This is a subject that we just had to write about. In the lightning protection business, we come into contact with many people who have had both dangerous and disastrous experiences with Mother Nature.
One that has perplexed antenna users for decades is the very common damage and destruction to radio equipment when connected to a so-called “DC Grounded” antenna system.
For many years’ antenna, manufacturers have touted the positive advantages of owning and operating a station with antennas whose feed systems are a direct DC short across the input terminals. Hence, both sides of the coaxial feeder cable are placed at the “ground” potential at the antenna site.
In reality, there are no such advantages to this kind of feed system, but it is singly the most dangerous ever used from a lightning perspective.
The reason is pretty easy to both explain and understand. Lightning bolts that streak from clouds to the ground frequently hit exposed metallic structures like towers and high antennas.
This is simply because the metallic nature of the object electrically shortens the striking distance between the ground and the sky. When a large voltage potential is reached between the two during a storm, the metal antenna acts as a prod, sticking up in the air and drawing the first arc.
Lightning wants to reach the ground, and that’s pretty much all it wants. And it will get what it wants in the easiest and least resistive way possible. Just about anything in the way can be easily vaporized out of the way by a good-sized lightning blast.
Suppose ten different paths to the ground are presented to a striking bolt (such as numerous transmission line conductors, the tower frame, etc.). In that case, the currents will divide quite nicely between all of them, with the larger amount of current flowing in the path of least resistance and so on.
“DC Grounded” type antennas provide a very neat dual path for those lightning currents. Some of the blasts will flow down the cable’s shield to ground-level earth terminal connections, while the rest will flow down the center conductor and ravage the radio connected at the other end.
Remember that at the point of impact, a bolt of lightning can easily deposit 50,000 volts or more to the ground.
And for an instant, the voltage at the radio equipment end will be the same.
By the time the balance of the surge ends, the equipment will have long since been toasted, probably beyond repair. The myth is that “DC Grounded” antennas offer good lightning protection. The legend is that antenna manufacturers have been claiming it for decades. The fantasy is that some of them still actually believe it.
But it’s not all hopeless. Here’s how you can tell if your present antenna is one of these and what you can do about it.
Disconnect the transmission line at the equipment end and measure across the center and outer conductors with a VOM on the R x 1 scale. If only a few ohms are measured, then the antenna at the other end is a DC-grounded type.
If you’re satisfied with the antenna’s performance and wish to continue using it, then you have two choices.
First, disconnect the antenna whenever a storm approaches and hope you’ll always be there to do it on time.
Or second, install a blocking-type lightning arrestor that will shunt center conductor voltage to the ground while blocking voltage from passing through the arrestor. Be sure to install the arrestor at ground level and ground the device’s body well.
If you’re in the market for an antenna and wish to enjoy some protection, select the ones that use capacitor or link feed systems. Capacitor feed systems such as gamma matches are excellent feed systems and lightning protectors as well. They isolate the center conductor and force lightning into the shield.
Based on the provided content from the link, here are 5 frequently asked questions and their answers:
What does it mean when an antenna is DC grounded for better lightning protection?
When an antenna is DC grounded, it means it has a direct electrical connection to the ground. This is believed to offer better protection against lightning strikes, allowing the electrical discharge to flow directly into the ground potentially reducing the risk of damage to connected equipment.
Is the claim about lightning protection through DC grounding accurate?
The claim about lightning protection is somewhat exaggerated. While there can be a direct path to ground for DC, it might not always offer adequate protection against lightning. The direct connection may not be heavy enough to handle the immense power of a lightning strike. However, a DC ground can help drain off static to some extent.
Is there a difference between a DC ground and an AC ground?
Yes, a DC ground isn’t necessarily an AC ground, and vice-versa. The type of grounding depends a lot on how the grounding is done. For instance, a coil can show a direct path for DC but no path for AC due to the inductance of that coil, which is frequency-related.
What is the misconception about DC Grounded antennas regarding lightning protection?
The misconception is that “DC Grounded antennas offer superior lightning protection. While they might contribute to lightning protection, they shouldn’t be solely relied upon. Lightning seeks the path of least resistance to the ground, and a “DC Grounded” antenna can provide a dual path for lightning currents, potentially leading to damage to connected radio equipment.
How can one determine if their antenna is a DC Grounded type, and what precautions should be taken?
To determine if an antenna is a “DC Grounded” type, one can disconnect the transmission line at the equipment end and measure across the center and outer conductors with a Volt-Ohm Meter (VOM) on the R X1 scale. If only a few ohms are measured, then the antenna is likely a DC-grounded type. For protection, it’s advisable to either disconnect the antenna during storms or install a blocking-type lightning arrestor at ground level, ensuring it’s well-grounded.