Why does the cable resistance jump from a low value to high value at a particular frequency?
Your tooling seems to be the cause there, not the cable. From https://www.keysight.com/main/editorial.jspx?cc=US&amp;lc=eng&amp;ckey=1428419&amp;nid=-32775.536879654&amp;id=1428419


  The 4294A extends its measurement frequency range up to 110 MHz by terminating each measurement terminal with 50 ohm in order to eliminate the resonance of test leads (including leads inside the 4294A). The measurement discontinuity is caused by the change in termination impedance at 15 MHz when the ADAPTER is set to NONE or at 5 MHz when it is set to 1m or 2m. The measurement discontinuity can be removed by performing LOAD compensation.