Grounding Radios

[desertengineer1]

An RF ground is a different animal entirely. HF antennas may require an enhanced ground, usually at the antenna site, especially for vertical antennae not having sufficient radials. Keep your cotton picken hands of RF radiators! These can shock and burn you , and unlike Gamma radiation, you won't develop superpowers as a result of exposure.

Major Lord

In this case, you become the ground.  RF burns are NOT fun. 

[arajca]

I've read the item a few times. I presume the strike was probably within 15'-20' of the cadet. Even if the antenna and trailer were properly grounded, if the cadet was by an open window (the article states the cadet was inside the encampment hq), he probably would have still received a minor shock just from proximity. The limited description indicates to me that the cadet did not receive the shock through the power system.

As for the equipment that was being used, remember, he was inside a building, not a trailer.

[desertengineer1]

CUL's and other radio heads:  The most recent issue of The Sentinel described an incident in which a cadet was zapped (not injured) while working in a comm trailer, and blamed a lack of proper grounding.

Given that our SAREX's and events are usually in borrowed buildings, I'm not about to pound an 8 foot grounding rod into the ground (or a parking lot.)  I'm curious what others typically do to ground radios and antennas in a temporary install.

If in a parking lot or concrete pad, the RIGHT thing to do is tie the trailer to the single point ground (or Earth Electrode System - EES) of the structure feeding it power.

If you are using a generator, it can get a little more complicated.  The simplest answer is to tie the generator and radio ground point to the single point ground of the structure (Radio --> Generator --> building EES) and KEEP power isolated as much as possible.  i.e. try to keep everything in the trailer on the generator and everything in the building on building power. 

Yeah, that's hard to do, but will lower your risk of damage/injury if there is a ground fault somewhere. 

[arajca]

If you can place something into the ground purchase the following items from your local hardware store:

10 feet piece of 3/4 " copper plumbing pipe
Electrical pipe ground connector
#10 electrical wire (length depends on how far your trench is from the radio)

Attach the electrical connector and wire to the 3/4 " copper pipe.
Dig a trench at least six inches deep and place the copper pipe in the ground and cover over with dirt. Make sure to frimly place all the dirt back over the 3/4" copper pipe. You will also need to wet and tamp the ground over the copper pipe.

Then hook up the wire to your radio.

I take it you are running the pipe horizontal, instead of vertical?

[desertengineer1]

Clamp to a Pad Eye on the ramp.  They are usually connected to a huge mesh of steel screen embedde in the concrete...

NO!

You do not have guarantee the pad and the power feeding structure are equal potential. 

Only if you used a ground continuity meter to verify NEC (10 Ohms, I think) can this be trusted.

[desertengineer1]

OK, here's a good place to start...

www.tscm.com/MIL-HDBK-419A.PDF

[arajca]

OK, here's a good place to start...

www.tscm.com/MIL-HDBK-419A.PDF

Nothing like a little light reading before bed. j/k

Thanks for the link.

[desertengineer1]

Nothing like a little light reading before bed. j/k

Thanks for the link.

NO KIDDING!  Yeah, it's one of the driest documents I've ever worked with - but good info.  Handbooks are intended to be guides for design and in this case, give you the info to keep risks as minimal as possible within your design constraints.

It ties well with what we do.  No, I don't have 200' of #6 AWG stranded laying around to ground everything to NEC.  But..  I can take steps to avoid the bigger landmines.

During our SAREX this weekend, I had minimal grounding resources and a single comm trailer provided by the local Emergency Management folk - generator fed and on concrete.  The only option was to either tell the IC can't do it, I'm going home, or lean into available options.  We leaned to procedure.  I briefed to the Safety officer, IC, and MRO's that under no condition will the trailer be operational in lightning hazards.  our procedure was to disconnect the antenna cables, pull them out away from the trailer and go in the hangar office.

[SarDragon]

Most hangars are tall metal objects, so stay away from the walls and supports if you use one for shelter...

Watch out for the same thing in concrete block structures. The walls retain enough moisture to give you at least a small zing if lightning strikes the building or nearby. BTDT. Not fun. No interest in a second experience.

[Airrace]

If you can place something into the ground purchase the following items from your local hardware store:

10 feet piece of 3/4 " copper plumbing pipe
Electrical pipe ground connector
#10 electrical wire (length depends on how far your trench is from the radio)

Attach the electrical connector and wire to the 3/4 " copper pipe.
Dig a trench at least six inches deep and place the copper pipe in the ground and cover over with dirt. Make sure to frimly place all the dirt back over the 3/4" copper pipe. You will also need to wet and tamp the ground over the copper pipe.

