Lightning Protection and Earthing

Did you know that about the most devastating thing that can happen in your shack is generally related to either a direct or near lightning strike. It actually happens more often than you might realise. In my day job, I probably assist a half a dozen hams a year replacing almost every aspect of the shack from antennas, coax, radios, amplifiers, SWR meters and power supplies. The most interesting thing though is that I can count on one hand the amount of lightning damaged sites I have worked on in the commercial radio sector, even though I have way more commercial customers than amateurs. Commercial installations are often far in excess of 30m off the ground, compared to the average amateur installation at around 10-15m. You would expect that with a higher mast, a commercial installation would be hit more, but it doesn’t seem to be the case. Why is this so?

There are a number of views on lightning damage and how lightning strikes on radio installations occur. Most of them actually sound plausible, but since I’m the one with the your ear, I’m going to give you my thoughts.

Firstly, let’s talk about why a lightning strike occurs. We all know that lightning is caused by a build up of electricity in the clouds, and once the potential exceeds an amount that is required to ionise the air – hence creating a path from earth to the clouds, which is in effect, the lightning strike. So lightning often occurs at a point that is above the ground, because positive charges built up in objects on the ground, until the potential between the negative charged clouds and positive charged objects on the ground, exceed the amount of potential required to ionise the air and allows an electrical path between the two.

The trick to reducing the likelihood of a strike, is to bring the potential of any objects above the ground, back to the same potential as earth (which is said to be at zero potential). How do we do that with a mast? Even if we ground the mast, there is still the potential for a build up of charge on the structure, particularly if the mast is an old one with a build up of rust between section etc. What is that is done on a commercial installation that maybe isn’t done so well on a ham mast?

Let’s talk about what is normal (and part of Australian standards).

Earthing the mast. This isn’t just a 3m copper rod hammered into the ground (but could be depending on the quality of the ground). Once an earth system is established, it is tested for earth resistance. Typically we would be looking for around 5 ohms between a peg hammered into the ground 10m or so away, and the earth system. How much rod do we need to achieve that? As much as it takes is the answer to that one. Typically this could be 3 x 6m stakes housed in a 50mm diameter hole, with an earthing compound used to bond the rod to the earth. If it is a guyed structure, you would also place a single 6m rod at each guy point, and bond it back to the mast base using a 40mm copper strap, also buried and bonded to ground using the earthing compound used for the copper rods. To top this off, check the entire structure for rust, particularly at junctions between sections of the mast. If these aren’t clean, you will have high resistance points that can isolate the mast from this great earthing system you have created. The take the thickets earthing cable you can afford (72mm is a good choice), and attaching it at the earthing system on the ground, take it to the top of the mast and place it on a lightning finial, that is above any other antenna on the structure. This bonds the top of the mast to something as close to humanly possible to ground potential, hence significantly reducing the likelihood of a strike. You can rent the earth resistance testers from any test equipment rental business.

The next step is to bond your coax cable to ground also. If your feeder is like most and connected to a balun and then to a balanced antenna, then you have actually created a competing path for a lightning strike and all your work on earthing the structure has protected your structure, but not the radio equipment in the shack! How do you do that? Easiest way to bond your cable is to use earthing kits. These are typically used for Heliax coax runs and used to connect the bared shield of the coaxial cable to the mast, which is then earthed to your mast earth. Typically you would bond the cable to the mast at the top of the mast where your antenna is, and where the cable leaves the mast. If the mast is over 50m in height you might also consider one halfway down the mast. That’s great but what if you can’t afford Heliax? That stuff is expensive! You can get earthing kits for LL400 which is a popular choice of cable for hams. Check out your local ham shop! I haven’t seen them for RG213 but given the physical specs for RG213 and LL400 are similar, you could probably use the same LL400 earthing kits on 213. If you are using RG58, well… you’re on your own there and I cant help you! ?

The next level of protection to consider is actually quite common in many ham shacks, and that is the Lightning protection device. This is best placed as the coax cable enters your shack. It must be bonded to the shack earth, which should also be bonded to the mast earth, where practical. If you don’t bond the lightning protector to your RF earth, you may as well not use one in the first place. As a buy product of using the device, it is also a handy way to terminate your cable on the outside of the shack, with something more flexible on the other side inside the shack to your equipment. These lightning protection devices are available from TET Emtron and their resellers and are very reasonably priced, or you can buy a gas capsule equipped arrestors that you can be re-used after a strike, by just replacing the capsule. The gas capsule arrestors are probably best for installations where you might be designing for that time when the ACMA lets us use 2KW.

The last step is to bond your equipment to the same RF ground used for the mast and feeder cable. If you can get away with a feeder gland plate coming into the shack, then this is generally a good point to collect all earth cables in the shack.

So, we do all this, we can reduce the likelihood of a strike to as low as practically possible. It also has another benefit. Differences in potential between your radio and the antenna, noise is created. Removing all these potential differences should also result in a lower noise floor on your receiver.

For more handy advice, you can talk to us here at Comtek Radio.