Directional Antenna Basics At the outputs of this network, or
networks, the RF will typically enter the transmission line
feeding each tower. At the far end of the transmission line is
the ATU. This portion of the system takes its RF at the
transmission line impedance and transforms it to the tower drive
point impedance using a specific phase shift. The drive point
impedance of each tower is the impedance of the tower when the
array is active. It is derived from the self-impedance of the
tower and the coupling that occurs with other elements in the
array when the array is active. Certain designs can result in a
negative tower, which is a condition in which the resistance of
the drive point impedance is less than zero. Instead of
delivering power to the array, the negative tower is parasitic
and absorbs power delivering it back to the phaser. Contrary to
popular wisdom, a negative tower is not a nine-headed Hydra
unless the impedance is within a couple of ohms of zero. Such
low impedances, regardless of the si