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#33
September 27th 05, 04:21 PM
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Owen Duffy wrote:
On Tue, 27 Sep 2005 02:54:31 +0000 (UTC), wrote: Owen Duffy wrote: On Tue, 27 Sep 2005 02:25:11 +0000 (UTC), wrote: SWR is nothing more than a dimensionless impedance ratio. The fundamental definition of SWR flows from the behaviour and properties of RF transmission lines. And power=EI. And it also equals I^2*R and E^2/R. SWR can be expressed in terms of power ratios, current ratios, and impedance ratios. When a transmission line is terminated in an impedance other than its characteristic impedance, there will be both a forward wave and a reflected wave of such magnitude to resolve the conditions that must apply at the termination. Irrelevant. The forward wave and the reflected wave sum at all points along the line having regard for their magnitudes and relative phase to produce a "standing wave". The Standing Wave Ratio (SWR or VSWR) is defined to mean the ratio of the maximum to the minimum of the magnitude of the standing wave voltage pattern along the line. Is is also defined as a current ratio and an impedance ration. The SWR on a lossless line can be calculated knowing the complex characteristic impedance of the line and the complex load impedance. What no waves, just impedences!! Now you are contidicting yourself. The SWR on the line does not depend in any way on some unrelated independent reference resistance as you suggest in your formula. Read it again. The R is the R of the thing at the end of the line. The X is the X of the thing at the end of the line. The X is the impedance of the line. You seem to be suggesting that your redefined SWR is a really good (obscure) way to talk about an impedance (independently of a transmission line) in terms of some standardised reference value, and you can throw away the fundamental meaning of SWR to support your SWR(50) concept. In your terms (independently of a transmission line), for instance, a Z of 60+j10 would be SWR(50)=1.299, and so would an infinite number of other Zs have SWR(50)=1.299... how is that of value. To know Z is 60+j10 is to know more than to know SWR(50)=1.299. The equations given are general and can be derived from first priciples. The Z in the equations is the Z of your reference, i.e. 50 for a 50 Ohm system. SWR is *ALWAYS* relative to some reference impedance. Owen -- -- Jim Pennino Remove .spam.sux to reply. |
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