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:
- 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
- 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:
- ground wave (possible at frequencies <=4 MHz) can somewhat surmount hills;
- 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;
- 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!
ADDENDUM
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
|
|
3.5
|
80
|
|
7.0
|
40
|
|
14.0
|
20
|
|
21.0
|
15
|
|
28.0
|
10
|
References:
[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
From the Survival Blog
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