Common-mode current is often misunderstood and often overlooked. It is important to reduce this unwanted RF current: reducing unintended RF exposure in the shack and helping prevent RF contact burns are key reasons to address it. [13, 14]
In a typical amateur radio station, eliminating common-mode current throughout the system is extremely difficult. We will focus on reducing unwanted RF current entering the shack and identifying other possible paths that are commonly overlooked. Achieving and verifying complete elimination is beyond the scope of this discussion.
Coax has two physical conductors, but at RF it is useful to distinguish three current-carrying surfaces: the center conductor’s outer surface, the shield’s inner surface, and the shield’s outer surface. The intended signal and its return travel on the first two. The third—the outside of the shield—can carry unwanted common-mode current and is the path we are addressing here. [1, 2]
A set of common myths or misconceptions we need to check
The coax may look like plumbing for RF, but its outside can have a job nobody assigned it. Confident advice can leave out the shield exterior, the grounding network, or the antennas’ intended return paths. We will follow the complete circuit and see where familiar shortcuts fall short.
- A lightning arrester is not a common-mode choke (a misconception I hear all too often). Its shield bond connects to the grounding network, which can provide an easily overlooked RF current path into the shack through equipment chassis and bonding conductors.
- Buried coax has not retired from RF duty. Soil losses can provide some attenuation, but burial alone should not be relied on for adequate common-mode suppression or elimination. Burying the coax is no substitute for a properly selected and installed common-mode choke.
- Placing a common-mode choke at the antenna feedpoint helps reduce unwanted RF current, but it does not make the rest of the coax immune to common-mode current. It suppresses exterior current at its location, but fields can still induce current farther along the feed line, including fields from nearby antennas in a multi-antenna installation.
- It is a poor choice to place the only entry choke in the coax between the grounded junction and station equipment when trying to suppress incoming common-mode current: RF can bypass it through the grounding and chassis-bonding network. Place the entry choke on the antenna side of the grounded junction to impede incoming exterior current before it reaches those branches. Station-side chokes can still be useful, as the scenarios below explain.
- Properly selected chokes placed at appropriate locations can help reduce unwanted common-mode RF current throughout the system, although they do not guarantee complete elimination.
- SWR does not measure common-mode current. Measuring that current requires other devices, which we will discuss later. Low SWR alone tells us nothing about whether significant common-mode current is present.
- An unselected antenna may still be on RF duty. Another transmitting antenna can induce common-mode current on its coax, and shared switch shields and bonds can join the paths.
Our station and the common-mode current we are following
Imagine a dipole, end-fed antenna, off-center-fed dipole, vertical, Yagi, or hex-beam serving an HF station operating from 10 to 80 meters. Its LMR-400 coax feed line runs underground for approximately 55 feet at a depth of one foot. The soil is described as poorly conducting. At the house, the coax connects to a grounded lightning arrester and continues to the amplifier and transceiver. Their chassis connect through braided bonding conductors to the same grounding system.
In our discussion, common-mode current means current flowing on the outside of the coax shield. Its return path may involve earth, antenna structures, equipment, bonding conductors, and capacitive coupling to nearby objects. The external circuit is larger than the coax alone. [1, 2]
The feed line can become an uninvited antenna element. That is why we follow the outside of the cable as carefully as the signal path inside it. Managing common-mode current deserves the same care as tuning the antenna—it should not be treated as a trivial detail.
A common-mode choke makes it harder for unwanted RF current to flow along the outside of the coax (by adding impedance to that path), while allowing the intended signal inside the coax to pass. Its effect is finite and frequency dependent. A matching transformer does not, by itself, establish common-mode isolation.
Again, SWR—standing wave ratio—does not measure common-mode current. (We’ll repeat this because SWR often gets treated as the whole station’s report card. It tells us about impedance matching, not whether unwanted current is flowing outside the coax.)
What happens underground
The coax’s outer jacket keeps its metal shield from touching the soil, but RF fields can still interact with the surrounding earth. Burial can attenuate—reduce—existing common-mode current and change its distribution along the cable. The effect depends on the soil, frequency, and installation. It is not equivalent to a specified common-mode choke. [3, 4]
An antenna’s RF field can induce current on the outside of the coax beyond the feedpoint choke. That current combines with any common-mode current that gets past the choke.
A separate study of buried wire at UHF demonstrates field-induced current and soil effects. It supports the mechanism described here, but does not establish its magnitude in our HF installation. [7]
Burial may attenuate some common-mode current, but we will not rely on that reduction. For our discussion, we will assume burial provides no common-mode suppression.
