Sunday, December 16, 2012

A Little Advanced Electronics

I had a good question in the email this weekend, from a friend reading about EMP protection.  He wanted to know if the advice he'd seen online made sense.  It centered on grounding a box.  I thought this might be something other people would find interesting.  

Every circuit has a return terminal, ground or "earth" as they call it in the UK.  It's pretty common to come across people talking about the importance of a good ground, but what makes a "good ground"?  The National Electric Code, here in the US, calls out a ground rod where the AC feeds come into your house; I think typically 8' long, and they specify wire size.  As it says over on the sidebar there, I design radio systems for a living - systems that go on commercial aircraft.  Kind of hard to sink an eight foot ground on a moving airplane, but we depend on good grounds in all of our stuff.  How can that be?

Here's the problem: as frequency goes up into the radio spectrum, you begin to find that ground doesn't really exist - in that sense.  Radio frequencies have a wavelength associated with them, and to figure out how to ground them, you need to know the wavelength.  Back when I taught Novice ham radio classes, it was pretty common for people to be confused about the way hams refer to some bands by their wavelength ("80 meters", "2 meters") and others by their frequency ("440 MHz", "10 GHz").  The wavelength is important because wires behave very differently when they're electrically very short or long.  A very short wire, under 1/20 of wave long, behaves just like you think: it's just a connection.  If that wire is quarter wavelength long and you ground the "far" end, it's not shorted to ground for that frequency, it's an open circuit - but it's still shorted to ground for DC.  You can't put that radio frequency energy into that wire.  This effect doesn't happen all at once, it gets progressively harder to achieve ground for the frequency you're working with as the distance to ground gets longer.  My usual rule of thumb is that effective grounds have to be less than 1/10 of a wavelength away.  Some folks will say you really need keep them under 1/20 wave.

What that means to your ham shack or radio comms is that the ground rod won't get any RF ("Radio Frequency") into it if it's more than a one or two feet from the radio.  The amateur "10 meter" band (28 to 29.8 MHz) requires grounds shorter than one meter (one yard for 'mercans).  While my system is well grounded at 3.5 MHz (80 meters), that ground rod is about 2 or 3 meters from my station, so it isn't grounded well at frequencies higher (shorter wavelengths) than 20 meters.  

Rules of thumb are handy to work with.  For a frequency, call it f, in MHz, 1/10 wave is just about 93/f.  The answer will be in feet.  So 1/10 wave at 93 MHz is one foot.  Approximately.

So how do you get a ground at 144 MHz or higher?  How do you get it in a moving car?  The chassis, the body is ground.  In radios with a metal chassis, the chassis is grounded.  In handheld radios, like FRS radios and amateur VHF/UHF radios, you become the ground that the antenna uses. (You are a bag of saltwater, you know...)

Getting back to the original question about grounding a box for EMP protection, what's the wavelength of EMP?  It is very broad spectrum, with energy from low frequency up to a few hundred MHz.  A ground wire can only help with part of that spectrum - the rest of the spectrum is unaffected by the ground wire.  Yes, you will shunt some energy away, and that can't hurt. 

What do I do?  I have my emergency back up 2m/440 radio in a metal ammo can, just sitting on my desk.  The desk is steel and grounded to my ground system, but that's not worth much as low in frequency as 20 meters.  A microwave oven should work as well as an ammo can.  Just wrapping it well in aluminum foil is probably just as good.  The most important point is to disconnect the antennas and power lines, to minimize pickup area - minimize signals to carry into the radio.  The power lines are where the majority of pickup from an EMP will come.

This plot of EMP voltage fields vs frequency shows that most of the voltage below 10 MHz, where a ground rod might do you some good. 

3 comments:

drjim said...

I keep my Elecraft K2, and a couple of HT's in steel ammo cans. Just to be safe, I used adhesive backed aluminum tape to "seal" the edges where the ammo cans close.

Back when I was in the Ground Station side of SatComm, we went to great lengths to get a good "Perimeter Ground" around the entire antenna farm. The highest points on all the buildings *and* antennas, had "lightning rods" connected to the ground field by *multiple* 1000 MCM copper cables, and NO right angle bends were allowed anywhere in the system.

Granted, it wouldn't protect us from a direct hit, but since the "lightning rods" bleed off any static accumulation, it reduced the risk significantly.

"CQ VHF" had an excellent article a few years ago about "grounding" repeater sites that were located on mountain tops.

Since it was futile to try and drive a ground rod into solid rock, they jack-hammered out long trenches, and used very thick, wide copper strap, held into the trenches with Bentonite clay.

I never heard how well it worked.....

Graybeard said...

While I know you know this, I want readers to know that the ground at your SatComm stations was not an RF ground, but a DC/low frequency ground. Also very useful for lightning protection, as you say.

The purpose of the safety ground with a ground rod, like the one the NEC requires for AC power into your house, is to provide a reference to the lowest potential that can be found, so that the risk of shock from everything in the house floating up to high potential with respect to ground is minimized. Your Station ground sounds more like this, along with lightning protection.

When you're using any sort of power transmitter without a good ground, it's common to get little shocks - burns, really - from the RF on metal surfaces, like a microphone grill, for example. The RF can also get in to things, creating noise on PC displays, TV or radio interference, and other things. A good ground can prevent this.

RF energy can behave in funny ways. In the industry, it's pretty common to hear RF design referred to as "black magic" because of the seemingly strange and non-intuitive ways it can act.

drjim said...

Yep, and we're practitioners of that fine art!

Since everything we did was in waveguide, an "RF Ground" was kind of pointless. Just keep all those flanges clean, square, and bolted tight!

As you say, it's damn near impossible to get a good RF ground unless you go to extremes. Most of the grounding you see touted in the Ham Radio magazines is really more of a safety ground than an RF ground.

About the best you can do *in your shack* is to bond everything together to a single-point ground, like a large copper pipe or flat buss, using 2" braid. At least that way, most of the gear will be at the same "RF Ground" potential.

One thing I've always done is to use a "choke balun" or "line isolator" where the coax comes into the house. At least it keeps any stray RF on the coax shield outside the shack.