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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 sign, tend to cause stability issues.
Now that the power is out to the tower, we have to have some way of measuring what is actually there. This is where the sampling system comes into play. Typically, current transformers or sample loops are used to feed a voltage sample back to the phase monitor. Section 73.68 of the Commission's Rules provides the requirements for approved sampling systems. In addition to the sampling system, many arrays still have ways of measuring the base currents. Although no longer mandatory, maintaining the ability to measure base currents is highly recommended as it provides an additional health check on the antenna.
Next time I'll delve deeper into the directional antenna array and look at tee networks.
Ruck is a senior engineer with D.L. Markley and Associates, Peoria, IL.
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