Electromagnetic Pulse (EMP) is a serious national threat with growing public awareness. A high-altitude atomic/nuclear explosion sends electrons plowing through the earth's magnetic field lines and thus generates powerful radio waves that impact the earth below the explosion within a radius of many hundreds to thousands of miles.

The peak field strength is immense, on the order of 50 kilovolts per meter, and covers a very broad frequency spectrum, from very low frequencies, past 100 MHz; but the first wave (named E1) is evanescent, over in a mere microsecond or so. There are additional, slower incident waves– E2 and E3, but these are more of a powerline problem and are not addressed in this article. (I will write on this aspect in a later article.)

The survival problem is the instantaneous destruction (by overvoltage) of transistors, diodes, integrated circuits, FETs, and microprocessors by the immense voltages. Millions of volts and thousands of amperes may flow on ordinary shortwave or Ham radio antennas and feedlines from the E1 wave, for an instant. Handheld VHF/UHF walkie-talkies with antennas only a few inches long won't be hit with nearly the same level of damaging power that will assault high-frequency (HF) rigs with scores of feet of antennas and feedlines. This article concerns those vulnerable high frequency stations with antennas more than two feet long. Of course, these are the very stations useful for getting interstate, national, and international news and messages flowing.