Scenario 1 No chokes anywhere
For this comparison, imagine removing every common-mode choke. The shield exterior is then free to participate in the antenna circuit, allowing unwanted RF current to reach the shack. That is a problem worth preventing. If you touch equipment carrying unwanted RF current, you can unintentionally become part of its current path, too. [1, 13]
Burial and grounding connections are not dependable substitutes for common-mode suppression. Leaving the chokes out is a poor choice for the system under discussion: it leaves unwanted exterior current without deliberately added choking impedance. The amount of current and any resulting interference or contact hazard still depend on the installation.
Scenario 2 A choke at the antenna
Begin with a suitable common-mode choke at the feedpoint, either on its own or as part of a hybrid balun. It suppresses feedpoint-driven current on the feed-line exterior according to its impedance and the rest of the circuit. Any coax between the feedpoint and choke remains on the antenna side of that impedance. Farther along the cable, field coupling can still excite exterior current.
The buried run changes how RF current flows along the outside of the coax and may attenuate some of that current. Current arriving at the house reaches a junction: the arrester bonds the shield to the grounding system, while the shield also continues toward the station. Current can divide among available paths according to their RF impedances.
The antenna choke has addressed one source of excitation. It has not isolated the entire station. Exterior coax current or current through the connected bonding network can remain.
Figure A. Possible exterior-current paths with an antenna choke. The grounded lightning arrester is a junction connecting the onward coax and the bonding network. Arrows identify possible paths. This drawing is a schematic only.
Scenario 3 Antenna choke and a choke after the arrester
In addition to the antenna choke, place a second choke in the coax between the grounded lightning arrester and the amplifier/transceiver. It adds impedance to the exterior-current path on the indoor coax and can reduce current on that segment.
Before declaring the indoor coax problem solved, follow another, often-overlooked connection: from the lightning arrester’s shield bond, through the shared grounding network and bonding braid, to the equipment chassis. This is a parallel electrical route around the coax choke; current need not travel first through soil. Unwanted RF current can still reach the equipment through the grounding and bonding system, even when the coax choke has reduced current on the coax itself.
That path does not automatically defeat the choke.
Reducing common-mode current on the coax is not the same as isolating the whole station.
Changing one branch can redistribute current and voltage among the coax, chassis bonds, equipment interconnects, and other cables. A lower reading on one coax segment alone does not establish less RF throughout the station.
Figure B. Adding a choke after the lightning arrester can reduce common-mode current on the outside of the indoor coax, but the chassis bond remains an additional path for unwanted RF current to flow.
Figure C. A more representative visual illustration of the same single-antenna concept shown in Figure B. Orange arrows emphasize the additional RF path through the shield-bonding network.
Scenario 4 Move the entry choke before the lightning arrester
Continue from Scenario 3 and keep the antenna-feedpoint choke in place. Move only the second choke from the indoor coax to the outdoor coax immediately before the grounded lightning arrester, on its antenna side. The indoor coax no longer has that second choke.
Moving the entry choke ahead of the grounded lightning arrester helps reduce incoming common-mode current before it can divide between the indoor coax and the grounding and bonding network. This addresses the incoming exterior-current path feeding both routes, instead of adding impedance only to the outside of the indoor coax.
The goal is reduction, not complete elimination. Results vary by station, and other connected cables can still pick up RF directly; other pickup paths or installation issues may remain beyond those addressed in this scenario. Grounding and chassis-bonding connections remain intact. [1, 2]
Figure D. A schematic showing the entry choke moved before the lightning arrester to reduce incoming common-mode current feeding both RF paths.
Figure E. A visual illustration of the arrangement shown above. Remember: keep the station and lightning-arrester grounding electrodes bonded to the home’s electrical-service grounding system, as illustrated. [5]
Keep grounding and bonding connections as short and direct as practicable. Failing to provide required bonding can violate electrical codes and be illegal in some locales. Electrical-code adoption and enforcement vary by jurisdiction. [15]
Scenario 5 Multiple antennas and additional RF paths
When you transmit on the primary antenna, its RF field can induce common-mode current on the outside of a second antenna’s coax shield—even if that second antenna has a feedpoint choke. This creates another possible RF path into the shack. Nearby control cables and bonding conductors can also pick up RF, creating additional paths into station equipment. These paths can complicate suppression, although they do not necessarily increase current or interference in every installation. Jim Brown discusses this pickup on coax shields in his article “Chokes and Isolation Transformers for Receiving Antennas,” published in NCJ, March/April 2019, page 11 (see bibliography entry 2).
