The directed radial envelope is commonly referred to as the antenna’s gain which compensates for losses created in the coaxial cable and connectors. Antenna gain may be expressed as either dBi or dBd and can be confusing as manufacturers do not always specify which metric is referenced.
When antenna gains are expressed as a comparison to the transmitter (TX) power/ isotropic antenna, they are referenced in dBi. Conversely, if the antenna gain is referencing a dipole antenna, it is referenced to dBd. The directed gain of a dipole antenna + 2.14 dB indicates there is an increased horizontal gain of 2.14dB (with consequent vertical loss), relative to power from the transmitter (TX power)/ isotropic antenna. Establishing the reference point as either dBi or dBd is of practical importance in system design given the increased reach of the dipole antenna relative to a reference isotropic antenna.
Irrespective of reference point, for effective communications reach, a dipole should be mounted vertically at it has a positive gain in the horizontal plane given the orientation of the electric field relative to the Earth’s surface.
As illustrated, the dipole antenna distorts the directed energy in the horizontal plane with consequent loss in the vertical plane (ie up and down).
As indicated, government regulations mandate the amount of radio power emitted from a wireless device in order to manage the spectrum. These bodies will reference the maximum transmitter power and overall gain relative to an isotropic antenna or gain relative to a dipole antenna.
Equivalent/ effective isotropically radiated power (EIRP) is a measure of overall gain (including coaxial and connector losses) referencing an isotropic antenna of equivalent signal strength, measured in dBi. As the isotropic antenna is a theoretical construct, a more practical measure is the effective radiated power or ERP. ERP also references overall gain (including coaxial cable and connector losses) relative to a half-wave dipole antenna and is measured in dB.
More generally gain may be expressed as: