User's Manual

Glossary
38
SpotCell 100, Release 1.0
dB The difference (or ratio) between two signal
levels. Named after Alexander Graham Bell.
(That is why the "B" is capitalized.) Used to
describe the effect of system devices on sig-
nal strength. For example, a cable has 6 dB
loss or an amplifier has 15 dB of gain. This is
very useful because system gains and losses
can be calculated by adding and subtracting
dBd The gain an antenna has over a dipole
antenna at the same frequency. A dipole
antenna is the smallest, least gain practical
antenna that can be made
dBi The gain a given antenna has over a theoreti-
cal isotropic (point source) antenna. Unfortu-
nately, an isotropic antenna cannot be made
in the real world, but it is useful for calculating
theoretical fade and System Operating
Margins.
A dipole antenna has 2.14 dB gain over a 0
dBi isotropic antenna. Thus, a simple dipole
antenna has a gain of 2.14 dBi or 0 dBd.
dBm A signal strength or power level. 0 dBm is
defined as 1 mW (milliWatt) of power into a 50
ohm load such as an antenna. Small signals
are negative numbers. This is useful since
dB shows values in a logarithmic scale, not
linear.
RULE OF THUMB: Every time you double (or
halve) the power level; you add (or subtract) 3
dB. For example, 125 mW is 21 dBm and
250 mW is 24 dBm.