"Meindert Sprang" wrote in
:

But doesn't this ground sleeve produce the "other end" of the dipole?



Yes. It performs the same function and gives the coax cable that must feed
the center of the dipole a shielded way to keep from being part of the
antenna and becoming a radiator itself, by putting it inside the sleeve.
It's still a dipole.

Impedance of the sleeve dipole is around 65-75 ohms, but that's "close
enough for government work", as we used to say in the Naval Shipyard. To
get 50 ohms of match, the sleeve needs to be a skirt out at around 45
degrees from the horizontal, like those ground plane base antennas with the
4 or so radials at 45 degrees, halfway between horizontal and vertical.
Flat out, like mounting a 1/4 wave whip against the metal top of a cabin or
car roof the impedance is near 30 ohms.