Cable Length Measurement (5)

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.