Ohm's Law & Power
V = volts · I = amps · R = ohms · P = watts
V = I × R (volts)
I = V ÷ R (amps)
R = V ÷ I (ohms)
Find any value
Know two of the four? Use the line that has the two values you already know.
To find Volts (V)
V = I × R
V = P ÷ I
V = √(P × R)
To find Amps (I)
I = V ÷ R
I = P ÷ V
I = √(P ÷ R)
To find Ohms (R)
R = V ÷ I
R = V² ÷ P
R = P ÷ I²
To find Watts (P)
P = V × I
P = I² × R
P = V² ÷ R
Resistor Color Code
| Color | Digit | ×Mult. | Tol. |
|---|---|---|---|
| Black | 0 | 1 | — |
| Brown | 1 | 10 | ±1% |
| Red | 2 | 100 | ±2% |
| Orange | 3 | 1k | — |
| Yellow | 4 | 10k | — |
| Green | 5 | 100k | ±0.5% |
| Blue | 6 | 1M | ±0.25% |
| Violet | 7 | 10M | ±0.1% |
| Grey | 8 | — | ±0.05% |
| White | 9 | — | — |
| Gold | — | 0.1 | ±5% |
| Silver | — | 0.01 | ±10% |
Read it: bands 1–2 = digits, band 3 = multiplier, band 4 = tolerance. (5-band parts use 3 digit bands.) Mnemonic: Bad Beer Rots Our Young Guts But Vodka Goes Well.
Schematic Symbols
Common components as they appear on a diagram.
Resistor
Capacitor
Cap. (polar)
Inductor
Diode
LED
Battery
Ground
Chassis gnd
Antenna
Switch
Fuse
Transformer
NPN trans.
PNP trans.
Crystal
Potentiometer
Op-amp
Connected
Crossover
Antenna & Wavelength
f in MHz. Use these as cutting starting points, then trim for lowest SWR.
λ = 300 ÷ f meters | 984 ÷ f feet
Half-wave dipole
Total (ft) = 468 ÷ f · each leg = 234 ÷ f
Total (m) = 143 ÷ f · each leg = 71.5 ÷ f
Vertical & loop
¼-wave vertical (ft) = 234 ÷ f
5/8-wave vertical (ft) ≈ 585 ÷ f
Full-wave loop (ft) = 1005 ÷ f
Coax stub / phasing line
Physical length = electrical length × VF
¼-λ coax (ft) = (246 ÷ f) × VF
¼-λ coax (ft) = (246 ÷ f) × VF
Example: a 40 m dipole at 7.150 MHz → 468 ÷ 7.15 ≈ 65.5 ft total, ~32.7 ft per leg.
Reactance & Resonance
f in Hz · C in farads · L in henries · X in ohms
XC = 1 ÷ (2π f C) capacitive
XL = 2π f L inductive
fres = 1 ÷ (2π √(L C)) at resonance XL = XC
Combining components
| Series | Parallel | |
|---|---|---|
| Resistors | R₁+R₂… | 1/(1/R₁+1/R₂…) |
| Inductors | L₁+L₂… | 1/(1/L₁+1/L₂…) |
| Capacitors | 1/(1/C₁+1/C₂…) | C₁+C₂… |
Two in parallel shortcut: (A × B) ÷ (A + B).
Decibels & S-Meter
dB = 10 · log(P₂/P₁) power
dB = 20 · log(V₂/V₁) voltage
| dB | Power ratio |
|---|---|
| +3 | ≈ 2× (double) |
| +6 | ≈ 4× |
| +10 | 10× |
| +20 | 100× |
| −3 | ≈ ½ (half) |
Antenna gain: dBi = dBd + 2.15.
S-meter: 1 S-unit = 6 dB. S9 on HF = 50 µV = −73 dBm. Reports above S9 read "+10", "+20", etc.
S-meter: 1 S-unit = 6 dB. S9 on HF = 50 µV = −73 dBm. Reports above S9 read "+10", "+20", etc.
SWR & Reflected Power
SWR = (1 + |Γ|) ÷ (1 − |Γ|)
Γ = (ZL − Z₀) ÷ (ZL + Z₀)
| SWR | Refl. power | Return loss |
|---|---|---|
| 1.0:1 | 0% | ∞ |
| 1.5:1 | 4.0% | 14.0 dB |
| 2.0:1 | 11.1% | 9.5 dB |
| 2.5:1 | 18.4% | 7.4 dB |
| 3.0:1 | 25.0% | 6.0 dB |
| 5.0:1 | 44.4% | 3.5 dB |
Most rigs fold back power above ~2:1. An ATU hides high SWR from the radio but doesn't fix loss in the feedline.
