dBm & Power Converter

Why radio uses decibels

Radio signals span an enormous range: a transmitter may put out 100 W while the receiver hears a signal a hundred billion times weaker. Decibels compress that range into manageable numbers and turn multiplication into addition. dBm is power relative to 1 milliwatt: dBm = 10 log₁₀(power in mW). 100 W is 100,000 mW, so it is 50 dBm, or 20 dBW relative to 1 watt; 5 W is 37 dBm and 1 mW is 0 dBm. Every 3 dB doubles the power and every 10 dB multiplies it by ten, so a 1,000 W amplifier at 60 dBm is only 10 dB louder than 100 W at the other end of the contact.

Applying gains and losses

A station is a chain of gains and losses, and in decibels they simply add. 100 W through a feed-line with 1.5 dB of loss arrives as 48.5 dBm, 70.8 W. A directional antenna with 6 dBd of gain then makes the effective radiated power 54.5 dBm, about 282 W, in the favoured direction. The same arithmetic runs on receive: a preamplifier with 20 dB of gain multiplies the signal and the noise arriving with it by 100, which is why preamps help on VHF, where the receiver’s own noise dominates, but rarely on HF, where atmospheric noise is already far stronger.

S-meter readings

The S-meter on a receiver expresses signal strength in S units. By the IARU convention, S9 on HF, below 30 MHz, is −73 dBm, a signal of 50 µV across 50 Ω, and each S unit is 6 dB, four times the power. S5 is therefore −97 dBm, and a report of “20 over 9” means −53 dBm. On VHF and above, S9 is defined 20 dB lower, at −93 dBm or 5 µV. In practice, many receivers’ meters are not calibrated to the standard below S9, and some compress or stretch the scale, so an S-meter reading is a rough comparison rather than a measurement. A common misconception is that S1 is a very weak signal; at −121 dBm on HF it is below the atmospheric noise in most locations.

Volts and watts

Across a 50 Ω load, power and RMS voltage are linked by P = V² ÷ R: 100 W is 70.7 V RMS, and the peak voltage is 1.41 times that, 100 V. At 1,500 W it is 274 V RMS, which is why high-power connectors, capacitors and tuners need adequate voltage ratings. Into a mismatched load, voltages along the line can be considerably higher at the standing-wave peaks.

How to use it

1. Choose whether you are starting from watts, dBm or an S-meter reading.
2. Enter the value, and for S-meter readings choose HF or VHF.
3. Optionally enter a gain or loss in decibels, such as cable loss or antenna gain.
4. Read the power in dBm, dBW and watts, the voltage across 50 Ω and the result after the gain or loss.