Header
Flag Counter

Practical Amateur Radio Grounding

The single point ground in the most important aspect of a good grounding system. The concept behind a single point ground is very simple, keep everything at the same ground potential. If one piece of your station has nothing but the electrical system for a ground then a lightning strike or surge will pass thru your equipment and thru your electrical system to get to ground. This photo shows what happens when a ground differential is present during a lightning strike.

Relying on the electrical ground as the sole source for your station ground can be dangerous and even deadly. I can recall a situation where a tower site where several commercial repeaters were located had no grounds connected to any of the repeaters at the site. The tower was grounded and the electric panel had a single ground rod. One day I was asked to check a repeater at that location. Upon arrival I found the repeater to be checked but I could not reach the coax connector to hook up my test equipment. I unplugged the repeater to move it out from the wall. Much to my surprise when I unplugged the repeater, all of the other repeaters in the building shut off. Checking the electric panel there was no tripped breakers . I opened the breaker panel and found that lightning had blown the ground and neutral buss bars separating them from the buildings power company neutral.

The result of this was that the entire breaker panel was getting its neutral connection thru the repeater I had unplugged, thru its coax to the grounded antenna on the tower. If the equipment had been grounded to a single point ground, along with the tower, breaker panel and all other equipment the problem would most likely not have occurred. I am just thankful that I could not reach that coax connector as that would have been deadly.

The way we make a single point ground is fairly simple but does take some work. You have to think about 3 separate terms, a electrical ground, a lightning ground, and an RF ground. Let us look at each form of ground and its purpose.

Electrical Ground

This is the protection path for faults in your electric power system. In the event of a short to ground it must be able to trip the breaker to protect the circuit. This ground must be able to pass current at a level equal to or greater than the main breaker of electric panel. If you have a 200amp service then your electrical ground must be able to pass 200amp to ground. While a branch circuit like the one feeding your radio room may trip the breaker at much lower currents we must still consider the worst case when considering an electrical ground.

Lightning Ground

The first thing to understand about a lightning ground system is the voltages, currents and frequencies involved in a lightning strike. While it is possible to reduce the likelihood of a lightning strike it is simply impossible to prevent them altogether. A direct lightning strike can be in excess of 100,000,000 volts and over 100,000 amps and a huge source of low frequency RF power. Even the best electrical ground becomes resistive when being supplied with 100,000 amps of ELF RF. This is why a lightning ground must be carefully thought out. The key is to supply as many points of contact and as large an area of contact as possible with the earth. Another key point to remember is that a lightning strike is a time limited event that last only a fraction of a second. While an electrical ground needs to dissipate current long enough for a breaker to trip, a lightning ground only needs to carry current for a fraction of that time.

RF Grounds

The next key point is that lightning also provides high RF currents. As a result you must keep in mind how a conductor responds to RF currents. Even if you have a good low resistance ground for electric circuits, at RF frequencies you have to consider the grounds impedance. Because of the skin effect most RF travels on the outer most portions of conductors. To reduce the impedance you must increase the surface area of your grounding conductors. A common method of accomplishing this is by using flat conductors. These should be used in connections at tower bases and cable entry points and should be connected to the grounding electrodes using as short of a run as possible.

The first step is to decide how we will achieve a single point ground. To do this we must create a ground electrode system. There is no way around it we will have to create a low resistance connection to the earth. Even the NEC code today requires at least 2 ground rods separated by 6ft. A ground rod has what they call a sphere of influence.

It basically shows the area equivalent of how much earth your ground rod is in contact with. As you can see from the image a single ground rod will not give you much ground. For radio towers and mast it is recommended that a tower ring grounding system be installed. A tower ring consist of a ring at the base of a tower with #2 or larger wire with ground rods every 10 - 15ft with connections to each tower leg. At my house I used #2 solid tin plated copper.

All underground connections should be made using exothermic weld (cad weld) or irreversible high compression connectors to prevent the connections for becoming resistive or just coming loose. You would not believe how many time I have found ground rods with the old " ground rod " clamp so loose I could just pull the ground wire out.

One word of caution, Always use the same type of ground rods if you use copper use all copper. However if you decide to ground your guy wire anchors and your guy anchors are galvanized use a galvanized ground rod closest to the guy anchor.

Now that the tower is grounded you need to think about the coax cables. Since coax cables are insulated they must have some way to ground them. One way of doing this is to remove the outer jacket and connecting the shield to the tower. They should be connected at the top and the bottom of the tower.

