Antenna dBi issue.

[ozaiobee]

Hi, it's my first time on the website.

I know how an isotropic antenna works. I work in the certification business and I keep seeing 0 dBi gain antennas. (Whip, Beamwidth 360 horizontal and 45 vertical.) Is this possible? I thought it was:
Beamwidth:
H, 360
V, 90
Gain = 3 dBi

Beamwidth:
H, 360
V, 360
Gain = 0 dBi

Please help, thanks.

 

[Higgler]

Looks right,
for an omni antenna like a perfect dipole, the gain formula is 112/Beamwidth (-3 dB rolloff angle in degrees).

There are many simple formulas out there for gain based on the angle in degrees at the -3 dB rolloff in power (relative to peak).

dBi is decibels relative to isotropic.
bel = Alexander Graham Bell who first plotted in log base 10.
deci is 10xlogbase10(power), seems other scientists thought multiplying by 10 was easier to read.

Common alternate units for dB() are;
dBiC (C=circuilar polarization),
dBiL (L=linear polarization),
dBiD (D=dipole, which has 2.1 dBi gain, so if you have 5 dBiD gain, it's actually 5+2.1=7.1 dBi or dBiL gain).

dBi really needs something else added to it for completeness, either the word linear, circular(Right hand or left hand) or dBiD (which is always linear).

 

[pstuckey]

I'm guessing that those formulas actually calculate directivity, where

gain = (antenna efficiency) * (directivity)

For instance, if based on beamwidth, you expect an antenna to have 3 dBi of directivity, but the manufacturer advertises the gain to be 0 dBi, that would imply the antenna efficiency is 50%.

Peter

 

[VEBill]

As Peter wrote, loss is the simplest explanation for the apparent anomaly in the 0 dBi gain and non-spherical coverage pattern beamwidth figures.