An online expert opined:
Whether your antenna is a perfect 1:1 or a 10:1, a 50 foot length of coax will have HALF the loss of the exact same coax on the exact same antenna system as measured with the 100 foot piece.
Is it true? Can we learn from it?
Let’s take a worked example of Belden 8259 (RG58A/U) with a load of 5+j0Ω at 146MHz. VSWR is approximately 10.
The loss of 100′ under that mismatch calculated with TLLC is 11.367dB, for 50′ it is 7.484dB, or 66% of that for 100′.
It is not “HALF” as shouted by the expert.
The results should be no surprise to anyone who understands transmission lines.
For a line of this type most of the loss even at 146MHz is conductor loss, and so more power is lost in regions of higher current than adjacent regions of lower current. The line is quite lossy, so although the VSWR at the load is 10, it reduces towards the source due to that loss, and as the VSWR reduces, so the loss per foot becomes lower and more uniform tending towards the matched line loss (MLL). In fact if you look at the loss in the first 50′ of the 100′ case, it is 11.367-7.484=3.883dB which is barely above the MLL for 50′ (3.348dB).
Be wary of what you read online.
The online expert expressed a flawed but very common opinion on transmission line loss under standing waves. You might ask where that comes from if it is so common.
The very expression of line loss in dB/length is the start of it. The manufacturer specified loss figures are more fully known as Matched Line Loss… and “Matched” is a hint to their limitation… they apply to a line that has a matched load, ie there are no standing waves and the ratio of V/I=Zo at all places along the line.
For most good practical transmission lines at HF through VHF and most of UHF, most of the loss is conductor loss, and so more power is lost in regions of higher current than adjacent regions of lower current. Whilst loss along a matched line is uniform, it is not so under standing waves.