Do you know How Good An Antenna Is?

Four ”figures” describe how good an antenna is compared to the required performance:

• SWR = Standing wave ratio
• D = Directivity
• G = Gain
• BW = Bandwidth

SWR
If the impedance of the antenna is different from the impedance of the cable, the antenna will reflect back some of the induced energy through the feeder cable to the transmitter, which naturally is undesirable.

Normally, the impedance of the cable is 50 Ω. If Ra indicates the impedance of the antenna, the standing wave ratio is defined as:

SWR = Ra/50 Ω (if Ra is more than 50 Ω) or SWR = 50/Ra Ω (if Ra is less than 50 Ω)

Examples:
If Ra = 50 Ω is SWR = 1.0
If Ra = 100 Ω is SWR = 2.0
If Ra = 25 Ω is SWR = 2.0

Consequently, it is of importance that the SWR is as close to 1.0 as possible thus obtaining the highest power being transmitted from the cable to the antenna.

Directivity D
The directivity D is an indication of the capability of the antenna to conduct the radiated power ”to a certain site”.

Normally, omnidirectional or directional antennas are mentioned.

Omnidirectional:
An omnidirectional antenna with high directivity has a radiation being similar to a pancake.

Directional:
A directional antenna with high directivity has a radiation being similar to the cone of light from a projector.

Gain G
The gain of an antenna is defined as G = η x D, where η indicates the efficiency of the antenna.

Consequently, in the gain value possible loss in the antenna is comprised. The η-figure is always less than the directivity.

For most antenna types the own loss is so low that G = D can be considered.

Bandwidth
The bandwidth of the antenna is the frequency range, in which it operates properly, i.e. both gain and SWR are within the more specified limits.