The test set-up of Fig. 24-1 was built for these measurements. The load box at. the antenna end of the transmission line was a little multi-2 dummy load that I use for various workshop applications and is shown in Fig. 24-3.1 can select an external dummy load, a short circuit, and a total of 10 discrete impedances. When the external dummy load is disconnected, the load box sees an open transmission line in that switch position. Why have a load box? It is not part of the TDR, but helped to calibrate the system and generate the pictures that follow. You dont need one for your own TDR, even though I recommend load boxes as a workshop adjunct.
Why have 12 different loads, you say? Because I had a single-pole, 12-position rotary switch in my junk box, that's why. The impedance values shown were selected to (hopefully) represent a wide range of actual impedances typically encountered in amateur antennas.
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