The coax cable for a 17m band dipole is terminated with a 100 ohm resistor at far end, temporarily removing the antenna elements.
(%i) V1:1.0+%i*0.0; (%i) t:2.0*3.1416*(-22.0e-9/(1/1.91e6)); (%i) s:2.175/2.682; (%i) V2:s*(cos(t)+%i*sin(t)); (%i) Vr:2*V1-V2; (%i) z:50.0*V2/Vr; (%i) realpart(z); (%o) 29.74885022942517 (%i) imagpart(z); (%o) -13.86642130849026 (%i) abs(z); (%o) 32.8218178941588 (%i) g:(z-50)/(z+50); (%i) abs(g); (%o) 0.30321141723506 (%i) SWR:(1+abs(g))/(1-abs(g)); (%o) 1.870311095029369
The impedance at the near end is therefore Z=29.7-i13.9 ohm.
With a 20m cable, the impedance at far end should be Z=93.4+i3.4 ohm, which sounds reasonable.