Transistor / QRP Equipment

The solutions shown so far may not adequately protect computer-based software-defined radios, hybrid transistorized-vacuum tube radios, or fully solid-state QRP or low-power stations, or transistorized shortwave radios. These radios will need to use protection that clamps at a much lower voltage than 700 volts! While transistor transmitters may use power amplifier transistors that may have breakdown voltages in the scores to hundreds of volts, the input circuitry of many QRP transistorized receivers will likely be fried with voltages well under 100 volts. There are a couple of solutions that can offer lower-voltage clamping more appropriate to these lower-power transistorized radios.

Certainly the grounding antenna switch described above, as well as some coax cable somewhere if possible, should be utilized. Shortwave radios with a collapsible antenna should have it at minimum size when not in use. The case of transistor-based equipment should ideally be constructed of metal, for its shielding ability, and ideally grounded, and any wires longer than a couple of inches (e.g., Morse code key, speaker wire, headphone wire) should have protection added. A typical electromagnetic-interference technique of adding a 0.01- 0.1 microfarad ceramic capacitor across those kinds of low-frequency signals wires (to ground/shield) will knock out a lot of the higher frequency components of an EMP E1 surge.

To provide further voltage clamping for those low-signal lines, a metal-oxide-varistor device ("MOV", as shown in Figure 4) can also be placed across speaker and similar wires. These semiconductor devices do not have a "polarity" and do not conduct until their breakdown voltage is reached. They come in a dizzying array of protection (breakdown) voltages and can protect repetitively unless the onslaught is extremely large. An 18-volt MOV suitable for speakers, headphones, many Morse code keys, and probably digital data lines as well, is available for less than $1.[1] One can select higher or lower voltage ratings as appropriate.