The selector knob says which antenna is selected; it does not necessarily give the other shields the day off. An indoor antenna switch can join these exterior-current paths inside the shack. In a switch that changes center-conductor connections while leaving shields bonded to a common enclosure, an unselected feed line remains connected to the shared exterior network. Current can divide among connected shields, the enclosure, and station bonds. A choke only between the switch and radio does not isolate all incoming paths from one another. An outdoor switch moves the junction outside; it does not automatically eliminate exterior current on the onward coax. This circuit interpretation depends on the actual switch construction, rather than an assumption about all antenna switches.
For each antenna system, use a suitable common-mode choke on the incoming coax before its grounded lightning arrester. This helps control each feed line before it joins the station’s shared grounding and bonding network.
Other shack wiring deserves attention too. Ethernet, USB, mouse, audio, and control cables can also pick up RF. Suitable ferrite chokes around those cables can help reduce common-mode current. Choose ferrite material and an arrangement effective at the frequencies causing the problem. These measures can also reduce received noise carried by common-mode current—including noise from shack electronics that you may not have recognized as a source. [1, 2]
Figure F. Adding a second antenna adds another RF path to consider. Even when unselected, its feed-line exterior can pick up energy from the transmitting antenna and connect to the station’s shared shield and bonding network. Purple indicates possible field coupling and induced exterior current; orange highlights shared bonding paths. Only the feedpoint chokes are shown here; entry chokes are omitted to highlight how RF from the transmitting antenna can induce common-mode current on the second antenna’s feed line. A feedpoint choke does not prevent pickup farther along the cable.
The second route through the grounding system
A conventional coaxial lightning arrester IS NOT a common-mode choke. This distinction deserves emphasis because grounding the shield is often mistaken for stopping common-mode current.
For each antenna feed line, place a suitable common-mode choke on the antenna side of its grounded lightning arrester. This helps reduce incoming exterior current before it can divide between the coax continuing into the shack and the grounding and bonding network. [1, 2]
The lightning arresters remain bonded to their shared grounding panel. Keep protective grounding and bonding connections intact. [5]
Remember: Ground is not a magical drain that makes all unwanted RF current disappear. Treating grounding as a substitute for common-mode suppression is a common misconception repeated online. Common-mode chokes add impedance to unwanted exterior-current paths; grounding and bonding serve their own essential safety functions. [1, 2, 5]
Figure G. Each incoming antenna feed line has its own entry choke before its lightning arrester. The physical grounding and bonding connections remain intact. RF arrows after the entry chokes are omitted to emphasize suppression, not complete elimination.
A practical approach with the existing equipment
I know some readers will say, “I’ve never used a choke, and I’ve never had a problem.” But no obvious symptoms does not prove that common-mode current is absent. Use suitable chokes to reduce unwanted current, and check the results with RF current measurements where practical.
First establish the antennas’ intended feed and return paths. Use an appropriate balancing arrangement for your antennas, including dipoles and beams. For end-fed antennas, provide the counterpoise or other return path required by the design before selecting choke placement. Some designs intentionally use part of the coax exterior as that return. For a vertical antenna that uses radials, connect the radials at the antenna feedpoint, on the antenna side of the choke. Here, “antenna side” means the electrical connection order—not a location outside the radial field. The choke can be at or immediately next to the feedpoint, within the area covered by the radials.
Select transmitting chokes using published impedance data for every band you actually use. A choke is not a universal item: different designs and manufacturers provide different common-mode impedance across the bands. Use the manufacturer’s published data for the exact model and installation. An ordinary operator does not need to measure choke impedance personally when suitable published data are available. A power rating or an “80–10 meters” label alone does not establish effective suppression throughout that range. [1]
Price alone does not determine whether a choke is suitable for your antenna system. Treat a choke’s power and duty-cycle ratings as seriously as a balun’s. Check that its voltage, power, duty-cycle, and heating limits suit your operation. A high-impedance antenna or high SWR can create high RF voltage or current at the choke’s location, so check the manufacturer’s mismatch limits and any required power reduction. A tuner showing low SWR at the radio does not establish the conditions at a choke elsewhere on the feed line. [16, 17]
In a coax-wound choke, loss in the coax produces heat, and high SWR can increase that heating for the same transmitted power. Common-mode current can also heat the ferrite. These are different mechanisms, and SWR alone does not tell you how hot the ferrite will become. Long transmissions and high-duty-cycle modes deserve particular attention: an unsuitable or overloaded choke can overheat or suffer insulation damage. Follow the manufacturer’s limits for the actual frequency, mismatch, and operating mode. A mostly reactive choke can also resonate with the rest of the exterior-current path and increase current, rather than reduce it. [1, 16, 17]
Use the antenna choke, outdoor entry choke, grounded lightning arrester, and shared protective bonding as a practical starting point. Keep the buried feed line in this layout, but do not rely on burial for common-mode suppression. Keep equipment bonding connections short and orderly, and use a local station bonding bus where appropriate. Avoid unnecessarily long, widely separated coax and bonding routes that form large loops.
