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Vna smith chart8/5/2023 ![]() ![]() What gives? The 8 ohms is only the expected load. Consider an audio power amplifier: it has a "8Ω" output, but if you measure its small-signal output impedance, you'll find it's most likely some milliohms. Also, note that there are two kinds of impedance we can measure. It should behave within the common passband - but the edges of those bands might not be as flat as expected, or you might end up with weird new passbands (producing images, or even overloading), or parasitic oscillation (more for transmitters). Who knows what the combination of the two will do. ![]() Often, transceivers are made with tuned circuits, and often antennas are resonant. It's worth noting that that madness sometimes shows itself, anyway. ![]() ![]() Even then, not actually madness, but precisely tuning the network between antenna, TL and transceiver isn't going to be a pleasant experience, nor is it general (in terms of swapping out different cable lengths, or antennas or transceivers). Put another way: because anything else, that way lies madness. So, 50 or 75 ohms for most coax, or 100 for twisted pair, or 300 for twin lead, etc. It's most convenient, when everything is matched to Zo of whatever the transmission line is between them. Because if it's not 50 ohm, it's a bad transceiver and you should feel bad. ![]()
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