AN4190

Antenna theory

Doc ID 023812 Rev 1

13/29

The quality of the input impedance match of the antenna is generally specified by one of two

parameters: return loss or standing wave ratio (SWR), sometimes called voltage standing

wave ration (VSWR). The return loss indicates how much of the incident power is not

reflected or doesn't return from a load. It is the square of the magnitude of the reflection

coefficient, usually expressed in logarithmical form as:

Equation 3

For instance, a return loss of -3.0103 dB indicates that half of the incident power is reflected.

Usually a return loss lower than -10 dB is acceptable for a good matching, in this case less

than 1% of the signal is reflected.

The standing wave ratio is defined as the ratio of voltage, minimum to maximum, on the

input transmission line. It is defined as:

Equation 4

One utility in the VSWR for describing input matching is that while the magnitude of the

reflection coefficient ranges from 0 to –infinity in logarithmical form, the magnitude of the

VSWR ranges from 1 to infinity in linear form. The VSWR then is particularly useful for

describing input match when the match is not very good. A VSWR of 5.83 corresponds to -

3.01 dB return loss. A good matched antenna is one that has a VSWR lower than 2.

1.9

Effective isotropic radiated power

Effective isotropic radiated power (EIRP), also called equivalent isotropic radiated power, is

the amount of radiated power measured in a single direction (that is, for a fixed

Θand Θ).

Typically, for an antenna radiation pattern measurement, if a single value of EIRP is given,

this is the maximum value of the EIRP over all measured angles.

The EIRP can also be thought of as the amount of power that a perfectly isotropic antenna

would need to radiate to achieve the measured value.

mismatch losses (L), and the antenna gain (G) by:

Equation 5

In built-up areas, regulations may restrict the EIRP of a transmitter to prevent exposure of

personnel to a high power electromagnetic field; however, the EIRP is normally restricted to

minimize interference to services on similar frequencies.

R.L.

20log

()

=

VSWR

1

|

Γ |

+

()

1|

Γ |

–

()

⁄

=

EIRP

P

t

L

–

G

+

=