Wednesday, March 1, 2023

Two ham-related perspectives on the decibel

A career working on radar equipment - first in the Navy, then for the FAA - has made me fluent with a unit of measure that many in this hobby are not entirely comfortable with.

I'd like to offer some "food for thought" regarding the decibel - and I want to do it in two ways that apply specifically to our hobby.


The FX-4CR and the Xiegu G90 are both 20-25 watt radios. This is significant because it represents the halfway point between a 5W QRP rig and a 100W radio. At first, this seems counterintuitive: 25 is not halfway between 5 and 100 - it's only 1/4 of the way there.

Enter the mighty decibel!

Going from 5 watts to 100 watts represents a 13dB increase in power. But - and here's the Wow! moment: fully half of that increase (6.5dB) takes place by going from 5 watts to 22 watts.

You don't have to go all the way up to 50-ish watts to become half as effective as 100 watts. Put another way, you can be half as effective as a 100W radio by using only 22 watts of power.

Think of what that means in terms of battery power required for portable (POTA/SOTA) operation.

It's this fact that made me snap to attention when I read the specs of the FX-4CR.


Every now and then, QRPers will tell you that 5 watts is only 13dB down from 100 watts. Therefore, only a bit over 2 S-units, as if this is a trivial amount.

Maybe they'll even add, "The math proves it!"

Mathematically, they are correct. Six goes into 13 a little over 2 times.

But what they infer - that a transmit power difference of 13dB is insignificant - is dead wrong, and here's how you can prove it to yourself, assuming you are set up to run digital modes (FT4/8, WSPR).

Run a screenshot's worth of FT8, then take a look at the dB level of all the stations you decoded.

Now, presume a 13dB reduction in power for all of them. To do this, subtract 13dB from everyone's actual decoded dB level.

Anyone who decoded at a level less than -12dB (or so) would not had been decoded.

In this example, seven of the 16 stations decoded, would not be there, including the DX I would like to have worked!

Power matters; decibels, not S-units, show the extent to which it matters.

The math proves it.




  1. When you use FT-8 to explain success at receiving, s/n ratio shown on the screen is calculated with noise bw of 2500 hz. The actual decode bw used by the decoder is 6.25hz. The s/n shown on the screen is much worse than the actual decoded s/n. This slide show explains it all. The narrow bw and the coding gain of the digital protocol all add to the decoding success. I wonder what the rate of change of success of decode with level of power transmitted. Please, note, this is not a critizing post, I am just curious how any of these factors change how much the decode sucess is dependant on the transmitted output power... There must be a s/n ratio/BER where transmit power plateaus, and the rest is wasted. Your articles sure make for thoughtful reflection...

    1. I welcome the criticism!

      I'll be making a posting of a method I've been working on to determine the sensitivity of digital-only transceivers like the QDX, etc. The procedure for these measurements for SSB/CW and AM receivers has been around for ages - not so for digital modes.

      I'll be interested in your input as to whether the method has validity. Coming up in a week or so.

      Thanks and 73,