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 in the order of 1000 times worse than sea water and is typically expressed in milli Siemens per meter.

As we're talking about the ground, the conductivity is seasonal, since rain comes and goes, and to add to the mix, this conductivity is frequency dependent.

So, In Australia, for a frequency up to 30 kHz, the conductivity varies from 1 to 10 milli Siemens per meter, or 1 cm of ground has a varying resistance between 1 and 10 Ohm. If you look at 1MHz, the conductivity varies much more, from 2 to 50 milli Siemens per meter, depending on where you are, how far you are from the ocean, a river or lake or what the ground is made up of.

Back to lightning.

Imagine an earth stake next to your shack for your radio and another stake next to your antenna. In a circuit diagram, both of them would show as being connected to earth and you could just look at that and think that all was well with the world. Both are earthed, so you're safe.

Unfortunately that's not the case.

If you drew the circuit diagram properly there would be a resistor between the two earth stakes. There would also be a conductor, namely your coax between the radio and the antenna. So you have a path of low resistance, the coax, and a parallel path of high resistance, something like 10 kOhm for 10m, between the earth stakes. No points for guessing which one the lightning will take.

But the coax is capable of handling that, isn't it?

If you have coax rated at 3kV, like RG213, a direct lightning strike will only exceed it's capacity by a million times. So, no,  coax is not a good earth path.

As an exercise, you can use 300kA as the current for a direct lightning strike. Based on the ground conductivity of 10 milli Siemens per meter, you can work out how far lightning needs to be in order for your RG213 to survive if your earth stakes are 10m apart and not bonded.

So the lesson is, bond all of your earth stakes together. Connect the coax shield to the tower and create a Single Point Ground by connecting them all together.

There are several online lightning maps showing real-time lightning activity which can also help. Weatherzone incorporates the Bureau of Meteorology Radar images and superimposed lightning strikes. Of course you can also use lightning data to check to see what the noise level might be like at a DX station you're trying to work.

I'm Onno VK6FLAB