Tools
Calculators
The arithmetic you need for the exam and for the shack. Every one shows the formula it used, so you can learn the method rather than just take the answer.
Ohm's law & power
Enter any two values. V = I × R, P = V × I.
Enter any two of the three values.
Frequency ↔ wavelength
λ (metres) = 300 ÷ f (MHz). The relationship behind every band name.
- Wavelength
- 21.13 m
- Half wave
- 10.56 m
- Quarter wave
- 5.28 m
- Band
- HF
λ = 300 ÷ 14.2 = 21.13 m
Antenna length
Practical wire lengths including the end-effect correction. Cut long and trim.
- Half-wave dipole, total
- 20.14 m
- Each dipole leg
- 10.07 m
- Quarter-wave vertical
- 10.07 m
Dipole = 143 ÷ 7.1 = 20.14 m
SWR from forward and reflected power
SWR = (1 + √(Pr/Pf)) ÷ (1 − √(Pr/Pf)). Under 2:1 is a fine practical target.
- SWR
- 1.58:1
- Reflection coefficient
- 0.224
- Return loss
- 13.0 dB
- Power reflected
- 5.0%
Resistor colour code
Four-band decoder. Black 0, Brown 1, Red 2, Orange 3, Yellow 4, Green 5, Blue 6, Violet 7, Grey 8, White 9.
- Value
- 1 kΩ
- Tolerance
- ±5%
- Range
- 950 Ω – 1.05 kΩ
(10) × 10² = 1 kΩ
Decibels
dB = 10 × log₁₀(P₂ ÷ P₁). +3 dB is roughly double the power, +10 dB is ten times.
- Change
- +13.01 dB
- Power ratio
- 20.00×
- S-units (6 dB each)
- 2.2
10 × log₁₀(100 ÷ 5) = 13.01 dB
Radio horizon (VHF/UHF)
d (km) ≈ 4.12 × (√h₁ + √h₂), heights in metres. Antenna height beats extra power.
- Approximate range
- 26.1 km
4.12 × (√10 + √10) = 26.1 km