Q Joe, NC4D, asks, “I’m curious about connecting two VHF antennas. Can you use a T connector to connect one feed line from a 6-meter beam, and another from a 2-meter beam, to a single piece of coax going back to the radio? Would this be any different than having multiple dipoles in parallel, all connected to the same feed line?”
A Parallel dipoles work as they do because the antennas that are nonresonant to the frequency of the transmitted signal provide a high impedance at the connection point while the antenna that is resonant provides a low (approximately 50 Ω) impedance.
A 6-meter beam may or may not offer sufficiently high impedance to 2-meter RF, and vice versa. Either way, you still have the issue of what happens in the coax. Coax that is terminated in its characteristic impedance will present the same impedance on the other end. Coax that is terminated in a high impedance will present a different impedance on the other end, dependent upon the length. Consider an open coax stub: the far end is about as high an impedance as you could want. If the coax is 1/2 wavelength (or a multiple thereof), the near end will also be a high impedance. However, if the coax is a 1/4 wavelength (or an odd multiple thereof), the near end will be a very low impedance. Lengths in between will give other impedance values.
So, to do what you describe, you would have to adjust the length of the coax going from the T to the 6-meter beam in such a way that it offers a high impedance to 2-meter RF. You’ll need to meet the opposite condition with the coax that runs between the T and the 2-meter beam. Perhaps an easier alternative would be to purchase a diplexer.
These matching/coupling devices are primarily designed to allow multiband VHF/UHF transceivers with single feed line ports to operate on several bands without changing antennas. Feed lines from each antenna connect to the diplexer, then a single coax feed line runs between the diplexer and the radio.
From QST July 2000