Then hook up the wire to your radio.

I take it you are running the pipe horizontal, instead of vertical?

Yes the pipe is horizontal.

[Airrace]

Try to remember that electricty or lighting will always follow the shortest path to ground.

If you are woried about lighting strikes then you will need to install a lighting sure protector. This will prevent the lighting from traveling past the surge protector.

If you have a generator or other power source make sure it is adequitely grounded which means connection from ground lugs on the equipment to the single ground point of a ground rod or metal structure. 

[desertengineer1]

Try to remember that electricty or lighting will always follow the shortest path to ground.

If you are woried about lighting strikes then you will need to install a lighting sure protector. This will prevent the lighting from traveling past the surge protector.

If you have a generator or other power source make sure it is adequitely grounded which means connection from ground lugs on the equipment to the single ground point of a ground rod or metal structure.

This is not correct.

The main lightning (return stroke) will follow the path established by stepped leader ionization paths, and is commonly NOT the shortest path.

Lightning strikes don't follow the same physics as we're used to with batteries and AC voltages.

When two opposing charges approach each other, an electric field is present.  The strength of the electric field is a product of the charge magnitude difference and the distance between them.  It has units of volts/meter.  Air at sea level pressure ionizes at around 10,000 V per centimeter.  The initial magnitude of stepped leaders can peak in the millions if the timing is fast enough.

Depending on the substance between the charges, at a certain field magnitude, electrons will begin to come out of their orbits and the material will ionize.  Free electrons means it is now conductive like a metal (factoid - metals conduct electricity because their electrons are freely moving to begin with).

Although metal objects are common targets of lightning strikes, they are not all the time.

The bottom line is this...  Anything will conduct electricity given a high enough electric field - and lightning paths are established by fields of enourmous magnitude.  Just prior to lightning strikes, charges move, the field adjusts, the charge moves again, and so on.  The magnitude of the electric field follows a 1/R^2 behaviour, which means extreme magitude at small distances.

Anything is a target in a thunderstorm if exposed to the electric field between terrain and cloud, and will happily conduct the return stroke if ionized.  This includes the wood in your house, fiberglass flagpoles, even the rubber of your tires.

My point is DO NOT ASSUME lightning will strike only metal objects.  We get at least one house fire per storm here due to the above mentioned physics.  And most of them are right next to a tree or grounded metal telephone pole.

Second serious misconception - SURGE ARRESTORS WILL NOT PROTECT YOU FROM A DIRECT STRIKE!!!!!!!!  THESE DEVICES ARE TO PROTECT THE EQUIPMENT FROM TRANSIENT VOLTAGES OF NEARBY STRIKES OR CONTACT WITH LOW VOTLAGE POWER LINES, ECT..

Surge arrestors are designed to connect the inner and ourter conductors of transmission line when the potential between them exceeds a certain magnitude, usually 300 volts.  They are usually electrodes mounted in a gas like zenon or argon, that ionizes (conducts) when a certain field strength is reached.  It will short the coax line, preventing distructive current between the center conductor and shield in the front end of the radio. 

If an antenna is struck directly, the initial potential can be in the hundreds of thousands of volts, with a peak current of hundreds of thousands of amperes.  There is enough energy in the larger strikes to explode trees, brick walls, and rocks. 

If your antenna line takes a direct strike, it, along with your equipment, has a survivability of about 0.01%.

Again SURGE ARRESTORS WILL NOT PROTECT FROM DIRECT STRIKES.

[desertengineer1]

I need to also throw out a disclaimer...

Commercially installed antenna systems have several required actions to prevent return stroke currents.  These are required by BCSI, NEC, and other similar standards...

Towers (depending on the type) usually have one or sometimes two large copper conductors from the top to the bottom.  This prevents excessive currents in the steel structure which can damage joining sections and reduce the operational lifetime.

Antenna mounts are metal and are bolted directly to the steel frame, as well as grounded to the ESS wire or bonded directly to the structure with a lightning down conductor.

The coaxial feedline shield also grounded directly to the structure or ground wire within a few feet of the antenna, immediately before the bend away from the antenna structure at the bottom, and (depending on run distance) grounded to the structure EES before entering the building.  Some codes require intermediate grounds between the antenna and the tower bottom.

Thirdly, surge supressors are installed before the coax enters the equipment.  Equipment racks are grounded to the EES as well - usually with #6 stranded copper or copper strips.

Altogether, these minimize voltage differentials to the coax cable and the radio.  remember it's a function of ohms law.  Even if the resistance is small, a current of 100,000 amperes can do some damage.