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David G. Nagel wrote:
If you want a quick lesson in high vswr find a ham with an old tube transmitter and see if he will hook it up to a mismatched load. The cherry red plates are the reflected energy being absorbed. Transistors will just turn to smoke under the same conditions. Unfortunately, you'd be learning the wrong lesson. The cherry color is due to the transmitter being loaded with an impedance it's not designed for, causing the final to run at low efficiency. You can disconnect the antenna and replace it with a lumped RC or RL impedance of the same value and get exactly the same result. Alternatively, you can attach any combination of load and transmission line which give the same impedance, resulting in a wide variation of "reflected energy", and get exactly the same result. All that counts is the impedance seen by the transmitter, not the VSWR on the line or the "reflected power". The problem is that the idea of "reflected energy" turning the plates hot is so easy to understand, that people aren't willing to abandon it simply because it isn't true. See http://eznec.com/misc/Food_for_thought.pdf for more. Roy Lewallen, W7EL |
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