EMP-Hardened Ham Radio Communications, by PrepperDoc


EMP attack is often considered the most rigorous of survivalist situations, due to the likely complete loss of electrical grid, many vehicles, and many transistorized/computerized products. Our group worked to provide post-EMP communications that would allow effective communications post-event. We had two major requirements:

  1. Short Range Communications. Two, separate, defense-hardened homes that were approximately 30 miles apart had to be able to communicate across a medium-sized city, and
  2. Long Range Communications. Both homes had to be able to receive news from in-state and out-of-state sources. These were considered necessary to receive adequate advance warning of defense issues, such as advancing bandits or armies.

This article describes how we accomplished these goals.

Three of our group possessed or obtained Ham radio licenses of varying levels. Passing the Technician license requires only a few hours of study, allowing voice communications in the VHF bands and limited communications in the high frequency (HF) bands. While long-range (>500 miles) communications can usually be easily accomplished by Ham radio communications in the high frequency bands capable of “skip” communication, survival of that equipment through an EMP attack had to be assured. Cross-city communications initially proved to be more difficult, because of the rolling terrain over the 30 miles between the homes. In fact, using hand-held VHF transceivers, we found it impossible even to achieve direct communications across a 6-mile range that included 200-foot hills.

We assumed that it is unlikely that established VHF repeaters will remain functional after one or more EMP attacks. Therefore, direct radio communications had to be achieved. While we could conceivably place our own repeaters, they would likely be destroyed by subsequent attacks. Thus, everyday 2-meter Ham radio contact via repeater stations does not solve either of our goals. High frequency (HF) communications became the ticket to success.

Thus, we sought HF Ham gear that would likely survive an EMP attack. Published articles suggest that voltages/currents developed on antenna feed lines may reach 1 million volts and 1000 or more amps (for an instant), with voltages on power lines only a bit less. While some transistorized mobile Ham radio units were found resistant in one set of tests (Ref. 1), we opted to go with used vacuum-tube Ham radio equipment. Vacuum tube equipment has been found highly resistant to EMP damage. Furthermore, while entry level software-defined computerized Ham radio gear often starts at more than $500, older, used, vacuum tube equipment often goes for $200 in working condition.

One of our members had extensive electrical engineering education and as a high school student had constructed Heathkit vacuum tube Ham radios. We went with that kind of gear.

HF Heathkit vacuum tube 5-band 180-watt transceivers capable of modern communications (single side band and morse code (CW)) were sold in two lines of products: the less-expensive HW-100 and later HW-101, and the somewhat fancier SB-100, SB-101, and the end-of-line SB-102 transceivers. All were new in the ’60s and ’70’s and are now available to varying degrees at Hamfests and on Ebay. The vacuum tubes utilized are very similar. All use an external–and heavy–power supply, connecting to the transceiver with a multi-wire cable. PDF versions of the manuals are available (Ref 2 and 3) and include not only construction but testing, operation, and repair information, including expected voltage and resistance measurements at various points; this is a gold mine for repairing used equipment. Similar equipment of that era include Drake and Collins brands, but we had lesser experience with these brands and were not as confident of the ability to repair them.

There are various idiosyncrasies of the Heathkit transceivers. The microphone connector is an unusual one, requiring effort to find. Also, the vacuum tube system requires a higher microphone output voltage than newer, lower-impedance, transistorized equipment. The chassis mic connector can be replaced if necessary. One can either find an older “ceramic” microphone or use a pre-amplified CB microphone [available at any truck-stop] with the pre-amp turned all the way up. Remember that the transistorized mike will likely be damaged in an EMP, so keep spares inside Faraday cages.

While our expert’s vacuum tubes had survived 30+ years, vacuum tube filaments do have a finite lifespan. Our group decided to develop backup supplies. The smaller tubes in the units are readily available from multiple suppliers (e.g. Ref 4), but some of the tubes are becoming scarce and extraordinarily expensive. In particular, the SB-line and later HW-line use a 6HS6 in the receiver for its extremely high gain, where earlier HW-devices used a lower-gain 6AU6. A possible replacement, which is far less expensive and more readily available, is the 6AH6, which also has the same filament current as a 6HS6, thus maintaining the carefully balanced series/parallel connections of 6-volt tubes to a 12-volt filament voltage. The final amplifier in all these and many other tube Ham radios is a pair of 6146 tubes. These come in a dizzying array of flavors and may best be bought from a more-specialized Ham radio supplier (e.g. Ref 5). You should use two tubes of the same “flavor”, while actual “matching” is probably of lesser importance. Your spares may be either two 6146’s, or two 6146A’s, or two of the higher-plate-dissipation-rating 6146B’s. The ruggedized 6146W’s can be problematic as they may be either “A” or “B”. We recommend these only if you can get two of a similar manufacture.

These old radios have some common problems, not all of which can be addressed here. In particular, the contacts on the multi-wafer band switch (80/40/20/15/10 etc) can become oxidized and you will notice dramatically reduced transmitter power on all bands other than 80 meters, if the contacts used in various stages of transmitter amplifiers are corroded. Careful work with a tiny bit of Brasso on a Q-tip on the band-switch rotating copper contacts and exercising the switch thereafter will repair this problem.

