Decibel (dB) Calculator — Ham Radio Power & Voltage Ratios

Decibel Converter

Enter value:

Key dB Values to Memorize for the Exam

dB Change	Power Ratio	What It Means
+3 dB	×2 power	Doubling transmit power
−3 dB	÷2 power	Halving transmit power
+6 dB	×4 power	4× power increase
+10 dB	×10 power	Ten times power
+20 dB	×100 power	100× power — two S-units on most receivers
−10 dB	÷10 power	One-tenth the power
0 dB	1:1	No change

Signal Path Budget Example

A practical dB calculation: You have a 50W (47 dBm) radio, 3 dB of coax loss, and a 6 dBi antenna. What is your effective radiated power (ERP)?

47 dBm − 3 dB (loss) + 6 dB (gain) = 50 dBm = 100W ERP. Your antenna gain more than compensates for the feed line loss, resulting in an effective radiated power of 100W — even though your radio only puts out 50W.

What is dBd vs. dBi for antennas?

dBi (decibels relative to isotropic) measures antenna gain compared to a theoretical perfect omnidirectional antenna. dBd (decibels relative to dipole) compares to a half-wave dipole. A dipole has 2.15 dBi of gain, so to convert: dBi = dBd + 2.15. The Technician exam uses dBi. Most commercial antenna specifications use dBi but some manufacturers still use dBd — check which reference is stated.
How many dB does doubling my power actually change my signal?

+3 dB — just barely perceptible on the receiving end. Going from 5W to 10W is +3 dB. Going from 5W to 50W is +10 dB — a meaningful but not dramatic improvement. The practical takeaway: antenna improvements of 3–6 dB (going from a rubber duck to a good base antenna) have more impact on your signal than doubling transmit power.