Common Mode Currents On Coax
Common Mode Currents
Common mode current is unwanted RF current flowing on the outside of a feeder, braid, mast, support wire, equipment lead, or station wiring. It can make the coax radiate, change the aerial pattern, bring RF back into the shack, and cause interference to audio, computers, routers, and nearby equipment.
Quick visual guide
In a clean feed system, equal and opposite currents stay where they should be. With common mode current, unwanted RF travels on the outside of the feedline and becomes part of the antenna system.
Balanced RF current
The wanted RF currents are equal and opposite, so the feeder does not become a major radiator.
Common mode current
The outside of the coax carries RF in the same direction, so the feedline starts to radiate.
RF in the shack
Unwanted RF can get onto microphones, USB leads, speakers, keyboards, and network equipment.
Choke / current balun
A choke gives common mode current a high impedance while allowing the wanted differential RF to pass.
Where the current should flow
What is common mode current?
In a normal coax feed, the wanted RF travels as differential current: one current on the centre conductor and an equal opposite current on the inside of the braid. Common mode current is different. It flows on the outside surface of the braid, on station earth wires, on metalwork, or on any connected cable that offers a return path.
Wanted differential current
The centre conductor and inside of the coax shield carry equal and opposite currents. This is the normal transmission-line mode.
Unwanted common current
RF flows in the same direction on the outside of the braid or connected wiring. The feedline can become part of the aerial.
Pattern distortion
Because the feeder is now radiating, the antenna pattern and take-off angle can change from what you expected.
Shack RF
RF can return to microphones, audio leads, computers, USB devices, routers, amplifiers, and touch-sensitive equipment.
Common causes
Common mode problems normally appear when the antenna system is not balanced, the feeder is routed through a strong RF field, or there is no effective choke at the correct point.
No current balun
A coax-fed dipole, inverted-V, loop, or off-centre antenna can drive current onto the outside of the coax if there is no suitable choke.
Asymmetrical layout
One leg near metalwork, buildings, guttering, towers, fences, or other cables can unbalance the system.
Poor radial or counterpoise
Verticals and end-fed antennas may use the feeder as the missing return path if the counterpoise is inadequate.
Feedline in the near field
Coax running close and parallel to the aerial can pick up RF and carry it back towards the operating position.
Long station wiring
Speaker, USB, HDMI, mains, network, keying, and audio cables can become accidental RF conductors.
High power operation
Small common mode currents that are barely noticed at low power can become obvious at higher power.
Symptoms to look for
Common mode current can look like several different faults at once. The clue is often that the problem changes when the feeder is moved, touched, coiled, or routed differently.
| Symptom | Likely clue | First thing to try |
|---|---|---|
| RF reports change when coax is moved | The feedline is radiating or detuning the aerial. | Add a choke at the feed point and improve feeder routing. |
| Audio feedback or hot microphone | RF is returning on mic or audio wiring. | Choke the feeder and add ferrites to affected leads. |
| Computer, USB, or network problems | Station wiring is acting as an RF return path. | Use ferrites, shorten leads, and separate RF and data wiring. |
| Unstable SWR near the shack | The measurement point is part of the radiating system. | Measure again with a choke fitted at the antenna end. |
How to reduce common mode current
The usual cure is not one single ferrite thrown anywhere on the lead. The best result normally comes from putting the right choke in the right place and then cleaning up the station wiring.
1. Choke at the feed point
Fit a good current balun or common mode choke where the coax leaves the antenna. This stops the feedline becoming the third leg of the aerial.
2. Add a shack-end choke
If RF still enters the operating position, add another choke before the feeder reaches the radio, tuner, or amplifier area.
3. Use ferrites on problem leads
Clip-on or wound ferrites can help audio, USB, network, keying, and speaker leads, especially when several turns are possible.
4. Improve layout
Keep coax away from parallel aerial wires where possible. Cross RF and data leads at right angles and avoid large cable loops.
Practical point: A choke is most effective when it has high impedance on the band causing trouble. One ferrite mix or winding style is not automatically best for every HF band.
Where should the choke go?
The first choke normally belongs at the antenna feed point. That is where the balanced antenna meets the unbalanced coax. A second choke may be useful before the feeder enters the shack or station patch panel.
| Location | Purpose | When useful |
|---|---|---|
| Antenna feed point | Stops the feeder becoming part of the aerial. | Coax-fed dipoles, loops, inverted-Vs, beams, and verticals. |
| Base of vertical | Separates feedline from radial/counterpoise system. | Quarter-wave verticals, inverted-Ls, and base-fed antennas. |
| Entry panel / patch panel | Stops remaining RF travelling into the shack wiring. | Multi-station rooms, contest shacks, or high-power setups. |
| Problem equipment lead | Suppresses RF already on a local cable. | USB dropouts, audio feedback, computer lockups, and router issues. |
How to check for it
The best method is to measure the current on the outside of the coax with a clamp-on RF current meter. If that is not available, look for repeatable changes when the feedline is moved, choked, shortened, or rerouted.
Important: A low SWR does not prove that common mode current is absent. The feeder can radiate and still show a reasonable match at the transmitter.
Key points to remember
- Common mode current is unwanted RF current on the outside of a feeder or on connected wiring.
- It can make coax radiate, distort the aerial pattern, and bring RF back into the shack.
- A low SWR does not prove that the feedline is clean.
- The first choke usually belongs at the antenna feed point.
- A second choke near the shack can help stop RF entering station wiring.
- End-fed, vertical, off-centre, and asymmetrical antennas need extra attention.
- Ferrites must suit the band and the amount of power being used.
- Good feeder routing and tidy station wiring are part of the cure.