I just hung and tested a new 20m minimal vertical with peak about 22', feed point just over 6', and two raised radials sloping down to about 2' at the ends. Coax terminates to a UHF connector, which is hooked to a choke made of twin power cable wrapped through two stacked 31-material ferrite toroids for 13 turns. I am using 25' of rg8x for current testing.
I originally cut all three wires for about 17'. Initial SWR measurements showed a nice dip to 1.5 around 13 mhz, so I "shortened" each wire by about 6" by bending them back on themselves at the end insulators. SWR was now dipping at around 13.250 mhz, so i bent back another 6". Another check showed the same SWR dip at 13.250, so I didnt bother to shorten it yet again.
Any ideas what might keep the lowest SWR where it was after the second shortening? The wire is solid bare 18 AWG, so it makes electrical contact where it loops back and wraps itself. Could the toroid choke be causing it? Or my length of coax?