What Use Is An F-call?

What use is an F-call? Last week I spent a little time talking about lightning. I discussed how lightning can affect many different things, not just by being a direct hit, but by having a nearby hit, that is, something that is in some way electrically connected to you or your station. We all know that the ground has some level of conductivity, just like air does - the lightning that you see during a thunderstorm is the visualisation of the conductivity of air. In the earth, you don't really see it that clearly, but the same thing happens. Conductivity is measured in Siemens per Meter. Deionised water has a conductivity of about 5.5 micro Siemens per meter, sea water is about 5 Siemens per meter, so, sea water is approximately a million times more conductive than deionised water. Since Siemens is a measure of conductivity and Ohms a measure of resistance, you can convert one into the other as their inverse. A resistor made of 1 cm of seawater at 20C has a resistance of 2 milli Ohm. Ground conductivity is