Understanding dB, aka go easy on your HF amplifier!
I was listening to a couple of guys on the 20m band today and they were discussing amplifiers…
“Yeah, this amplifier will do 1500W. The last one I had would only do 1000W so I got rid of it and bought this new one”.
Random Ham
500 extra watts! That’s gotta make a ton of difference!
The difference between 1000W and 1500W in decibels is about 1.76 dB.
Compare this to an S-meter vs dBm scale…
All other things being equal, that extra 500W he paid for is moving my S-meter by maybe a half of an S-unit?
I’ve ranted about this before, but this one highlighted the misunderstanding when using a high powered HF amplifier. Decibels are logarithmic, not linear.
Increasing an output power in dB by 3 dB doubles the power. 6 dB quadruples it. 10 dB is an order of magnitude higher, i.e: 10 times higher.
Don’t believe me? Try it here. Enter 0 in the dBm field and increase it by 3 dB or 6 dB for example to see the difference in linear terms (watts).
When it comes to HF amplifiers used in ham radio, there really is no need to push for the maximum power permitted. This saves a lot of stress on the components of your amplifier and any transformers or baluns you have connected in your antenna system.
In the USA, the power limit for general and extra class ham radio licenses is 1500W. This used to be specified as an input power – i.e: how much power was drawn from the mains power. Now it is specified as an output power.
There is some debate about what an S-unit is. Collins specified 6 dB per S-unit (so, a quadrupling of power) and other sources (like the table above) specify around 4 dB per S-unit. For this example, I’ll use the table above.
Lets say we are using an Ameritron AL-811H which is capable of around 800W. We are being given a signal report of S3. The following table shows the receiver S-meter reading as the power increases.
| TX Power in W | TX Power in dBm | RX Signal in dBm | RX S-meter reading |
|---|---|---|---|
| 100 | 50 | -118 | S1-2 |
| 398.10 | 56 | -112 | S2-3 |
| 794.32 | 59 | -109 | S3 |
| 1000 | 60 | -108 | Slightly above S3 |
| 1500 | 61.76 | -106.24 | S3-4 |
So running only the radio at its full power of 100W gets us S1-2. Lets hope there’s not much noise on the band!
Running the AL-811H at 794W gets us into the S3 range.
Running the full legal limit of 1500W moves us from S1-2 with only the radio, to S3-4 which really can make the difference in making a contact vs not. However, 794W gets us a solid S3, which is pretty respectable!
Don’t sell your 800W to 1000W amplifier for a 1500W amplifier. There’s really no need do to that unless the various authorities raise the power limit for ham radio. I understand that running a 1500W amplifier at 800W is possible and maybe will extend the life of its components, but doing this just for power reasons doesn’t make sense.
