A Quiet Receiver Is Not a Current Probe
A Quiet Receiver Is Not a Current Probe
A terminated coax cable, a loose connector shell and disappearing broadcast signals: a useful club experiment opens a much bigger question about shielding and common-mode current.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
A club email forwarded to me described a striking receive test. Disconnect the antenna, terminate the far end of a 35 m coax cable in about 50 Ω, and connect only the centre pin at the receiver. Medium-wave stations appear. Complete the connector’s shield connection, and they disappear into the displayed noise.
There is useful engineering in that observation. I would keep the experiment and build on it. Where I part company is the conclusion that the cable is therefore free of common-mode current. A quiet receiver input and a current-free cable exterior are not the same measurement.
The email explicitly leaves the final RG400 jumper and dipole outside its conclusion. That is a fair boundary, and I am keeping it. The question here is whether the narrower claim about the terminated 35 m cable follows from the observations.
First, Keep What the Experiment Gets Right
Replace the antenna with a known load. This is a useful way to separate cable-and-receiver behaviour from the wanted antenna signal. A properly screened 50 Ω termination gives the intended coaxial signal path a defined load. It also makes an unexpected broadcast signal easier to investigate.
For a receive-only check at roughly 1.5 MHz, a short-leaded, suitably low-inductance 47 Ω resistor can be a reasonable approximation. It is not automatically an RF load at every frequency, and its construction still matters.
Deeper engineering: for an ideal 47 Ω resistance on a real 50 Ω line, |Γ| = |(47 − 50)/(47 + 50)| ≈ 0.031, corresponding to an SWR of about 1.064. That small differential mismatch is not the central weakness of this experiment. A suitable connector-mounted load avoids adding an exposed pickup structure.
Take the connector seriously. Reconnecting the shell makes a large observable difference. The shield connection is part of the RF circuit, not merely something that stops the plug falling out. Good cable, sound connectors and a continuous enclosure connection deserve their place in a receiving installation.
Control the receiver. Keeping gain and processing settings consistent is sensible. Turning off noise blanking and noise reduction can make comparison easier; avoiding unnecessary preamplification helps preserve headroom. But “all filters off” is not a recipe for linearity: suitable filtering or attenuation may be needed to prevent overload. Keep the measurement bandwidth fixed and record it.
Build on That: One Cable Has Two Different Jobs
Inside the coax, the wanted signal travels between the centre conductor and the shield’s inner surface. Outside, the shield is also a conductor in the station’s wider electromagnetic environment. It can carry current involving equipment cases, other cables, nearby structures and distributed coupling.
The 50 Ω resistor closes the intended inside route. It does not automatically terminate the outside route. That is the missing distinction. A well-shielded cable can keep exterior disturbances out of the wanted signal path while still carrying appreciable exterior current.
Deeper engineering: at one defined cross-section, use the same positive direction for the currents in the centre conductor and the complete shield. Then ICM = Icentre + Ishield, with complex phasor currents. The intended differential currents cancel in that sum. At RF, the inner/outer-surface picture helps distinguish the coaxial mode from the external mode; these are not three physically separate wires.
The exterior mode’s impedance depends on geometry and its return environment. It is not fixed at 50 Ω simply because “50 Ω” is printed on the coax.
Shielding and common-mode control therefore solve related but different problems. Shielding limits coupling into the enclosed signal path. A suitable common-mode choke adds impedance to an unwanted exterior-current path. One is not a substitute for the other.
What Changes When the Receiver Shell Is Disconnected?
With only the centre pin connected, this is no longer the normal two-terminal coaxial connection to the receiver. Its reference connection has been changed. The open connector region can pick up fields, and current or voltage on the cable exterior can couple into the receiver differently.
Completing the connection restores shielding and changes the return network and the conversion of exterior disturbances into a centre-to-shield signal. Several mechanisms may contribute. The experiment does not isolate where each unwanted signal entered, so I would not claim that every signal came solely through the exposed pin—or solely through the braid.
Deeper engineering: a receiver detects differential voltage at its input; it does not directly report the cable’s common-mode current. In a simple small-signal description, one contribution is Verror = Zconversion ICM. Reconnecting the shell can change both quantities, as well as other pickup paths. A smaller Verror cannot tell us which factor changed. The same distinction between shield current and its conversion into circuit interference is illustrated by Jim Brown’s explanation of the pin-1 problem; that is a mechanism example, not a diagnosis of this particular receiver.
