Coax Cable Grounding: One Path to Earth, at the Entry Point
Quick answer / TL;DR: Short answer: bond the coax shield to earth at the point where the cable enters the building, using a ground block connected to the building grounding system. Do not add a second ground connection further along the run, because the potential difference between the two drives current on the shield and produces hum or noise.
The shield's path: drain, panel bond, rack, building earth.A coaxial cable entering a building is a conductor that reaches outside, which makes it a path for lightning energy and for ground potential differences between structures. Grounding addresses both.
The rule that matters is that the shield is bonded to earth at the building entry, and that the installation does not create a second path that carries current where it should not.
This article covers the entry point rule, the ground block, and the hum and noise faults caused by a bonding mistake.
A coaxial shield is a conductor, and a conductor that leaves the building and returns is an antenna for potential differences between the two ends.
Bonding it to earth at the entry point gives that energy a defined path. The mistake is to bond it again somewhere else.
The Two Faults
| Arrangement | Result | Symptom |
|---|
| Grounded at entry only | Correct | None |
| Grounded at both ends | Current flows on the shield | Hum bars, noise, intermittent faults |
| Not grounded | No defined path | Damage risk to equipment |
| Grounded through equipment only | Energy passes through the device | Equipment failure |
| Ground block not bonded to the building system | No effective earth | No protection |
The both-ends row is the common error, and it is made with good intentions: someone adds a ground at the far end to improve reception, and introduces a hum loop.
Doing It Correctly
- Fit the ground block as close as practical to the cable entry point
- Bond it to the building grounding electrode system, not to a separate rod
- Keep the bonding conductor short and direct
- Do not add a second ground connection further along the run
- Where two buildings are connected, break the shield path with a suitable isolator or use fiber
Real-World Applications
An antenna installation is grounded at the entry with a ground block bonded to the building earth, and the run continues ungrounded to the outlets.
A camera installation between two buildings shows hum bars. The shield is bonded at both ends, and the potential difference drives current along it.
A satellite installation is grounded at entry, and the technician who later adds a ground at the dish creates a loop that appears as interference on specific transponders.
A distribution amplifier introduces hum after installation. The amplifier's chassis provides an unintended second path to earth through its supply, which is corrected by ensuring a single bonding point.
Standards and Verification
Grounding and bonding requirements for antenna and cable systems are set by the electrical installation code applicable to the location, and the ground block is the conventional component for the entry point.
Verification is a visual check of the bonding conductor and its connection, plus a continuity measurement between the block and the grounding system.
Where hum appears after a modification, look for a second ground before replacing any cable. The cable is rarely the cause.
Why a Separate Earth Rod Is Wrong
Bonding to a separate rod creates two earths at different potentials, and the coax shield becomes the conductor between them.
Current then flows on the shield continuously, which appears as hum on analogue video and as noise on digital services.
The correct connection is to the building's grounding electrode system, so that there is one reference potential for the whole installation.
When Two Buildings Are Connected
A coax link between buildings places the shield across two grounding systems, and the potential difference between them can be substantial during a fault.
The solution is to break the conductive path: a suitable isolator in the coax, or fiber for the link.
Attempting to solve it by improving the grounding at one end usually makes the current larger, not smaller.
Where an isolator is fitted, confirm that it passes the frequencies in use. A component specified for one service may not suit another.
Photograph the ground block and its bonding conductor at commissioning. It is the single connection most often disturbed by later work in a service cupboard.
Testing the Bond
A continuity measurement between the ground block and the grounding electrode confirms the bond is present and is the check an inspector will make.
Measure with the coax disconnected so the reading reflects the bonding conductor rather than a path through equipment.
Record the reading. A later value that has risen indicates corrosion at a termination, which is common in damp service cupboards.
Where the building has no accessible grounding electrode, the antenna system cannot be correctly bonded, and that is a question for an electrician rather than for the cable installer.
Related reading: Coax cable for HD antenna — blogs/news/coax-cable-for-hd-antenna
Related reading: EtherCAT cable — blogs/news/ethercat-cable
Related reading: Coax cable for CCTV distance — blogs/news/coax-cable-for-cctv-distance
A second ground connection does not improve protection. It creates a current path on the shield and shows up as hum.
Frequently Asked Questions
Where should coax be grounded?
At the point where the cable enters the building, using a ground block bonded to the building grounding system.
Can I ground the coax at both ends?
No. The potential difference between the two grounds drives current on the shield and produces hum or noise.
Why does my system hum after adding a ground?
Because a second ground path was created. Remove the extra bond and keep a single entry-point connection.
What if the coax connects two buildings?
Do not bond the shield at both ends. Use an isolator or fiber to break the conductive path.
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