The lower formulas below calculate the current at the load (termination) based on the current at the input on the left using the impedance at the load, on the right using the impedance at the input. Yet another method is convenient if you also want to calculate the current and voltage at either end of the line. #Tlw transmission line program for windows full#The full K6OIK presentation can be downloaded here.It includes a very nice explanation of the flawed assumption in the ARRL formula and why the formula is accurate only in special cases, none of which have anything to do with the stated warning of "SWR levels less than 20:1". Here's page 43 from the K6OIK presentation. That's not the same as a negative SWR which happens when the magnitude of the (normal) reflection coefficient is greater than one.Īnother method was described by Steve Stearns, K6OIK, in a presentation titled "A Transmission Line Power Paradox and Its Resolution" given at Pacificon 2014. Note that "LOSSswr" calculated above can be negative in some cases. #Tlw transmission line program for windows software#The one that has been used in all AC6LA software (except JavaScript) for the past 15 years or so was described by Charles Michaels, W7XC (SK), in the November 1997 issue of QST. Once you know the impedance at both ends of the line there are several different ways to calculate the total dB loss. The SolutionThe first step is to use the basic "Transmission Line Equation", also called the "Telegrapher's Equation", which calculates the impedance at the input end (near end) of a line given the impedance at the load (far end, termination) and the characteristics of the line itself. And there's no way to know without doing more rigorous calculations. The disadvantage of the ARRL formula is that the results may or may not be particularly accurate. And they all copied that script directly or indirectly from the original. That's why so many online calculators all look the same they all use the same script code in the html of the web page. The advantage of using this formula is that it is simple and thus easy to implement using JavaScript. The ProblemMost of the online calculators use the ARRL's "Additional Loss Due to SWR" formula, shown (with minor edits) below. Online coax loss calculators that use SWR as one of the inputs certainly are convenient but can you trust the results?Alternative tools which are much more accurate are readily available. Think Twice Before Using a Web-Based Line Loss Calculator "Additional Loss Due to SWR" is in Quotes for a Reason
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