Additional loss due to VSWR – a quite flawed concept

The concept of “additional loss due to VSWR” is so engrained in Ham mythology.

Screenshot - 01_06_2014 , 16_30_10

Here it is detailed in the latest version of TLW announced in QST June 2014. On the second last line TLW lists “additional loss due to SWR” as 0.003dB, which seems ok in relative terms as the cable is nominally matched (VSWR=1.03).

This basically informs us fo the matched line loss (MLL) of 0.1dB for this scenario.

Screenshot - 01_06_2014 , 16_30_16

Above, the load is changed to 5Ω, so VSWR is approximately 10. Total loss is now 0.863dB incorporating 0.1dB of MLL and 0.761dB of “additional loss due to SWR”… all good, this result is believable.

Screenshot - 01_06_2014 , 16_30_25

Above, the load is changed to 500Ω, so VSWR is again approximately 10. Total loss is now 0.06dB incorporating 0.1dB of MLL and 0.04dB of “additional loss due to SWR”… wait a minute, this does not add up, 0.1+0.04=0.14, not confidence inspiring!

The 0.04dB result is believable even though the displayed figures don’t add up.

Flicking to an article by Silver in the same issue, he gives a table of “Additional loss due to SWR”

Screenshot - 01_06_2014 , 16_29_28Above, the chart given by Silver. Using that chart, following from SWR=10 on the horizontal axis up to the MLL=0.1dB line and left we find that “Line Attenuation” is about 0.49dB, 1225% of TLW for the 500Ω VSWR=10 case.

So, Silver predicts 0.49dB and TLW predicts 0.86dB and 0.04dB for the same VSWR.

Something is wrong here!

The failure to reconcile is that the chart is only valid under certain circumstances, and those limitations were not published.