OHMS LAW - A potential difference of 1 volt will force
a current of 1 ampere through a resistance of 1 ohm. OR....
P=(IxV) or P=(I2R)
Potential Difference (V) = The difference in voltage between the two
ends of a conductor through which a current flows. Also known as a voltage drop.
CURRENT (I) -The quantity of electrons passing a given point (Unit of
measure: AMPERE)
VOLTAGE (V)- Electrical Pressure or Force. (Unit of measure: VOLT)
RESISTANCE (R)- Resistance to the flow of current (Unit of measure:
OHM)
POWER (P)- The work performed by current (Unit of measure: WATT)
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RESISTANCE - The opposition to flow of charge through a
material, expressed in ohms ( ). Resistance is similar in many respects to mechanical friction. The
resistance of a wire depends on its material, length, thickness and temperature.
RT = Total Resistance
Series circuit RT=R1+R2+R3+...
etc.
Parallel circuit RT=(R1xR2)/(R1+R2)
or 1/[(1/R1)+(1/R2)+(1/R3)+(...)]
Resistivity of
various materials |
(higher =
more resistance) |
| Silver |
9.9 |
| Copper |
10.37 |
| Gold |
14.7 |
| Aluminum |
17.0 |
| Tungsten |
33.0 |
| Nickel |
47.0 |
| Iron |
74.0 |
| Carbon |
21,000 |
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CAPACITANCE - Measured in farads, sometimes referred to
as a condenser. May be defined as the property of a circuit to oppose any change
in voltage.
Series circuit capacitance = 1/[(1/C1)+(1/C2)+(1/C3)+(...)]
or (C1xC2)/(C1+C2)
Parallel circuit capacitance = C1+C2+C3+...
etc.
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INDUCTANCE - Measured in henries, sometimes referred
to as a choke. May be defined as the property of a circuit to oppose any change
in current. Inductance can be in phase or out of phase. As the current increases through
an inductor, the magnetic field increases, exactly in step. At the instant the current
reaches it's maximum positive value, it's rate of change is zero. This means maximum field
strength but zero magnetic-field movement and therefore zero counter electro magnetic
force due to moving magnetic fields. Thus maximum current and zero counter emf occur at
the same instant with an alternating current applied.
I=(V/X), V=(IX), X=(V/I)
I = Current, in A (usually rms)
V = emf, in V (usually rms)
X = Reactance, in ohms (or w)
Series circuit inductance (Field of inductors uncoupled) = L1+L2+L3+...
etc.
Parallel circuit inductance = 1 /[(1/L1)+{1/L2)+(1/L3)]
or (L1xL2)/(L1+L2)
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REACTANCE - The opposition to a flow of charge due to
inductance and capacitance, similar to resistance.
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RESONANCE - Resonance is the basis of all transmitter,
receiver, and antenna operation. Without resonant circuits there would be no radio
communication. Resonance is the condition when XL=XC or (2 fL)=(1/2fC) or wL=1/wC
XL = inductive reactance in ohms
XC = Capacitive reactance in ohms
f = Frequency
L = Inductance in Henries
C = Capacitance in farads
w = 2f
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The "Q" of a circuit is a measure of quality when
inductance and capacitance are involved. Q=(XL/R)=(2fL/R), Q=(XC/R), Q=(R/X).
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Frequency of a Tuned Circuit - f=(1/2)x sqRt[LC], BW=(FO/Q)
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Transformers - Voltage ratio: (TP/TS)=(VP/VS),
Current ratio: (TP/TS)=(IS/IP)
TP = Primary Turns
TS = Secondary Turns
VP = Primary Voltage
VS = Secondary Voltage
IP = Primary Current
IS = Secondary Current
Transformer Efficiency as a percent = (PO/PI)x100
PO = Power output
PI = Power input
(TP/TS)=(VP/VS)=(IS/IP)=sqRt[ZP/ZS]
Z= Impedance as measured in ohms
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Audio
Speed of Sound in air (27o C) = 1,139.67 ft./sec.
Intensity measured in decibels (dB). Frequency measured in hertz (Hz).
Range of human hearing is approximately 20 Hzto 20 KHz.
120 dB = Pain
120 dB = Aircraft engine at 20 ft.
110 dB = Amplified rock music
110 dB = Thunder
108 dB = Piezoelectric buzzer at 12 in.
90 dB = Air Force T-38 2,500 ft. overhead
90 dB = C02 pellet gun at 12 in.
85 dB = Digital alarm clock at 12 in.
80 dB = Electric typewriter at 18 in.
70 dB = Air Force T-38 at 1 mile
65 dB = Typical conversation
62 dB = Paper clip dropped on desk from 12 in.
61 dB = Computer keyboard at 18 in.
56 dB = Telephone dialtone
54 dB = Pencil eraser tapped on desk at 12 in.
45 dB = Average residence
30 dB = Soft background music
20 dB = Quiet whisper
0 dB = Threshold of hearing
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Frequency (f) vs. Wavelength ( ) - =(c/f), F=c/ where c=speed of
light. Example: The wavelength of a 108 MHz signal is (3x108)/(1.08x106)
or 2.78 meters.
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Antennas - Adding inductance to an antenna lengthens the
antenna. Adding capacitance shortens it. Adding inductance and capacitance allows tuning
over a broad range.
Z=276 log10(d/r) for two parallel wires or flatlead, 300 ohm
line
Z = impedance in ohms
d = center to center separation (inches,feet,millimeters,whatever)
r = conductor radius (same unit as in d)
dB = 10 log10(P2/P1)
Z=138 log10(di/d) for round lead or coaxial, 75 ohm line
Z = Surge impedance in ohms
di = inside diameter of hollow tubing
d = diameter of center conductor
in other words a coaxial line can be measured to determine impedance.
=(V / f ) or
(300,000,000 / f ) = (300,000 / f in KHz) = (300 / f in MHz)
= Wavelength (Meters)
c = Speed of light
f = Frequency (Hertz)
V = velocity of radio waves, meters/second = 300,000,000
The length of a 7-Megahertz antenna is: (/2) feet = (468/ f )
/2 feet = (468 / f
)= (468 / 7) = 66.9 Feet
Antenna wavelength in inches =11,811/f
For wavelength in coaxial cable multiply by velocity of propagation.
Approximate length for a quarter wave whip antenna in inches = lambda/4 or 2775/F in MHZ
Length of a quarter wave line (matching transformer-no end effect)
L = 246(vf) / f
L = Length in feet
vf = velocity factor of transmission line (see below)
Velocity Factor
Air insulated parallel line - 0.975
Air insulated coaxial cable - 0.85
Polyethylene parallel line (twin lead) - 0.82
Polyethylene coaxial cable - 0.66
Voltage Standing Wave Ratio (VSWR) - VSWR=(Vmax/Vmin)
or SWR=(Imax/Imin)
The ratio of current or voltage delivered to an antenna to that reflected back
down the line is the reflection coefficient, p. It is equivalent to
p=SWR-1/(SWR+1).
Power reflected back to the source is a mismatch meaning not all power is being absorbed
by the load as wanted.
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