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Understanding Tee Networks
This month we take a more in depth look at the tee network. This impedance matching network, aptly named, has the appearance of a T. The vast majority of AM transmission facilities will have at least one of these networks somewhere in the facility. The substantial versatility of the network is the primary factor in its ubiquity.
The tee network seems almost magical as it has the ability to transform between almost any two complex impedances using only reactive elements. Remember that reactive elements are capacitors and inductors, and have essentially zero resistance. The absence of resistance in the elements comprising the network is critical as it eliminates power dissipation and associated heating. Even though ideally there is zero resistance in the components, the reality is we live in an imperfect world. Thus, some small quantities of resistance are unavoidable. The presence of these miniscule resistances is one of the reasons power is normally measured through the use of current meters at the antenna. In directional antennas, this is accounted for by a fudge factor in the mathematics.
In addition to transforming two impedances, the tee network also creates controlled phase shifts. The sign and magnitude of any shift in phase across the network is a function of the arrangement and value of components in the various legs. For instance, networks where the shunt leg is capacitive tend to induce a negative, or lagging, phase shift. Conversely, networks in which the shunt is inductive, or has a positive reactance value, tend to result in a leading phase shift.
Use of particular phase shifts, in combination with power division is how directional patterns are created. Although the phase shift across the matching network for a non-directional antenna is irrelevant with regard to the radiation characteristics, it cannot be totally ignored. In cases where the antenna impedance varies wildly from the transmission line impedance, changes briskly from one sideband to the other, or other out of the ordinary cases, judicious selection of the phase shift can improve bandwidth.
Attention to details
In addition to bandwidth considerations, care should be paid to the current in the shunt leg of the network. Changing the network phase shift will affect the current in the shunt leg for a given set of impedances. For a non-directional station, networks with a phase shift of 90 degrees are fairly common. The reasoning here is the mathematics work out quite nicely. Sometimes, however, the impedances may vary enough on the sidebands that a phase shift in this range results in a shunt leg current high enough to warrant larger components. Swinging the phase shift closer to zero degrees can result in a lower shunt leg current translating into greater savings in the component price.
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