What use is an F-call?

You've come across the term dB, or Decibel. Likely you've heard of dBi as well. These terms are all related to Decibels, but mean completely different things.

A decibel, named after Alexander Graham Bell, is a RELATIVE measure of two different power levels, that is, one power level compared to another power level. 3dB is about twice as much power, 6db is about four times as much, and 10dB is exactly 10 times as much power. This means that you can say that a feedline has 6dB loss, that is, you need to put 20 Watt in at one end to get 5 Watt out at the other. In short, a dB is a ratio between two levels of power, in the feedline case, the power in vs. the power out the other end.

In antenna land, you'll have heard dBi as the measure of the amazingness of an antenna. A dBi is a measure of gain of an antenna when compared to an ISOTROPIC source. This is a theoretical reference, that cannot actually exist in nature, but at least it's always the same, which allows you to compare two antennas to each other when their gain is both expressed in dBi.

You might also come across a dBd, or antenna gain when compared with a dipole. A dipole in itself can be compared to an Isotrope. Its gain is 2.41 dBi or 0dBd. Which incidentally goes to why many antenna manufactureres play silly games with dBi and dBd.

An antenna described as 24 dB should send you back to the manufacturer to ask them, 24 compared to what? If it's 24 dBi, it's compared to an isotrope, if it's 24 dBd, it's compared to a dipole. This means that there could be a 4.81 dB difference between two incorrectly named "24 dB" antennas.

There's more than this, think about dBW, dBV, dBu, dBmV, dBA, dBZ and many, many more. 

The thing to take away is that a dB is a relative term. One compared to another. If only one's specified, you don't nessicarily know compared to what? dBi references it to an Isotrope and dBd references it to a dipole.

I'm Onno VK6FLAB