On Sun, Oct 13, 2019 at 11:03 AM, W5DXP wrote:
If we replace the balun with a 50 ohm dummy load, disconnect the antenna, and
measure the impedance looking back down the feedline, what do we measure?
TLDetails says very close to 189+j1000 ohms, actually considering losses,
195+j994 ohms, i.e. a near-conjugate match.
I agree. This is true of any lossless (or essentially lossless) matching network (be it a tuned feedline or a lumped-element matching network) that has been tuned to match an impedance attached to the network's output to, say, 50 ohms, If you then disconnect the load and attach 50 ohms to the input of the network, and then measure the impedance presented by the tuner at its output (using your NanoVNA, for example), the measured impedance should be the complex conjugate of the impedance that had been attached at the network's output. See attached drawing.
I've used this technique to determine the "match-space" of various antenna tuners (that is, the range of impedances a tuner can match to an SWR of 1:1). If I attach 50 ohms to a tuner's INPUT and then measure the impedance at its output terminals as I vary its tuning controls, I am measuring the complex-conjugates of the load impedances that this tuner can match to 50 ohms. For an example, see:
Of course, if a network is lossy, the measured impedance presented by the tuner at its output (when its input is terminated with 50 ohms) will diverge from the actual complex-conjugate of the load impedance.
- Jeff, k6jca