The Mode (TUNE/LSB/USB/CW) switch has a similar problem and switches 300-volt plate connections, leading to some scary arcing. Very careful Brasso work and possibly gentle tensioning of delicate contacts may be needed if you have this problem. As a last resort, replace the switch sections, which switch the 300-volt line to the 12AU7 carrier oscillator halves, with toggle switches to choose which plate of the dual triode gets power (one for LSB and the other for USB/CW).

The delicate string system rotating the final amplifier “load” variable capacitor in the transmitter final amplifier can be replaced with a suitably sized O-ring or rebuilt with braided fishing line and tensioning spring.

All of these transceivers include a 100 kHz crystal oscillator, providing calibration signals every tenth of a MegaHertz (MHz). Nevertheless, the frequency ticks on the HW-series tuning dial are close together. If this is truly a problem to your becoming adept at HF Ham radio, using a series 47 pF capacitor one can pull the variable frequency oscillator signal from the cathode of the 6EA8 first transmitter mixer and route it with small coaxial cable (even microphone cable will work) to a connector on the back panel, and read it with a digital frequency counter without damage to the transceiver. Just remember that EMP will probably destroy the digital frequency counter. Its reading goes UP in frequency as the actual dial frequency goes DOWN, because the VFO output is subtracted from a higher-frequency oscillator’s signal.

Finally, to complete your EMP hardening of this brand of Ham radio transceiver, you should add common metal-oxide-varistor (MOV)-based surge protectors to the AC power line, and also inline with the antenna feed (your most dangerous EMP gatherer). The reference article (Ref 1) below describes testing where MOV devices did successfully protect Ham radio equipment. Nevertheless, it would be very wise to unplug AC and antenna connections (at a minimum) from the transceiver when not in use, either by removing the connectors several feet away or at the very least by using commercial power-strip switches and antenna-switches. With the voltages possible by an EMP, arcing of these commercial switches is a real possibility.

While working on your newly-purchased HF Ham rig, you can avoid interference to others from your transmitter by using a 100-watt incandescent light bulb as your “antenna.” The load seen by your transmitter should be acceptable at least on the lower-frequency 3.5 & 7 MHz bands.

Once your equipment is working and EMP-hardened, you are ready to move on to suitable antennas. This will often depend on your location– unfettered rural versus very restricted urban environments. Optimally, a new HF Ham would be best served by a very simple resonant half-wave dipole antenna, as this will have an acceptable standing wave ratio (SWR) at resonance, and can be easily fed by commonly available coaxial feed line. (See Ref. 6.) As documented below, we found 3.5-4.0 MHz 80-meter band was crucial. It requires an antenna of roughly 130 feet total length, split near the middle by an insulator, with an insulator on both ends. (If you haven’t that much room, try next for a 65-foot length 40 meter resonant dipole.) Simple RG-58U (or even more easily available 75-ohm RG-59 from home supply stores) can be used, with the center conductor soldered to one wire at the center conductor, and the braided shield carefully moved away (if in doubt, simply unbraid and then twist all the strands together) and soldered to the other half of the antenna at the center insulator. The height of the antenna is not that important– 10 to 25 feet is fine– and the antenna can sag or bend as needed. Many of us have used dipoles for years fed successfully by coax without the use of a “balun,” so consider it optional. The connector on the other end of your feedline should be a PL-259 to connect to the SWR bridge you will need to adjust your antenna length; the published formulae are only approximate, as every antenna is in a different environment. Read the directions on your SWR bridge and, if possible, seek help from any nearby Ham radio operator; tuning antennas is a learned skill. If you are fortunate enough to have access to an antenna tuner (Ref. 7), you can very quickly adjust your antenna, but this is not required. Ideally you will get your SWR below 2:1 in the frequency range of interest; – typically that will be maintained across only about 0.2 MHz (on 80 meters), so pick whether you want to use single-side-band (requiring the General or higher class license) at the top end of that band, or morse code (requiring only the Technician license, in a special area in the middle of the band). Have your helper-Ham teach you how to quickly “tune” your transmitter so that you do not damage (by overheating) the expensive power-amplifier 6146 tubes. If you don’t have a helper, follow very closely the instructions in the excellent Heathkit manual, and stick to lower powers– in the range of 100 watts input (50 watts output) until you are really proficient. Besides the power level adjustment control, there are only three tuning capacitors that must be properly adjusted to have your transmitter working well on a given frequency. The first of these– the driver amplifier tuning– can be roughly set by adjusting for maximum received signal, as the stage has double duty. You have to actually be generating transmitter power to tune the remaining two– the power amplifier TUNE and LOAD capacitors. Be attentive to the plate current meter reading; you want that MINIMIZED quickly with the final amplifier TUNE adjustment, which will occur nearly simultaneously with a MAXIMUM output observed on your SWR bridge. The setting of the LOAD capacitor is much less important and more broad in effect. Start with its plates fully meshed and pay more attention to the TUNE knob. It will all become easy with practice.

