I made a vertical whip for 70MHz by adapting the fibre glass chassis and the radials of a dead Watson 300. The internal colinear of the Watson 300 was a bit corroded and the capacitor on the base loading coil had disintegrated.
I extended the three radials with sliding alloy tubes from a scrap VHF yagi and took off the top cap. Then instead of the base loading and colinear I fitted a low loss 75 Ohm coax with inner and outer shorted. This was nearly 4m long. I cut it shorter till the nanoVNA indicated a reasonable match when it was on a 2m pole in the yard. I’d earlier tried to to a combo colinear for 50 MHz and 70 MHz and a colinear for 70 MHz only. I think 50 MHz and 70 MHz in a single vertical is nearly impossible. A Yagi-Uda or log periodic is possible. So the simple vertical seemed best. The exposed coax (really a wire) was accommodated by gluing a length of white PVC pipe on the top of the fibreglass tube and gluing a tight fitting bottle top on the open end. The radials were also adjusted and unsurprisingly 1 m (about 1/4 wave) was best. They were then clamped. The vertical wire part must be about 3.5 m. I’d expected it to be shorter. A 5/8ths whip for 52 MHz is maybe a little longer. Obviously a whip that’s electrically 3m on 6m band won’t tune at all with an FC-40 without a base-of-whip matching autotransformer as the FC-40 can’t tune a 1/2 wave or exact multiples on any band.
It has been sitting the rear garden since last year. Today it was put on pole on the chimney previously having a discone. An old 12V relay unit with three SO238/”UHF connector” sockets was added on the roof on the output of the FC40 tuner which feeds a long wire from the roof to a flagpole at the front gate. As expected the whip tunes well on 10m but is poor SWR on 6m (50MHz), though usable. Possibly the coax feed cable from the whip to the relay/FC-40 on the roof is radiating at 50 MHz. The longwire tunes well enough at 50MHz, and mostly well enough from 1.8 MHz to 24.5 MHz bands. The 20m band is most tricky for exact length of longwire and it needed lengthened as front gate to chimney is too close to one wavelength at 20 m (14 MHz) band.
Now I need a parasite power connection to main rig 13.8V power and an on/off switch for the remote relay. The supply cable is a 5A mains appliance PVC-PVC that was used as a speaker cable some years ago.