"Ralph Mowery" wrote in
:

....
Way too much time and money is being wasted in the change over from
the rg58 coax. Even with a 20 to 1 SWR the rg-58 will loose about 3
db of signal due to the large mismatch.

The OP mentioned a 2.5m whip down to 40m. VSWR(50) of such a thing is
likely to be around 1000 so your estimates of loss for VSWR(50) are not
applicable to that scenario. (Estimating loss based on VSWR is prone to
error in any event.)

An NEC model suggests that feedpoint Z might be something like 10-j800.
Taking that for example with 25' of RG58, line loss is more like 17dB.

RG213 is better, but it does not solve the fundamental problem that R at
the feedpoint is very low, and a large current is required to deliver
power. Large currents contribute to high loss in feedlines.

Owen