Now and then, someone accuses the KAT500 of disturbing their impedance match even when it is bypassed. I wanted to see just how much residual mismatch the KAT500 has when it's in bypass mode, so I made a couple of measurements today.
I took measurements at 3.6, 7.1 and 28.3 MHz; in other words, on 80, 40 and 10 meters. I suspected that 10 meters would be close to a worst-case scenario, which is why I included it in this test. In Step 1, I connected my dummy load to the VNA and measured its impedance on the three frequencies. In Step 2, I connected the dummy load to the ANT #3 output of the KAT500 in bypass mode and measured the input impedance of the tuner. Let us begin at 10 meters and get the bad news over with. The dummy load by itself, at 28.3 MHz, measured: 48.954 - j*0.032 Ω It's hard to digest this, so let's convert that impedance to SWR: 1.021. Now, the same dummy load through the bypassed KAT500 measured: 46.866 + j*1.498 Ω, or in terms of SWR: 1.074. Are you alarmed? Let's break this down a bit to understand if this is really, really bad, or just kinda bad. In an ideal world, when you put a KAT500 into bypass it should 'disappear'. It shouldn't change anything. It should feel as if the tuner wasn't there and you were directly connected to the dummy load or whatever is on the output of the tuner. And you should measure the same exact impedance as when the tuner really was out of the picture. But in reality, this is impossible. Even when bypassed, a device like an antenna tuner adds length -- the distance between the input and output connectors-- and this length, like all conductors, has a little bit of inductance. The tuner is also going to add stray capacitance. Imagine shrinking down to the size of an electron and entering the tuner through the coax. The first thing you see is a coax connector, followed by a cable, a relay, a circuit board, another thingy maybe, and eventually you emerge from the tuner at another rear panel coax connector. By traversing this path, you were subjected to the stray capacitance and inductance of all the stuff you touched or passed by. By virtue of being in the real world, the tuner doesn't disappear but in fact adds a measurable perturbation to an otherwise pristine path. How much did my KAT500 add in this real and imperfect world? In order to bump the dummy load's impedance to the new value I measured, the KAT500 had to add the following: 24.2 pF of stray capacitance -- a totally believeable value, and 64.1 nH of inductance from the path -- again, a value in line with reality. In case you're still laboring under a pretty good shock right now, don't be too scandalized. Perhaps it's helpful to recall this: First, you've heard of LMR-400 coax, right? Well, LMR-400 coax has a capacitance of 24 pF per foot. That's right, just 1 foot of this pretty good coax has 24 pF (actually 23.922 pF, according to renowned transmission line guru AC6LA). Second, that same piece of coax has 61.5 nH of inductance. Holy smokes, those numbers are darned close to the KAT500's!! The KAT500, when all is said and done, looks pretty much like a short 1-foot section of very good coax when it is bypassed, with a tiny bit of mismatch thrown in for good measure. In addition, I wish I could post a screenshot of my Smith Chart with the before and after impedance values plotted on it. The distance between the two points is far less than 1/100 of the diameter of the Smith chart. The two points are virtually on top of each other. It's only because of the resolution of the test equipment that we can even see the difference between 1.021 and 1.074. (On 80 and 40 meters the difference is literally two-tenths of an ohm-- not even worth mentioning here.) I hope that this put a new perspective on something that we see here on the reflector rather often. Regards, Al W6LX/4 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
