Application Note 16: Quick Method of Finding "T" Network Values

It is often helpful to know approximate values of reactances in “T” networks without resorting to mathematics. The table below has been developed as a quick reference to get network components close enough so that an empirical adjustment may be made with an RF Bridge.

When using this method, bear in mind that actual components must be properly rated in current and voltage to guard against failure. The table also lists some typical voltages and currents for 1,000 watts. These can be adjusted to actual operating power by multiplying by the square root of the power ratio. Voltages and currents include a 3.5 voltage and 1.3 current safety factor, adequate for normal 125% modulated AM.

The values provided are for a network feeding pure resistance loads. If the antenna is reactive, that value must be added algebraically to the value given for Z2. The current through Z2 will not change, but the voltage across the component will and must be computed. Also, note that voltages across components assume a single component per leg. Series or parallel combinations to achieve these values of Z must be individually examined for component voltages and currents. The 1,000 watt voltage to ground at the input for 50 ohms should be assumed to be about 800 volts, with safety factor. Safe values for different output resistances are given in the table.

"T" NETWORK DIAGRAM

VALUES FOR 50 OHM LINE IMPEDANCE