Lesson 1/Learning Event 3

Learning Event 3: ATTENUATION AND ATTENUATORS.

Attenuation can be defined as loss of signal power, and may be

classed as either intentional or unintentional. Loss of signal power can

be attributed, in most cases, to impedance mismatch (reflection loss) and

to heat loss. Reflection loss is unintentional as is also heat loss due

to resistance in lines. However, heat loss is intentional when values of

resistance are willfully inserted in the circuit.

Heat loss is usually

called

I2R loss.

a. Importance of Impedance Matching. A communication system consists

of many parts which must be connected together by means of telephone lines

or other facilities.

Each element of the system has a given input and

output impedance which must be matched to the line facilities.

The

purpose of impedance matching is twofold: To transfer maximum power with

minimum loss, and to prevent reflection loss with consequent phase

distortion of the signal elements.

(1) Maximum transfer of power is demonstrated in figure 1-2 by three

simple DC circuits consisting of a battery and two resistors.

Each

circuit represents a transmission line, with R1 being the source impedance

and R2 the load impedance. The battery represents the source of energy.

Calculations shown on the figure prove that power (0.24 watt) delivered to

the load is greatest when impedances are matched (both 600 ohms).

When

load impedance is greater or lower than source impedance, power delivered

to the load is reduced (0.18) watt.

Figure 1-2.

Maximum Power Transfer from Source to Load.