Do not attempt to solder to the shield as that will likely damage the dialect of the coax. Coax grounding kits are available online or you can make them yourself.

Be sure to seal up the connection with coax seal and tape to prevent moisture from getting into the coax. Keep in mind that coax lightning protectors only remove the surge voltage off of the center conductor of a coax. If a lightning strike occurs and the coax shield is not grounded then voltage from the lightning will be the same on both the shield and the center conductor and lightning protectors only works if the voltages are different.

Next we will need to look at the point where the coax cables and rotor cables enter the building. This is the point where most hams like to put a ground rod. While this is a very good place it is only part of the grounding system. In order to maintain a single point ground, all ground rods must be connected together to form one ground electrode.

In commercial installations a ring is installed around the building with multiple ground rods. While this is ideal it is somewhat impractical for an amateur station. In the most recent versions of the NEC codebook it does say however that all ground electrodes should be bonded together so the electric service ground rod and any other service ground rods should be connected together. PLEASE NOTE, you should never disconnect an electric service ground. You can tie into the service ground without disconnecting it by using an irreversible clamp. In addition to the ground rod at the coax point of entry and electric service ground the tower ground ring must be connected together so that it makes one singe common ground.

Per the NEC codebook all cables entering the building should be grounded at the point where they enter the building. This can be accomplished by adding one additional coax ground kit on each coax just before they enter the building. These ground kits will need to be connected to an external ground bar (EGB). This grounding bus is then connected to the common ground electrode system.

When it comes to rotor control cable the NEC says that unshielded conductor should be run thru metal conduit and the conduit should be grounded. I have yet to see a single ham installation where this has been done, including my own. Perhaps I will convert to a shielded rotor cable at some point. For now I am just using a Polyphaser lightning protector grounded to the MGB.

At this point all cables are now inside the building and the outer conductor of all cables should have a low resistance path to ground. It is at this point where the lightning protectors come into play. We need to install a Master Ground Bar (MGB) at the point where the cables come inside. The MGB will be grounded to the outside ground electrode. The lightning protectors should be installed as close to the cable entry point as possible. Each of the lightning protectors should have a separate ground wire connecting them to the MGB by as short a wire as possible. If all of the previous suggestions were followed correctly the lightning protectors should now be able to do their job and provide a path to ground for any lightning strikes or surges on the center conductor of the cables.

It is the MGB that is the sole grounding connection point for all equipment inside the shack. This IS your single point ground. If the shack is a short distance from the MGB then all equipment in the shack should be connected with separate individual ground wires to the MGB. If the distance from the shack to the MGB is more than about 8ft you may want to install a Sub System Ground Bar. This SSGB will then need to be connected to the MGB and all equipment in the shack connected to the SSGB.

While it is not specified how long the ground wire from a MGB to the equipment should be for electrical and lightning grounding we should take into consideration the effect of RF on the ground wire. In a typical amateur HF station we should always avoid long runs of ground wire. If long runs cannot be avoided we should at least avoid any wire length that is close to any multiple of ¼ wavelengths of any HF Band. The last thing we want is for our ground wire to become a radiating part of the antenna system.

So what equipment in the shack needs to be protected? Basically if it is conductive it should be protected but more specifically anything that connects thru a conductive material from outside the room to inside the room. This includes power lines, phone lines, network cables, TV cables and etc. There are separate devices for protecting each of these devices. Each of these protection devices should be connected to either the SSGB if used or the MGB using the shortest possible ground wire as possible.

A few final notes I would like to make in regards to grounding. You should always keep the grounding conductors in as straight a line as possible. If there must be curves in the ground wire, keep them slow and gentle. Lightning likes to jump from sharp corners. Also keep the ground wires flowing in the direction of the ground bar.

Finally to reduce the RF resistance in your grounding system I recommend using flat copper strap instead of round wire. This will increase the surface area of the ground and will reduce losses due to skin effect at RF frequencies.

Recommendations outlined in this document are based partially on the following documents:

Motorola R56 Standards and guidelines for Communication Sites Rev 2005

MACOM /Harris Site Grounding and Lightning Protection Guidelines Rev D

NEC Codebook Rev 2011

Polyphaser Guide First Edition



Amateur Grounding Law

Reducing Damage

Electrical Shock and RF Burns

Predictable Performance

Recommendations for Amateur Grounding



  Copyright © Radio2way.net