For those who are more severely restricted, a random length single wire antenna (ideally more than 30 feet), or open-wire-fed (ladder line) non-resonant dipole may be the best choice, though both involve the addition of an “antenna tuner” and two or three more knobs that must be adjusted adroitly and quickly (starting at lower power settings initially). (Ref. 8) MFJ manufactures high quality antenna tuners, but these devices last forever, can even be manufactured easily at home, and many models old and new can be found easily on Ebay or at Hamfests. Seek help to become familiar with them, if you have no instructions. They require trial and error. Once you find adjustments that work for one band, write them down!

As an aside, I should mention that morse code still has a place in limited-power communications, as you will find it carries much farther than equal-powered single side band, because it has only two states (ON or OFF) meaning that the ear at the other end has a much simpler job than deciphering a weak voice amongst crashing interference and static. Once learned, like a bicycle, the skill lasts for a lifetime, as older Hams can prove easily. There are even computerized cheater-devices that can translate it for you now! (However, they will likely fail after an EMP.)

Finally, you must assure that your planned post-disaster power source will not create its own interference to radio communications! If you are planning a generator (even a 900 watt unit should suffice), this may not be a problem. However, we found that fancy inverters (you have one stored in a Faraday cage, don’t you?) generate wide-spectrum NOISE from their highly-efficient switching power supplies used to construct each point on the sine wave output, and “modified sine wave” inverters automatically create wide-ranging interference. We have successful experience dramatically reducing the radiated inverter-generated power line interference by placing a low-pass power-line-capable filter in series with the inverter output right at the output of the inverter. (An example of such a filter is the Chinese JR-1230-R 30A Alternating Current Power Line EMI Filter AC 115/250V.) Be certain to test your actual grid-down complete system to see if your communications radio actually works as intended in both transmit and receive conditions!

Now armed with the requisite license, hardened equipment, and an antenna, you are ready to gain the skills that you will need to effectively meet an impromptu “net” of survivors to pass crucial information after a disaster. Do not be fooled into thinking you need no experience. These nets will spring up, starting from previously existing state and local HF nets, and they will have appointed times and frequencies, requiring you to have the skills to communicate in a fast-paced environment on an exact frequency at a certain time. Volunteer net controls operating in high-stress times may not be helpful to a newcomer. It’s best to gain the expertise NOW.

How did we reach our two communication goals? As background, three modes of radio wave propagation are most common:

  1. ground wave (possible at frequencies <=4 MHz) can somewhat surmount hills;
  2. line-of-sight (LOS) is normally the only method possible for VHF walkie-talkies, but trees induce huge losses per mile, and hills obliterate the signal;
  3. long-range “skip” occurs as frequencies between about 2MHz and 50 MHz (depending on time of day and sunspot cycle) are refracted back to earth by ionized layers above us (and then potentially back upwards by sea or ground). Ground wave communications simply did not work for our 30-mile requirement (the 160-meter band might succeed) and line of sight failed even at six miles. Using 3.5-4MHz (80 meter) at night with dipole antennas at low heights (10-20 feet), we had acceptable “near-vertical-incidence” refraction/reflection by the F layer directly above us, and we succeeded at our cross-city goal. During the day, absorption by the D layer caused failure. The lower height antennas actually send more of their energy out at higher angles, allowing our success at cross-city distances while still giving us adequate strength at lower angles to reach stations hundreds of miles away. While 40 meters during the day provides intra-national communications, 20/15 meters provide international communications during the day when the sunspot number is above minimal. At night, 40 meters provides international communications. Very short range communications (intra-neighborhood) would be provided by (previously Faraday-protected) hand-held VHF transceivers, which is a subject for another essay.

Thus, it is quite possible for you or your group to create short- and long-range post-disaster communications that are likely to survive even multiple EMP attacks. Using older tube-type transceivers, simple antennas, and careful purchasing of spare tubes, we were easily able to accomplish this for $300-$500 per station, or about the price of one firearm. What are you waiting for? Get started!


In Ham radio lingo, frequency bands are interchangeably denoted by their frequency or related wavelength. This table gives the equivalences for the HF bands:

Frequency Band

Meters of Wavelength












[1] QST article in EMP/ham radio: http://williamesimpson.com/wp-content/uploads/2013/04/QST-Electromagnetic_Pulse_and_the_Radio_Amateur.pdf

[2] HW-101 manual online: http://www.wmsinc.org/N7EBG/heathkitpdf/HW-101%20Manual%20KB2LJL.pdf

[3] SB-102 manual online: http://tubularelectronics.com/Heath_Manual_Collection/Heath_Manuals_S/SB-102/sb102gif_v6.pdf

[4] One of several vacuum tube suppliers: https://www.tubedepot.com/

[5] Ham radio power amplifier tubes: http://www.aesham.com/ham-radio-accessories-tubes/

[6] ARRL page on building a simple half-wave resonant dipole: http://www.arrl.org/single-band-dipoles

[7] An example of an antenna analyzer: http://www.mfjenterprises.com/Product.php?productid=MFJ-213

[8] ARRL page on random length antennas: http://www.arrl.org/random-length-multiband-dipoles

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