AN4190

Antenna theory

Doc ID 023812 Rev 1

9/29

greater the energy transmitted or received. According to the IEEE standard, gain doesn't

include losses arising from impedance mismatches (reflection losses) and polarization

mismatch (losses). This means, therefore, that the gain takes into account only the loss of

the dielectric and conduction system of the same antenna. The reflection losses and the

polarization mismatch are very important losses and they need to be included in the link

calculation of a communication system to determine the received or radiated power.

1.5

Antenna efficiency

If an antenna is taken as a device which accepts power from a source and radiates it into

space, the ratio of the power radiated into space to the power accepted from the source is

the efficiency (

reference as the ratio of the total power radiated by an antenna to the net power accepted by

the antenna from the connected transmitter. The power that is accepted by the antenna but

not radiated is dissipated in the form of heat.

The total antenna efficiency

within the structure of the antenna. In general, the overall efficiency can be written as:

Equation 1

where:

line].

Usually

The radiation efficiency is rarely, if ever, published in any antenna manufacturer's literature.

There are several reasons for this: first, radiation efficiency is exceedingly difficult to

measure accurately. Second, the radiation efficiency of an antenna is implicitly contained in

the complete specifications of the gain of an antenna.

1.6

Antenna bandwidth

The bandwidth of an antenna is defined as the range of frequencies within which the

performance of the antenna, with respect to some characteristics, conforms to a specific

standard. The bandwidth can be considered to be the range of frequencies, on either side of

a center frequency (usually the resonance frequency for a dipole), where the antenna

characteristics are within an acceptable value of those at the center frequency.

Because the characteristics of an antenna do not necessarily vary in the same manner or

are even critically affected by the frequency, there is no unique characterization of the

bandwidth. The specifications are set in each case to meet the needs of the particular

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