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Since the question was asking about feeding antennas on the tower rather than separating signals at the bottom of the feedline, the use of a diplexer would require that the diplexer and it's connections be installed inside a suitable weatherproof enclosure on the tower. Also not all diplexers are created equal; I would avoid any diplexer with coaxial leads and stay with a connector only type. Also consider the total amount of RF signals that will be passing through the diplexer and don't forget to leave some headroom for reflected power.

This case would actually require the use of two diplexers; one in a suitable enclosure on the tower and the other at the equipment end of the feedline.

Using a coaxial T connector simplifies weatherproofing however the use of the folded dipole antennas would most likely require critical lengths of coax cable between the T and the antennas. These critical lengths might also need to be something other than 50 ohm coax.

The fan dipole is not a good comparison since in the case of the fan dipole the wire making up each side of the dipole is either resonant or non-resonant and the antenna is an open circuit at DC and is resonant only over a relatively small bandwidth. The folded dipole antenna is a DC short circuit that has a wide bandwidth over which it can be resonant.

VNA and Smith charts anyone?


Milt
N3LTQ