If you cannot install a feedpoint choke, an entry choke before the lightning arrester can still help reduce incoming exterior current at the shack entrance. It does not provide the same control of the outdoor feed line as a choke at the antenna. A choke after the lightning arrester can also be useful if testing shows an improvement, but the grounding and bonding network remains another possible path into the equipment.
Additional chokes may also reduce received noise carried by common-mode current on the coax exterior. This is an option to evaluate, not an assured improvement; it depends on the noise source and the current paths. Do not add chokes at arbitrary intervals and assume their suppression will add together. Check the result on each band you use, and keep changes that reduce unwanted current or interference. Check both the coax and bonding conductors: a lower reading at one location does not prove that current has decreased everywhere in the station.
Measuring current at the shack
Clamp an RF current sensor around the entire coax. The intended internal outgoing and return currents approximately cancel in the sensor, leaving an indication associated with the exterior mode. Check accessible positions near the entrance and equipment, and measure the individual bonding braid separately without disconnecting it.
A relative indicator can detect current in an already assembled station; a before-and-after comparison is not required. Comparisons help assess a modification. Keep frequency, transmitter power, sensor settings, and marked positions consistent. Position the probe with transmission off, start at low power with the amplifier bypassed, and follow the instrument limits.
Check several positions and each used band. A low reading can indicate a current minimum or inadequate sensitivity. Diode detectors can be nonlinear: half the indication need not mean half the RF current. An ordinary electrical clamp meter or volt-ohmmeter generally lacks the HF response required.
Actual readings in amperes require a known probe response and compatible readout. A calibrated RF probe converts current into output voltage; its transfer impedance at the operating frequency supplies the conversion, with the specified termination and cable corrections. A suitable spectrum analyzer or RF detector can provide the readout. [6]
Commercial and inexpensive measurement options
A homemade detector offers a low-component-count approach for presence and relative checks; Figure 1 in the ARRL QST article by Larry Lamano gives a published circuit (see reference 1). Absolute readings require calibration.
RF.Guru CurrentScout is listed at EUR 139 and is explicitly a relative RF current indicator, not an absolute calibrated ammeter. It can detect current and compare changes, and its opening accommodates LMR-400. [9]
Palomar PCM-1 is listed at USD 499.99 and marketed as a direct-reading RF current meter covering 0.2–30 MHz. Direct-reading capability should not be equated with demonstrated precision. [10]
Tekbox TBCP2-250 is a snap-on RF current probe with calibration data and requires a suitable separate readout. As of the date of this article, the cost is approximately USD 700 for the probe alone. Total cost and uncertainty depend on the readout and measurement setup. [11, 12]
For basic troubleshooting, a relative indicator may be sufficient; quantitative reporting justifies an absolute measurement system only when its accuracy and sensitivity are suitable.
The short answer
The immediate recommendation is to control excitation near the antenna, consider an outdoor choke before the bonded entry junction, retain protective bonding, and check more than one RF path. Also consider local RF pickup in the shack and, where needed, add suitable ferrite chokes to other connected cables.
My strongest recommendation is to read the relevant technical literature carefully, starting with the references below and seeking additional well-supported sources. Do not rely solely on claims from self-described experts, amateur-radio videos, or even this article. Follow the evidence, examine the details, and consider how the findings apply to your own station.
References and links
8 Times Microwave Systems. LMR-400-DB product specifications. Manufacturer source.
9a RF.Guru current meter operating manual.
11 Tekbox. TBCP2 snap-on RF current monitoring probes and TBCP2-250 manual. Manufacturer source.
12 Saelig product list. TBCP2-250 dealer price reference; not a calibration authority.
14 ARRL. RF Exposure. Station assessment guidance and resources.
Evidence boundary: references 3, 4, and 7 are peer-reviewed research. References 1 and 2 are published amateur-radio technical articles, not presented as peer-reviewed trials of these configurations. Reference 5 is a technical book reference.