Circuit Theory •

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Circuit theory deals with electrical circuit.

An engineer can predict the performance of complicated electrical networks with the help of circuit theory. But this theory has certain limitations like : It cannot be applied in free space. It is useful only at low frequencies.

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This theory is unsuccessful in explaining the radiation of electromagnetic waves into space in radio communications. It cannot be used to analyse or design a complete communication system. Example: Radio Communication System.

Field theory has following advantages in comparison to circuit theory •

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It is also applicable in free space. It is useful at all frequencies, particularly at high frequencies, The radiation effect can be considered. This theory can be used to analyse or design a complete communication system. Example: Wireless Communication, Radio Communication.

Electromagnetic Field Theory •

Although electromagnetic Field Theory (EMFT) is complex in comparison with circuit theory but EMFT is simplified by using appropriate mathematics. This theory deals with E and H vectors, whereas circuit theory deals with voltages and currents.

Relationship between Field Theory and Circuit Theory •

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At lower frequencies where physical circuit dimensions are small compared to the wavelength of electromagnetic waves, the behaviour of circuits is accurately modelled using “lumped element ” component models, together with Kirchhoff’s laws At higher frequencies where the distances between components are a significant fraction of a wavelength and greater, the signals carrying information or power from one place in a circuit to another are treated as waves.

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Signals must be routed from one point to another using transmission lines, modelled using transmission line theory. If the component dimensions be comparable to the wavelength then accurate understanding and prediction of behaviour may require modelling using electromagnetic field and wave theory

why dB is used in microwaves •

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A decibel is a convenient logarithmic ratio of two RF power or RF voltage levels (usually input and output levels). The beautiful thing about log ratios is that multiplication of "linear" numbers becomes addition, and division becomes subtraction. The conversion of linear ratios to dB is: 10xlog(power level2/power level1), or 20xlog(voltage level2/voltage level1)

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Decibels are very useful for talking about increases (gains) or decreases (losses) without talking about the actual power or voltage levels. Remember, though, that dB by itself isn't a unit like millimeters or inch, it's all relative. A negative number of dB indicates loss or reduction in signal strength, while a positive number indicates gain or increase in signal strength. When you refer to a loss in dB, it is customary to eliminate the negative sign The decibel system is used when quantities can vary by massive amounts.

Table I. Conversion between Linear Units and Decibels Linear Value

dB Value

1e-10

-100 dB

1e-9

-90 dB

1e-6

-60 dB

1e-3

-30 dB

1e-1

-10 dB

0.5

-3 dB

1

0 dB

10

10 dB

100

20 dB

1e3

30 dB

1e6

60 dB

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