Basic Electronics


OHMS LAW - A potential difference of 1 volt will force a current of 1 ampere through a resistance of 1 ohm. OR....

    V=(IR), I=(V/R), R=(V/I)

    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)


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


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.


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)


REACTANCE - The opposition to a flow of charge due to inductance and capacitance, similar to resistance.

    Capacitive Reactance XC=2fC
    Inductive Reactance XL=2fL
    = 3.141592654 or 3.14 rounded


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 (2fL)=(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


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).


Frequency of a Tuned Circuit - f=(1/2)x sqRt[LC], BW=(FO/Q)

    BW = Bandwidth
    FO = Frequency of resonance


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
    Z= Impedance as measured in ohms


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


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.


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.