Noise reaches your receiver by four different paths. They come from different places and need different fixes, and a common-mode choke reaches only one of them. So measure first — on a quiet station the right answer is to buy nothing at all. Every number below is marked C when you can look up the source, or 算式 when you can redo the arithmetic yourself.
You cannot pick the right fix until you know how the noise gets in. The ARRL gives its own interference hunters the same instruction: work out where it is coming from before worrying about what is making it.
| Path | What it is | How it couples | What it looks like |
|---|---|---|---|
| ① Antenna wire | The antenna picks the noise up the same way it picks up signals. | Near-field electric field, and far field | Cut 2 leaves it almost unchanged |
| ② Coax shield | Current on the outside of the braid, flowing along the whole length of coax between feedpoint and shack. | Along the braid, and from nearby fields | Cut 2 kills it; a clip-on bead changes it |
| ③ Mains wiring | Noise arrives on the power line, goes through the supply, and ends up on the chassis the receiver measures against. | Conducted | Cut 4 changes it |
| ④ Near field | The rig, the SDR, a USB lead or a network cable is picking it up directly. | Near field | Cut 5 makes the noise follow the device |
All five fit into an afternoon, and none of them needs equipment you do not already own.
| Cut | What you do | What it tells you |
|---|---|---|
| 1 | Screw a 50 Ω dummy load onto the antenna socket and listen on the same frequency. | Noise gone → it arrives through the antenna system (paths ① and ②). Noise still there → it comes in on the mains or from nearby equipment (③ and ④). This one cut halves the search. |
| 2 | Disconnect the antenna wire at the transformer. Leave the coax connected and running back to the shack. | Noise still there → the braid is receiving it, and a choke has something to bite on (②). Noise collapses → the antenna wire is doing the listening and no choke will help (①). |
| 3 | Clip a ferrite bead on the coax and take it off again, several times. | Level steps with the bead → you have found a common-mode path and a permanent choke is worth building. Level moves but does not come back, or only moves on some bands → you are moving a resonance, not removing a current. Section 03 explains why. |
| 4 | Run the radio from a battery, and unplug every charger, wall adapter and network cable in the room. | Noise changes → it is coming in on the mains (③), and the fix belongs on the power lead, not on the coax. |
| 5 | Walk around with a portable shortwave receiver and hold it close to things that might be guilty. | Noise jumps near one device → you have found it, and it is path ④. Noise is the same everywhere → go back to ①, ② or ③. |
You will read everywhere that an EFHW station needs a separate receiving loop. On this station it does not, and the numbers say so. They come from a year of PSK Reporter records analysed for BG1SB (grid ON80da), and they describe this site only.
| What was measured | Result | What it means |
|---|---|---|
| Weakest signal decoded, quietest direction | −23 dB SNR | The FT8 decoder stops around −24 dB, so the station is within a hair of theoretical sensitivity |
| Noise floor by direction | Even, with no direction worse than the rest | No local noise source is swallowing any part of the sky |
| Share of long-distance contacts, receive vs transmit | 29.9% receive against 20.6% transmit | The station hears further than it talks, so receiving is not the weak link |
Each card below follows the same four headings, so you can read the choke verdict straight down the column and see where it differs.
Where it comes from. Arcing across the utility's own hardware. A gap opens between two conductors — usually because fittings are loose, broken or spaced too closely — current crosses it, the air ionises, and broadband energy radiates away.
How to recognise it. On a 50 Hz grid it fires 100 times a second, because every cycle has two voltage peaks and two zero-crossings. On 60 Hz it is 120 Hz. On a scope you see two bands of noise drifting slowly upward, one from each half of the cycle. Two tests need no equipment at all: it goes quiet in the rain, because water bridges the gap, and it varies in the wind, because loose hardware moves.
A common mistake worth correcting. Corona discharge is almost never the cause. The ARRL is explicit: corona sits at low frequencies, it is a low-current effect, and it does not travel far. Blame corona and you will spend the day looking at the wrong thing, on the wrong pole.
