Explaining Noise Figure and Gain

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Explaining Noise Figure and Gain

Explaining Noise Figure and Gain

We do see various offers for any kind of preamplifiers that are promising wonders, so let me give an explanation for the true important factors when using such devices. During my active time at ham radio I was very active in earth-moon-earth communication, that is sending signals to the moon and getting the echo back, up to 5.6 GHz, far beyond the ADS-B frequency of 1090 MHz. This is one of the most sophisticated operational modes in amateur radio, and needs high transmission power as well as most sensitive receivers. This knowledge was brougth into the receiver design of the Mode-S Beast as well as the Radarcape.

Now we do see a lot of preamps on the market. Most of the time gain is given for them, but the more important value of noise figure is missing. There are synonyms for both that can be understood easier:

  • Gain is like brightness of a monitor
  • Noise figure is contrast or somehow the minimum black level

You may quickly understand by these synonyms, once you have destroyed the black level of your picture into a kind of grey, you no longer never can see the small nuances of weak black symbols on your screen. It is the same with radio signals: Once you have destroyed the noise figure, you never will get it back. Even turning the brightness on (= adding gain), you never will recover the true cold black on your screen but instead simply amplify your grey even more.

This brings me to the cable: A cable is attenuation. With our video screen, this is similar to a milky glass in front of it. So you don't see the nice pure black any more but a little bit of grey. Of course, your bright symbols still appear readable. If now you insert an amplifier, the bright symbols will become brighter, but also the grey becomes brighter. But the weak nuances of black are lost.
However, if you amplify before passing the lossy section, you will amplify the nuances of black we're talking about, and they may be still readable behind the attenuation.

Even worse, electronic devices may become overloaded by the shiny bright parts. So this means with an amplifier after the cable and in front of the receiver, you may make things even worse. Only if your device is really deaf or has a lot of internal losses, an amplifier in direct front of the antenna input will improve the situation.

Our Active Diapason antennas for 868MHz (FLARM) and 1090 MHz (ADS-B) are equipped with an amplifier directly connected to the antenna element, which is the pure optimum. Their noise figure is somewhat 1 dB, a quite reasonable value for an antenna that partly sees warm earth within its diagram (less than, say 0.8 dB, only makes sense for space pointing antennas). They are sold with 20 m cable because the gain should not overdrive the receiver, and because this cable attenuation is completly knocked out due to the low noise figure.