dB and dBi gain

1. Antenna gain is measured in decibels. It is the ratio between the gain of the
antenna compared to the gain of an isotropic antenna. An isotropic antenna
is a theoretical antenna which radiates power uniformly in all directions.
When we calculate the gain of an antenna and compare it to an isotropic
antenna the unit of the gain is dBi (i stands for Isotropic antenna). An
Isotropic antenna has a 0 dB power rating, i.e it has no gain/loss when
compared to it self.
Dipole antennas have a different radiation pattern compared to isotropic antennas.
Though they are known to be omni-directional, their radiation pattern is 360
degrees in the horizontal plane and 75 degrees in the vertical plane (if standing
vertically). It resembles the shape of a donut. Because the beam is slightly
concentrated, dipole antennas have a gain over isotropic antennas of 2.15 dB in the
horizontal plane. Which means they have a gain of 2.15 dBi.

2. Some antennas are rated in comparison to a dipole antenna. In this case
the unit of the gain would be dBd. So an antenna that has 0 dBd gain will
have a gain of 2.15 dBi.
dBi = dB(isotropic): The forward gain of an antenna, measured in decibels
(dBi), The dBi value reflects the antenna's directional / beamwidth
characteristics, i.e., directional as opposed to omnidirectional: Generally,
the higher the gain (dBi), the narrower the beamwidth - the more directional
the antenna.
dBi is the amount of focus applied by an antenna with respect to an
"Isotropic Radiator" (a dispersion pattern that radiates the energy equally in
all directions onto an imaginary sphere surrounding a point source). Thus
an antenna with 2.1 dBi of gain focuses the energy so that some areas on
an imaginary sphere surrounding the antenna will have 2.1 dB more signal
strength than the strength of the strongest spot on the sphere around an
Isotropic Radiator.
dBd refers to the antenna gain with respect to a reference dipole antenna.
A reference dipole antenna is defined to have 2.15 dBi of gain. So

3. converting between dBi and dBd is as simple as adding or subtracting 2.15
according to these formulas:
 dBi = dBd + 2.15
 dBd = dBi - 2.15
Specifying antenna gain in dBd means that the antenna in question has the
ability to focus the energy x dB more than a dipole.
Q-factor

4. What Is the Q Factor of an Antenna?
The Q factor or quality factor of a tuned circuit (which includes antennas) measures how
many times the current passes through the circuit. If an antenna has a Q factor of 10,
then the current passes through its coils and capacitors ten times before the energy is
radiated.
The Q factor of an antenna is a measure of the radiation efficiency of the antenna.
With high-Q antennas, any loss not only lowers the Q but also widens the bandwidth of
the antenna. To avoid this, loading coils for antennas and inductors are usually made
from heavy gauge wire.
A number of mobile antennas have loading coils made from heavy gauge wire in order
to keep the resistive losses down, maximizing the Q factor. Cleaning the joints of the
antenna can improve its efficiency, seen by a rise in the Q factor.
There is usually a tradeoff between Q factor and bandwidth—the broader the antenna’s
bandwidth, the lower the Q factor. The higher the antenna loading, the higher Q factor
and the narrower the bandwidth.
Shortening an antenna increases its Q factor. This is one of the benefits of using an
antenna that is half a wavelength or quarter of a wavelength long over one that is the
full wavelength of the intended signal frequency. For low frequencies, you have to use a
¼ wavelength antenna simply due to the size.