Does a choke help? No. The energy runs along the power wiring and reaches you by radiating into the antenna wire — path ①. A coax choke works on the braid's common-mode path, and the noise is not on it.
What to do. Complain, with evidence. This is the one category where somebody else has to do the work, and in many places they are legally obliged to.
Sources: ARRL, Power Line Noise · IEEE Std 1897-2024
Where it comes from. A switching supply chops DC tens or hundreds of thousands of times a second. The harmonics of that chopping, plus the ringing on every edge, run right up through HF. The noise leaves through two routes: differential, between live and neutral, which an X capacitor suppresses; and common mode, between the conductors and earth, which needs a Y capacitor and a common-mode choke. Cheap supplies save money by leaving out those last two parts — the only two that do anything at HF.
How to recognise it. A comb of harmonics spaced at the switching frequency, drifting around as the load changes. On light load you often hear a slow wobble as the supply enters and leaves burst mode.
What "compliant" actually means. Household equipment has to meet the Class B conducted-emission limits. Here they are, with the voltage limit converted to the power it represents across 50 Ω:
| Frequency | Limit | Voltage | Into 50 Ω |
|---|---|---|---|
| 0.15–0.5 MHz | 66 → 56 dBμV | 2.00 → 0.63 mV | −41.0 → −51.0 dBm |
| 0.5–5 MHz | 56 dBμV | 0.63 mV | −51.0 dBm |
| 5–30 MHz | 60 dBμV | 1.00 mV | −47.0 dBm |
| for reference | quiet rural HF noise floor | typical level | ≈ −120 dBm |
| for reference | suburban HF noise floor | typical level | ≈ −100 dBm |
Compare the top three rows with the bottom two. A device that scrapes through with no margin puts about the same level on its mains port as a quiet band puts at your receiver. Even after the house wiring re-radiates that at a small fraction of its conducted level, it can still be far above a quiet band. Passing the standard is not the same as not interfering. The limit is a deal between manufacturers and spectrum managers, not a promise that your HF reception will work.
Does a choke help? Not on the coax. Noise arriving on the mains never touches the coax, so a choke there does nothing at all.
What to do. Move the same component to the right conductor: a common-mode choke on the mains lead or the DC lead. Then check the USB and network cables, which are the ones most often overlooked.
Sources: GB/T 9254.1-2021 (≈ CISPR 32) · standard EMC practice for X and Y capacitors and common-mode chokes
Where it comes from. Slow charging runs the mains through a charger inside the car: a switching converter of 3.3 to 11 kW. Fast charging does the conversion inside the charging post, at higher power and increasingly with silicon-carbide devices that switch even faster. Either way, the charging cable is several metres of multi-conductor cable running from a building to a vehicle — and its protective earth conductor is exactly as long as the antenna it forms. A kilowatt-class switching converter driving a long unshielded conductor, with no RF ground at either end.
How to recognise it. Broadband, strong while a car is charging, gone when the plug comes out. That in-and-out relationship is the useful part: it dates the interference and identifies the source even when you cannot see the charger.
Does a choke help? Partly. Most of it arrives through path ①, radiating from the charging cable into your antenna wire. A coax choke only reaches the small share that arrives on the shield.
What to do. Talk to somebody, and change your own geometry. Establish the charging-times correlation first — that is the evidence that makes a conversation with the owner or the building manager possible. Then move the antenna, or accept those hours as lost.
Sources: GB/T 18487.1-2015 · published EV-charger EMC remediation case reports · the 125 kHz keyless-entry reports
Where it comes from. Most modern string inverters have no transformer, so the panels and the grid share a common-mode path with nothing in between. Add the long DC run down from the roof, which forms a big loop, and switching devices with fast edges, and you have a strong common-mode source bolted to the same building as your antenna.
How to recognise it. It follows the sun and the inverter's output. Strongest around midday, gone at night, and it changes when a cloud passes.
