Richard Clark wrote:
"This analysis is derived from Schelkunoff`s work that was later (1952)
offered in his "Advanced Antenna Theory"."

Kraus acknowledges this at the beginning of his 1950 "Antennas" chapter
8:
"This problem (input resistance and reactance) is most simply approached
by Schelkunoff`s treatment of the biconical antenna (S.A. Schelkunoff,
"Electromagnetic Waves"1943) which will be outlined in the following
sections."

As Reg Edwards says, these guys all copied each other`s work. But, they
nearly always gave each other the credit due.

Other than a few discones, which have 180-degrees as one of their vertex
angles, I don`t see many amateur conical antennas despite the obsession
with bandwidth and SWR. Skeletal designs might retain desirable
characteristics without extreme cost and complexity.....Nah, the plain
thin wire dipole is too simple and easy.

Best regards, Richard Harrison, KB5WZI