2
$\begingroup$

My wife is a teacher at a school about 15 miles from our home. My growing concern about a possible EMP event (natural or man-made) led me to buy a pair of Baofeng UV-5RV2+ radios in the hopes of allowing us to communicate after such an event (I intend to form a Faraday cage by wrapping them in foil in order to survive the EMP).

I have a 35 foot high rotating TV antenna mast at my home. My question is what would be the best directional antenna to mount on this mast to maximize my range for transmission and receiving? Also, what would be the best portable antenna for my wife's radio in order to maximize her range (it could be a collapsible unit that could be stored in her car).

$\endgroup$
  • 1
    $\begingroup$ if you can EMP-proof a ~100 MHz device by wrapping it in tin-foil, then you certainly didn't see a proper EMP event. $\endgroup$ – Marcus Müller Oct 3 '17 at 12:18
  • $\begingroup$ tthanks to both you guys regarding the shortcoming of my aluminum $\endgroup$ – Capt. Nemo Oct 3 '17 at 19:39
  • $\begingroup$ I also did not consider the failure of the rotator mast. I currently have a Discone D-130J antenna mounted on the mast (at slightly less height). It claims to cover both the VHF and UHF frequencies of the Baofeng. Does that sound like a good match? $\endgroup$ – Capt. Nemo Oct 3 '17 at 20:05
  • $\begingroup$ Thank you all for pointing out the shortcomings of the foil Faraday cage approach. I was not aware of the skin affect. It sounds like this can be remedied, however, with an ammo can or similar container to keep the radio in the car (with antenna removed). Most of my shielded equipment at home is both foil wrapped and then stored in galvanized garbage cans with an insulating liner and tight lid. Hopefully, that will suffice. $\endgroup$ – Capt. Nemo Oct 3 '17 at 20:09
  • $\begingroup$ The NoKos are gonna EMP you for sure. Place all electronics in a faraday cage - in your bunker. $\endgroup$ – user2497 Oct 14 '17 at 8:02
2
$\begingroup$

Nuclear initiated EMP events are by nature, low in frequency (below 100 MHz). The estimated energy of such an EMP event is less than 1 J/m2 at the ground level. Antennas and power lines are the major collectors of this energy. Due to the low frequency nature of the EMP spectrum, an 80 meter dipole could deliver a 1 MV pulse to the feedline. The best protection for your VHF radios is to simply remove the antenna and any other cables since these act as collectors of EMP energy.

The aluminum foil may actually increase the susceptibility of the radio to EMP damage if not made and maintained correctly. Here are a few of the basics: The skin depth in aluminum at 1 MHz is 82 um. It is considered best practice to use a material with at least five times this penetration depth or 410 um. Common household aluminum foil is 16 um thick and heavy duty household foil is 24 um thick. As a result, a generous portion of the 1 MHz energy will pass through the foil. This effect is multiplied by the fact that larger surface area of the foil allows it to more effectively couple to the 1 MHz energy. The net result is that more 1 MHz energy could be coupled to the radio with this faux Faraday cage than with simply leaving the antenna jack open circuited.

Regarding antennas and range, keep in mind that on VHF, most of the communications takes place on a line of site basis. You may get "lucky" and get just the right reflection or scattering from the geography or man made objects but this should not generally be relied upon.

A few watts EIRP will allow point to point communications on VHF with line of site conditions. You should first focus on height of the antennas and then secondarily on gain. The gain of the antenna may help you overcome minor fringing, foliage attenuation, and interfering signals or noise sources but it cannot typically and reliably make up for a blocked line of site. Put another way, you could invest in expensive, high gain antennas and still not be able to reliably communicate without line of site conditions.

There are two approaches you can take. You can do empirical work by experimenting with various antennas and antenna heights. This could be done with some simple home built 1/4 wave ground plane antennas placed on extendable painters poles held up by hand. Once you have communications, experiment with the power level and height required to maintain the link. Keep a log book of the results of your experiment so you can translate this into the final solution.

The alternative approach is to study the topology between the two locations in order to reach some general conclusions. An on-line topographical map and a drive through the area can be quite instructive. Once you have a sense of what should be possible, you can then proceed to experimentation to verify your findings.

$\endgroup$
  • $\begingroup$ FYI, the risetime of the initial pulse is from zero to full value in about 8 to 9 ns. That correlates with the part sizes within ICs. Mil-spec chips often use zener diodes internally on each pin of the chip package. $\endgroup$ – SDsolar Oct 4 '17 at 5:41
1
$\begingroup$

The easiest EMP protection I know of for hand-held radios is an ammunition box (available from Army/Navy Surplus stores).

Some people grind the top edge of the box and remove the rubber waterproofing gasket from the top part. This makes a Faraday cage for warm storage.

Then keep the radio(s) charged, and in between charging store them with any spare batteries in the ammo box.

The 701-style 8-inch flex whip antenna is the best fit for such boxes. They are available from the usual sources.

Obviously, a handheld antenna will not give as much range as a base or mag-mount car antenna - and those antennas, when disconnected, themselves are generally not going to burn out in an EMP event, so can be kept on standby to be connected to the radios after the event.

Rotating antennas are the same - as long as they are disconnected they present no path for voltage spikes to come into the radios. However, the directional controls themselves could well end up disabled. It may turn out that having an omnidirectional antenna is preferable.


Having said all that, I'm sure it would be obvious that any EMP event would be so devastating that having comms just for the time batteries can last will be very brief compared to the outages of methods to recharge the batteries. Plus, any car built after about 1990 would likely be disabled due to the internal computers that control the ignition system, et al.

$\endgroup$
  • 1
    $\begingroup$ I agree. My plan is to use my old 1991 Cummins diesel after an event. The only susceptible components (I believe) are the voltage regulator, alternator and starter, and I have spares for each of these in faraday cages. The reason I want to communicate with my wife in this scenario would be to coordinate me coming to get her with the truck (where and when, etc.). $\endgroup$ – Capt. Nemo Oct 3 '17 at 20:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.