Does a choke help? Partly. The paths are mixed. The part radiated into your antenna is out of reach. The part conducted into the building wiring can be dealt with, but with a choke on the mains, not on the coax.
What to do. Almost all of it is source-side work, which means not yours: LCL filters, common-mode reactors, active filtering and gate-drive tuning all live inside the inverter. What is yours is the same on/off evidence, and the same conversation.
Sources: ARRL RFI programme, which tracks solar alongside power lines and lighting
Where it comes from. Broadband over Power Line is defined as carrying data over power wiring between 2 and 30 MHz. That is not near the amateur HF bands. It is the amateur HF bands, plus everything in between.
How to recognise it. This is the only entry here that is not a by-product. It is a modulated signal transmitted on purpose, inside your band, and it sounds like one.
Does a choke help? No, and it never will. The signal arrives in band, on the antenna, exactly where a wanted signal would arrive. There is no common-mode path to interrupt.
What to do. Frequency coordination, or a complaint. This is a regulatory problem, and treating it as an antenna problem just costs money.
Sources: the BPL band definition (2–30 MHz) · the national radio-interference complaint procedure
Where it comes from. Every LED lamp contains a driver, and that driver is a small switching supply built to the tightest possible budget. The same cost pressure as card 2, at a smaller scale and with less room for filtering. High-power ballasts for grow lights are the industrial version of the same thing; the ARRL lists them as a category it gets a steady stream of questions about.
How to recognise it. It follows a switch. Noise appears when the lamp is on and disappears when it is off, which makes cut 5 decisive and makes this the easiest source on the list to identify.
Does a choke help? Partly. It depends entirely on which path the noise used, which is why the checking comes before the buying.
What to do. Replace the lamp with one that behaves. Cheaper and more reliable than filtering the consequences.
Sources: ARRL RFI programme (lighting devices category)
Eight rows. Only the last one is a job a coax choke can do properly.
| Source | Main path | Does a coax choke help? | Where the fix is |
|---|---|---|---|
| Power-line arcing | ①, by re-radiation | No | Complaint, and antenna position |
| Your own switching supply | ③ | No | A choke on the mains or DC lead |
| Your own PC, USB and network cables | ④ | No | Ferrites on those cables |
| A neighbour's LED or supply | ①, mostly | Partly | At the source, plus antenna position |
| EV charger | ①, mostly | Partly | A conversation, plus antenna position |
| Solar inverter | ① and ③ | Partly | At the source, plus the mains |
| PLC / BPL | ①, in band | No | Frequency coordination, complaint |
| Noise the shield picks up | ② | Yes, completely | A coax choke. This is what it is for. |
Get this part right and everything else follows: where to fit the choke, why one sometimes makes things worse, and how many turns to wind.
One is your signal: a voltage difference between the inner conductor and the inside of the shield. The receiver sees that and nothing else. The other is current on the outside of the shield — your feedline acting as an antenna, picking up whatever is around it.
That second current cannot walk into the receiver on its own. Something has to turn it into a voltage difference first, and the turning happens wherever your system is unbalanced: the 49:1 transformer, the feedpoint, the chassis ground. Your feedline is an antenna wired into a converter you cannot remove.
A common-mode choke does one thing. It puts a high impedance in the path along the outside of the shield, so less current flows there. Your signal runs on the other side of the shield and is unaffected. The choke does not select a frequency, it selects a conductor.
That is also why it does nothing else. It will not improve your match, raise efficiency, or change SWR, because none of those depend on the common-mode path. If your SWR moved after you fitted a choke, something else moved — and it is worth finding out what.
