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Protecting Both Tube and Transistor HF Communications Equipment From E1 EMP Pulse- Part 2, by PrepperDoc
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.
Figure 4: Metal oxide varistor designed to pass 140VAC (RMS) and to clamp at 198 volts, capable of shunting 6,500 amperes for 20 microseconds (which is much longer than an E1 EMP pulse). These are available in many different breakdown voltage levels, including levels suitable for Morse code keys, speaker, and headphone wires.[1]
To protect a transistorized receiver, “back-to-back” silicon diodes (see schematic, Figure 5) can be connected at the receiver input between the center conductor and the shield, either connecting inside the feedline connector or added inside the receiver’s enclosure. The reverse breakdown voltage rating is of less importance. One diode conducts and limits the voltage to 0.6 V when the center conductor is driven positive, and the other conducts and limits the voltage to -0.6 V when the center conductor is negative, so the peak-to-peak AC voltage is limited to 1.2 VAC. Because the transmitter may well produce more than this voltage, there has to be some relay or other mechanism to disconnect the receiver from the transmission line during transmitting, so these diodes don’t short out the transmitter output.
Source: http://survivalblog.com/protecting-both-tube-and-transistor-hf-communications-equipment-from-e1-emp-pulse-part-2-by-prepperdoc/