Coax & Feedline
Velocity factor (VF) sets the physical length of resonant coax sections.
| Cable | Z₀ | VF | Notes |
|---|---|---|---|
| RG-58 | 50 | 0.66 | thin, lossy |
| RG-8X | 50 | 0.82 | mid-size |
| RG-213 | 50 | 0.66 | low loss |
| LMR-400 | 50 | 0.85 | very low loss |
| RG-6 | 75 | 0.85 | TV / 75 Ω |
| Ladder | 450 | 0.91 | open-wire |
Q-Codes
A "?" after the code makes it a question.
QRGExact freq.
QRLFreq. busy?
QRMInterference
QRNStatic/noise
QROMore power
QRPLow power
QRTStop / close
QRVReady
QRXStand by
QRZWho calls?
QSBFading
QSKBreak-in
QSLConfirm RX
QSOA contact
QSYChange freq.
QTHLocation
RST Signal Report
Spoken/sent as three figures, e.g. "599". Tone (T) is CW only.
R — Readability (1–5)
1Unreadable
2Barely
3Difficult
4Readable
5Perfect
S — Strength (1–9)
1 = faint, barely perceptible … 9 = extremely strong signals.
T — Tone (1–9, CW)
1 = rough, AC hum … 9 = pure, clean DC note.
ITU Phonetic Alphabet
AAlfa
NNovember
BBravo
OOscar
CCharlie
PPapa
DDelta
QQuebec
EEcho
RRomeo
FFoxtrot
SSierra
GGolf
TTango
HHotel
UUniform
IIndia
VVictor
JJuliett
WWhiskey
KKilo
XX-ray
LLima
YYankee
MMike
ZZulu
CW Abbreviations & Prosigns
CQCalling any
DEFrom (this is)
KGo ahead, any
KNGo ahead, named
AREnd of message
SKEnd of contact
BTNew section
BKBreak
RReceived OK
ESAnd
TUThank you
FBFine business
HILaughter
OMOld man
YLYoung lady
XYLWife
WXWeather
AGNAgain
CULSee you later
73Best regards
88Love & kisses
5NN599 (CW)
Morse Code
A·−
B−···
C−·−·
D−··
E·
F··−·
G−−·
H····
I··
J·−−−
K−·−
L·−··
M−−
N−·
O−−−
P·−−·
Q−−·−
R·−·
S···
T−
U··−
V···−
W·−−
X−··−
Y−·−−
Z−−··
Numbers
1·−−−−
2··−−−
3···−−
4····−
5·····
6−····
7−−···
8−−−··
9−−−−·
0−−−−−
Punctuation & prosigns
.·−·−·−
,−−··−−
?··−−··
/−··−·
=−···−
+·−·−·
AR·−·−·
SK···−·−
U.S. Band Plan
FCC allocations. Mode privileges depend on license class — check the full ARRL chart.
| Band | Frequencies (MHz) |
|---|---|
| 160 m | 1.800 – 2.000 |
| 80 m | 3.500 – 4.000 |
| 60 m | 5 channels (USB) |
| 40 m | 7.000 – 7.300 |
| 30 m | 10.100 – 10.150 |
| 20 m | 14.000 – 14.350 |
| 17 m | 18.068 – 18.168 |
| 15 m | 21.000 – 21.450 |
| 12 m | 24.890 – 24.990 |
| 10 m | 28.000 – 29.700 |
| 6 m | 50.0 – 54.0 |
| 2 m | 144 – 148 |
| 1.25 m | 222 – 225 |
| 70 cm | 420 – 450 |
30 m: CW & digital only, 200 W PEP max. 60 m: channelized, USB, 100 W ERP.
Popular Frequencies
FT8 dial (MHz, USB)
3.57380 m
7.07440 m
10.13630 m
14.07420 m
18.10017 m
21.07415 m
24.91512 m
28.07410 m
50.3136 m
Calling frequencies
7.03040 m CW QRP
14.06020 m CW QRP
50.1256 m SSB
144.2002 m SSB
146.5202 m FM nat'l
446.00070 cm FM
Grid Squares & UTC
Maidenhead locator
Field (2 letters) + Square (2 digits) + Subsquare (2 letters).
| Part | Format | Covers |
|---|---|---|
| Field | AA–RR | 20°×10° |
| Square | 00–99 | 2°×1° |
| Subsq. | aa–xx | 5'×2.5' |
This station: EL29da — Sweeny, Texas.
Universal Time (Zulu)
UTC = local time + offset
Central: CST +6 · CDT +5
Central: CST +6 · CDT +5
Always log contacts in UTC to keep both stations' records aligned.