Use nanoVNA Partner v0.20? link: Kind regards Kurt -----Oprindelig meddelelse----- Fra: [email protected] <[email protected]> P? vegne af Bob Albert via Groups.Io Sendt: 27. januar 2020 00:39 Til: [email protected] Emne: Re: [nanovna-users] characteristic impedance Ah now I have found it, or eureka!? The trick is not to measure at the eighth wave (6 o'clock) point of the Smith chart; I was doing that and getting precisely 50 Ohms no matter what. The trick is to go to the quarter wave (9 o'clock) point as precisely as possible, then measure the capacitance at half that frequency.? The reactance is the numerical value of characteristic impedance. My version of firmware doesn't support a Reactance menu item (I use Phase instead) so I have to measure capacitance and then the formula 1/(2*pi*f*C).? My scientific calculator does it almost instantly. When I did that with various cables, I see that none are exactly nominal Z, but plenty close enough, within an Ohm or three.? Much more believable than what I was getting. Thanks to all who submitted comments and ideas. Bob ? ? On Sunday, January 26, 2020, 02:09:58 PM PST, hwalker <herbwalker2476@...> wrote: On Sun, Jan 26, 2020 at 01:29 PM, Oristo wrote: OK, since you ask, but I find yours much simpler to follow ================================================== Kind of you to say Oristo.? The comments that followed my post, regarding my mixing up the wavelength terms, were right and require correcting.? Andy's explanation is easy to understand.? My steps are easy for me to remember and have worked every time, but now I have added Andy's explanation to them in my notebook. Best regards, - Herb |
