
3-6
REFERENCE
DATA
FOR
ENGINEERS
and steradian, are given to the number
1
in
these two
cases
to
aid understanding, e.g., to indicate that a par-
ticular plane angle is expressed on the basis of arc
length divided by radius rather than on the basis
of
a
full circle divided into
360
degrees.
Decibels
and
Nepers
The decibel, symbol dB, represents the ratio of two
values of power. The number
n
of
decibels is
10
times
the logarithm to the base
10
of
the power ratio:
n(dB)
=
10
log,,
(P2/P1)
Where the power ratio is less than
unity,
it is
usual
to invert
the fi-action and express the answer as a loss
in
decibels.
In the electrical case of voltage
V,
current
I,
and
impedance Z (when
2
is a pure resistance):
P2/P1
=
(VZ/V,l2(Zl/Z2)
=
(12/~1)2(z2/z11
Thus,
n,(dB)
=
20
log,, (V2/V1)
+
10
loglo (Z1/Z2)
and
n,(dB)
=
20
log,,
(12/11)
+
10
log,, (Z2/Z1)
Only
in
the special case
of
matched impedances (Z2
=
2,)
does
Greek Alphabet
The neper, symbol Np, is a dimensionless unit for
expressing the ratio
of
two values
of
amplitude. The
number
n
of neper is the natural logarithm of the
amplitude ratio:
n(Np)
=
In
(A2/A1)
To convert decibels to nepers, multiply by
0.1 151.
Numerical Values
To
facilitate the reading
of
numbers, the digits may
be separated into groups of three, counting from the
decimal sign toward
the
left and the right. The groups
should be separated by a small space, but not by a
comma or a point.
In
numbers of four digits, the space
is
usually not necessary. For example:
2.141 596 73 772 7372 0.133 47
T
=
3.141 592 653 589
e
=
2.718 281
828
459
American-English documents use a period for the
decimal sign. British-English documents use a raised
dot
(.).
For all other languages, the recommended deci-
mal sign
is
a comma.
If the magnitude of a number is less than unity, the
decimal sign should be preceded by a zero.
The sign
of
multiplication
of
numbers is a cross
(x)
or a raised dot
(.).
Name
Capital Small Commonly Used to Designate
Alpha
Beta
Gamma
Delta
Epsilon
Zeta
Eta
Theta
Iota
Kappa
Lambda
Mu
Nu
Xi
Omicron
Pi
Rho
Sigma
Tau
Upsilon
Phi
Chi
Psi
A
B
r
A
E
Z
H
0
I
K
A
M
N
0
n
P
z
T
Y
cp
X
w
-
c
angles, coefficients, attenuation constant, absorption factor, area
angles, coefficients, phase constant
complex propagation constant (cap), relative density, angles, electrical conductivity,
increment or decrement (cap or small), determinant (cap), density, angles
permittivity, electric intensity
coordinates, coefficients
intrinsic impedance, efficiency, surface charge density, hysteresis, coordinates
angular phase displacement, time constant, reluctance, angles
unit vector
susceptibility, coupling coefficient, thermal conductivity
permeance (cap), wavelength, attenuation constant
permeability, amplification factor, prefix micro
reluctivity, frequency
coordinates
propagation constant
3.1416
(see more precise value in text)
resistivity, volume charge density, coordinates
summation (cap), surface charge density, complex propagation constant, electrical
conductivity, leakage coefficient, deviation
time constant, volume resistivity, time-phase displacement, transmission factor, density
scalar potential (cap), magnetic flux, angles
electrical susceptibility, angles
dielectric flux, phase difference, coordinates, angles
Omega
n
w
resistance in ohms (cap), solid angle (cap), angular velocity
Note: Small letter is used except where capital (cap) is indicated.