Rigs & thunderstorms

Back in October 2024 I had my QRSS  TX connected to a 33ft end fed wire in the backyard. At the time a thunderstorm was approaching. The closest I think it came was about 5 miles away. It was not a forked lightning storm but the type that has plenty of  sheet lightning / intercloud stuff. Clearly that was enough to induce enough voltage into my QRP Labs U3S TX and destroyed the BS170 FET power output device.

Thankfully no other damage was done to the rig. There was no PCB damage at all in this instance. The ATMEL 328P survived and so did the synth board. Other users have reported much worse in the past. But as you can see, there's not a trace of any sillicone substrate left in the FET! I was using the rig with an external T match ATU, the kind that has the inductance going to ground, and I've usually assumed that any induced high voltages would be shunted to ground. Maybe most of it was, but did some get across the variable polyvaricon  capacitors ? I think they are rated at about 100 volts. They didn't suffer any damage either, so I guess that the induced voltage was somewhere over the 60v level which is the BS170's maximum Drain to Source breakdown voltage, but a bit below 100v. 

Now then lets think a little further.  As you any radio operator will know, when you have a thunderstorm you can hear the effects from low frequencies (for example on an AM/Medium Wave receiver) and often up to maybe 50Mhz. That means that there is energy spread about around a broad spectrum. In the circuit below of a T - Match, each L and C will impose a value of reactance which is dependent upon frequency.  We end up with a fuzzy potential divider circuit, and the bottom line is that there must always be some EMF making it's way into our equipment from one of natures untuned spark transmitter. So the idea of shorting out EMF through  the inductance is only relevant in a pure DC condition.

In some 40 years of playing radio, and usually leaving antennas connected I have never experienced a blow out before. But clearly there is no room for complacency. One day it WILL happen, so don't take any chances. This also brings into question the advice from the past about using things like lightening arrestors or having a bleed resistor across the terminals of an antenna. I'm not yet convinced that they would ever work properly.

Remember that an antenna is a collector, or a scoop for EMF. A 200ft long wire antenna collects more EMF than a 20ft long wire etc at HF.  It's like fishing with a small net or a big net when it comes to this problem. More metal connected to the FEEDPOINT is more EMF.

Now the thinking about things, maybe the effects of EMF are not always about what the antenna collects ? Let's say that a lightning strike hits the earth, then travels UP the earth from a buried grounding system and then upwards.  My advice is isolate the rig entirely from both antenna and earth, and that includes any line / mains cables too.