# Additional loss due to VSWR

Hams often refer to Additional loss due to VSWR, indeed some ham publications contain graphs that show the Additional loss due to VSWR. Let's take an example from the award winning article Understanding SWR by Example (Walraven 2006). The example and its analysis is quoted below for reader's convenience.

Lets review the example. The stated matched line loss of 0.5dB for 300ft of 450Ω ladder line at 29MHz is probably optimistic, as is the assumption that Zo is 450Ω. The following analysis uses N7WS's characterisation of Wireman 551 ladder line, matched line loss 0.9dB and Zo=400Ω. Using TLLC for the ladder line case, line loss for Wireman 551 under the stated conditions is 3.5dB, more than double K5DVW's 1.4dB. K5DVW does not state the specific coax line analysed, but a practical line that has 0.25dB matched line loss for 40ft at 29MHz is LMR400, and it is used in the following analysis. Using TLLC for the coax case, line loss for LMR400 under the stated conditions is 5.9dB, less than half K5DVW's 12dB.

 K5DVW VK1OD 300' ladder line 1.4dB 3.5dB 40' coax 12.0dB 5.9dB Difference 10.6dB 2.4dB

The outcomes are quite different. There are two factors that contribute to the difference:

• K5DVW's matched line loss of nominal 450Ω ladder line is optimistic (possibly based on ARRL figures, see A review of QST article "The Lure of Ladder Line" for more information); and
• the Additional loss due to VSWR Graph is not correct in this scenario.

So, what is wrong with the Additional Loss Due to VSWR Graph? The graphs are usually based directly or indirectly on the formula LossRatio=(1+S^2)/(2*S) that depends on a set of assumptions that are rarely stated. Some of the graphs are 'adjusted' for source end VSWR, but the adjustment adds more assumptions that again, are usually not stated. Smith (Electronic Applications of the Smith Chart) states:

If a waveguide is one or more wavelengths long, the average increase in dissipative loss due to standing waves in a region extending plus or minus one-half wavelength from the point of observation may be expressed as a coefficient or factor of the one-way transmission loss per unit length.

and goes on to give the above formula. So, the graphs are often misused in ignorance of the underlying assumptions. The ongoing reference to Additional loss due to VSWR in our handbooks, tools, language etc engenders and reinforces a flawed concept that line losses where VSWR>1 are necessarily greater than the matched line loss.