Two Letters Re: Questions on Faraday Cages and Radiological Survey Meter EMP Resistance

Mr. Rawles,
I am an RF and EMC engineer. I’ve worked nuclear EMP issues for a couple of decades.
You were fairly right on – a Faraday cage (or “shield room”) is hard to build. It can be done with fine mesh – similar to window screen but made from copper wire – but the penetrations and doors are always the problem. Mesh will not protect you from large magnetic fields, but for much of the affected area, they are not the problem.
Your plan to use a steel ammo can has merit – as long as you close the seam created by the lid and rubber gasket! Otherwise you don’t get much shielding. You could sand off the paint and use copper or aluminum tape to cover the gap from lid to case. It’s sold for use in EMC testing and features a conductive adhesive.

Transistor gear is susceptible – vacuum tube gear is much more hardened. But the prospect of having to replace tubes in a major SHTF scenario drives many people to buy solid state gear. So there are trade offs to consider. – Sun Dog


First, the misconception on the effectiveness of chicken wire. It is correct to state that any wavelength that is shorter than the holes in the chicken wire will pass through it as if it isn’t there. However, EMP is a broadband RF signal and most of the energy is in the lower frequencies. Almost no energy will remain at frequencies high enough to penetrate the wire, making damage highly improbable.
The basic design of a Faraday cage is to have a space that is entirely enclosed by a conductive surface with the lowest possible resistance. The best Faraday cage would be a hollow cube of gold. Of course, with no door, this is not a terribly practical design. More typically, top end Faraday cages were made of two layer of copper screening attached to a wooden frame with no electrical connection between the layers. The door would have the same two layers and some sort of copper fingers would be on the doors to make the electrical connection to the rest of the cage. Any power going into the cage would have some big filters on it to prevent any RF from going into or out of the cage.
The most common Faraday cages that are in people’s homes are microwave ovens and computer cases. Microwave ovens were originally called radar ranges because they heat your food with the radio waves from a radar transmitter. Needless to say, it designed to heat only food and not the user so these are designed to prevent the RF from escaping. Just make sure that no one can plug them in if you have your electronics stored in them. Similarly, computers are notorious emitters of RF interference due to their use of square wave clock signals. All the cases that I have seen are pretty much solid and the newer ones have the fingers to insure that any removable panels maintain electrical connection.
Ammo cans can be used but are not perfect because the gasket and paint prevent good electrical connection between the lid and the box. Aluminum foil is an amazingly convenient material to make a cage out of. Chicken wire can be used, but you will have to solder it together to get the good electrical connection. Virtually any metal box will provide some protection, including ovens and refrigerators. As stated in the original comments, the cage will not be effective if penetrated by antenna or power lines. Cars, being made of metal, will also provide some protection. Multiple layers of metal will provide additional protection, but must not contact each other. Put your electronics in a plastic bag with desiccant, wrap it foil, another plastic bag, and put it in the modified ammo can.
The basic rule of thumb is that susceptibility to EMP damage is proportional to size of the collection area (usually the antenna or power line) and inversely proportional to the size of the electrical component [gate]s. The first thing that anyone can do is put all their sensitive electronics on power strips or better yet surge protectors with EMI filtering. Doing this will allow a quick disconnect from the power lines should a threat arise. The filters will also provide a small amount of protection should an unanticipated event occur. Back in the 1980s, QST magazine did a three part article on EMP. As part of this series, they exposed a handheld radio to an EMP simulator, with no damage to the radio. Based on this, it reasonable to expect that a fair amount of electronics will survive, so long as they are not plugged in.
For the record, I am an RF Engineer. While I do not work in the EMP field, I have read up on EMP from what sources I could.
Note that Wikipedia has entries for both Faraday Cage and Electromagnetic Pulse. – R.H.