Lesson 1/Learning Event 3

Figure 1-3.

Schematic Diagram of T- and H-Type Pads.

(3) A pad will maintain its characteristic impedance only if terminated

in a like impedance. To prove this point we will design a symmetrical T

pad and then apply it to a theoretical circuit. Table I shows the shunt-

and series-arm factors for T pads between circuits of equal impedance.

Assume that we must design a 12-dB, symmetrical T, and 600-ohm pad.

According to table I, the series-arm factor for 12 dB is 0.5985 and the

shunt-arm factor is 0.5362.

R1 is the product of 600 times 0.5985, or

359.1 (approximately 360 ohms). R2 is the product of 600 times 0.5362, or

321.72 (approximately 320 ohms).

R1 will be the series arm and R2 the

shunt arm in figure 3-4. The total resistance presented to the generator

will be 600 ohms when the load resistor RL is attached. If RL is removed,

the total resistance presented to the generator will change to 1,280 ohms.

The circuit impedance (resistance, in this case) will no longer match the

output impedance of the generator, and reflection loss will occur. If an

H pad is desired, the value of R1 is one-half the value determined in the

above problem.

The series-arm resistors in an H pad will each be 190

ohms.