10-19-2011, 12:58 AM
Crotchety Old Bastard
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Join Date: Apr 2005
The amount of power used to check SWR isn't going to matter, SWR doesn't change with power. The accuracy of the meter you use to check that SWR can, and does change with the power level. A good idea is to use the least amount of power necessary for that meter to read anything (for several reasons). Remember SWR is a -ratio-. That ratio between forward and reverse power won't change with the amount of power used.
That instruction manual is a necessity, and so is reading and understanding it! The root of the whole thing is understanding that impedance (Z) is a combination of both resistance(R) and reactance (X), or, Z = R + X. An SWR meter only reads 'Z', it can't tell you anything about 'R' or 'X', just the combination of those two things. The 'catch' to that is that there's an infinite number of 'R's and 'X's that can in combination can equal any particular SWR reading. The higher that 'X' is, the worse the antenna will behave because reactance 'X' doesn't contribute to radiating anything. It's what causes power to be reflected. Another 'catch' is that a typical antenna will almost never have an input resistance 'R' that will be the 50 ohms that's normally desired. It may be 'close', which will certainly work, but don't expect it to be 50 ohms exactly. (One reason why an SWR of 1.5:1 is considered acceptable!)
The biggest problem with all this is that the importance of SWR has been blown way out of reason. It is NOT the 'end all' of tuning antennas. It's only a very inexact indication of things.