Put an impedance in the common-mode loop and less current flows. To a first approximation:
A ≈ 20 · log₁₀( 1 + Z_cm / Z_loop )
| Choke impedance | Reduction, Z_loop = 50 Ω | Reduction, Z_loop = 200 Ω |
|---|---|---|
| 50 Ω | 6.0 dB | 1.9 dB |
| 100 Ω | 9.5 dB | 3.5 dB |
| 250 Ω | 15.6 dB | 7.0 dB |
| 500 Ω | 20.8 dB | 10.9 dB |
| 1000 Ω | 26.4 dB | 15.6 dB |
| 2000 Ω | 32.3 dB | 20.8 dB |
| 5000 Ω | 40.1 dB | 28.3 dB |
The division sum is a rough picture. It gets three things wrong, and all three matter in the field.
More turns give more impedance — it goes up with the square of the turn count. But the ferrite's permeability also changes with frequency. Measured values for the two mixes you are most likely to use:
| Frequency | Mix 43 μ′ | Mix 43 μ″ | Mix 51 μ′ | Mix 51 μ″ |
|---|---|---|---|---|
| 7 MHz | — | — | 364 | 156 |
| 10 MHz | 293 | 340 | — | — |
| 14 MHz | — | — | 235 | 186 |
| 28.6 MHz | — | — | 129 | 163 |
| 30 MHz | 117 | 202 | — | — |
Windings that are known to work. These are quoted, not invented:
| Covers | Mix | How |
|---|---|---|
| 80–20 m | Mix 31 | FT240-31, 5–9 turns of RG-8X |
| 40–10 m | Mix 43 | FT240-43, 6–10 turns of RG-8X |
| Portable | Snap-on Mix 31 | 3–5 spaced along the coax. Convenient, and clearly worse |
Three of the four paths are out of a coax choke's reach. Each has its own giveaway, and none of the fixes is a ferrite on the feedline.
The noise source shares a supply with your rig. Its rubbish travels down the wiring, through the power supply, and onto the chassis — and the chassis is what the receiver measures against. None of this route involves the coax, which is why cut 1 does not clear it.
An end-fed half wave is fed at the point of highest voltage on the wire. A high-voltage point is naturally sensitive to nearby electric fields, which is how lamp drivers and small switching supplies mostly couple. The shape that makes an EFHW convenient to hang is also the shape that makes it a good listener for everything in the building.
Your rig, your SDR, a USB lead, a network cable: any of these is a small antenna sitting on your desk, listening to whatever is switching next to it. It is the least exotic path and the easiest to overlook, because the culprit is usually something you installed yourself and therefore trust.
The two rules from section 01 are still in force. Everything here is about making them easy to follow.
You do not need a laboratory. If your station runs through the MRRC stack you already have an S-meter feed and a DSP spectrum display, and both are enough to A/B a choke as long as you freeze the receiver settings. The spectrum display is the better of the two, because it shows the shape of the noise as well as its level — and a choke that moves a resonance looks completely different from one that removes a current.
For a number you can compare across days, the PSK Reporter analysis this station uses is open to anyone with a callsign and some patience. It works out a noise floor from which stations you can hear, and the method is written up on the measured antenna profile page. Run it before the change and again after, then compare.
Section 02 gave you a table. This is the same table read as a list of things not to waste a ferrite on.
If your station really is buried in local noise, the standard answer is to split transmit from receive: keep the EFHW for transmitting, add an active magnetic loop for receiving, point its null at the noise, and switch between them with a T/R relay or the rig's own receive antenna socket. It works. The products exist.
Power-line arcing and a neighbour's charger are the two cases you cannot fix at the source yourself. Both need evidence and a channel. In China that channel is the 12381 hotline, opened on 20 March 2023 as the radio-interference complaint centre under the MIIT public-service platform.
Build the log before you call: when it started, which bands, which modes, what it sounds like, and a 30 to 60 second recording in both AM and SSB. This is the same list the ARRL recommends to its own members, and it is what turns "my radio is noisy" into a case.
Every number above carries one of two labels, or a dash.
| Label | Means |
|---|---|
| C | An engineering claim with a source you can check. They are all listed below. |
| 算式 | Arithmetic you can redo. The formula is printed next to the numbers, so you can work out every cell yourself. |
| — | Not measured, and not guessed at either. |