Where the Interpretation Goes Further Than the Evidence
Disappearing Signals Do Not Establish Zero Exterior Current
The defensible conclusion is positive and useful: with its shield connection completed, the terminated cable did not produce a distinguishable response from those broadcast signals above the displayed floor in that setup. The connector correction worked at the observed receiver port.
It does not follow that the cable exterior carried no RF current. Current could remain while coupling to the receiver became much smaller. Nor does the test establish behaviour across all HF bands, after reconnecting the antenna, or during transmission. Those are different excitations and, potentially, different boundary conditions.
This is not a reason to fit chokes by superstition. It is a reason to reserve “no common-mode current” for a current measurement with a stated sensitivity, frequency and position.
The Reported 47 dB and 24 dB Are Not Interchangeable
The email reports a feature near 1.467 MHz at about −73 dBm before the shell connection, disappearing into a panadapter floor around −120 dBm afterwards. It also reports S7 falling to S3, with a 3 kHz receive bandwidth.
The arithmetic −73 − (−120) = 47 dB is straightforward. The interpretation needs care: when the carrier disappears into noise, −120 dBm is not an independently resolved measurement of that remaining carrier. It describes the displayed floor. With consistent settings and a defined detection threshold, the observation can place a bound on the suppression; it does not establish an exact 47 dB insertion loss.
Elecraft specifies S9 at −73 dBm and 6 dB per S-unit for the K4, with preamp/attenuator compensation. Four S-units therefore represent 24 dB on that scale. This is not a reason to dismiss the meter as uncalibrated.
A spectral feature and an S-meter indication need not describe the same signal-and-noise quantity. The K4 documentation distinguishes spectrum-bin data from receiver metering. The stated 3 kHz receive filter does not establish the panadapter’s resolution bandwidth. Without the relevant settings and detection definitions, I cannot assign the difference to one specific DSP mechanism.
Deeper engineering: for approximately flat noise density, measured noise power scales with equivalent noise bandwidth: ΔPnoise = 10 log10(B2/B1) dB. A narrow carrier and integrated noise behave differently as bandwidth changes. Rohde & Schwarz explains this resolution-bandwidth effect. Neither displayed change is, by itself, attenuation of common-mode current.
A Small Laboratory Does Not Prevent Good HF Shielding
The email rightly asks what a manufacturer’s shielding number actually covers. A value specified at 1 GHz is not an all-frequency guarantee. But the proposed explanation—that a laboratory needs half a wavelength of room length, so comparable shielding cannot be achieved at HF—does not follow.
Cable-screen measurements need not use a free-space antenna range. Standardised triaxial fixtures surround a cable sample with an outer conductor and establish a controlled coupling circuit. Their electrical length and terminations matter; the room is not required to be half a free-space wavelength long.
At lower frequencies, surface transfer impedance is particularly useful. It describes how exterior shield current couples voltage into the protected circuit, normalised to cable length. Screening attenuation is a different, dB-valued quantity determined under its specified test conditions. Neither is a universal measurement of installed common-mode current.
Deeper engineering: cable transfer impedance ZT(f) has units Ω/m. For a short, approximately uniform test length l under the specified conditions, the coupling relation is Vcoupled ≈ ZT(f) · l · Ishield. For electrically long or non-uniform installations, currents, phase and propagation must be accounted for along the length; multiplying a single sample value by 35 m is not automatically valid.
IEC 62153-4-3 addresses triaxial surface transfer impedance; IEC 62153-4-4 addresses triaxial screening attenuation. Their different operating conditions are the reason to choose the appropriate metric—not a reason to assume HF shielding must be poor. Through-line attenuation in dB/100 m is yet another quantity: it describes loss along the intended signal path, not screening against external coupling.
Cable Quality Matters; a Universal League Table Does Not Follow
The Ecoflex 10 manufacturer’s specification lists shielding attenuation of at least 90 dB at 1 GHz. That supports the email’s caution about the stated frequency. It does not establish the shielding of the complete 35 m installation at 1.467 MHz.
Foil coverage, braid construction, conductor material, connector termination and mechanical condition all matter. A short list ranking RG58, RG8, RG213, RG400 and branded cables from universally worst to best leaves out exact constructions, frequency curves, methods and assembled connectors. Buy against the specified job, not just the thickness of the jacket or the number of braid wires.
The email’s replacement example also deserves fair treatment. Messi & Paoloni identifies Ultraflex 10 Competition as the replacement for Hyperflex 10. The documented Hyperflex construction has 192 braid wires; the Ultraflex sheet lists 144—indeed 25% fewer. Both declare screening attenuation above 105 dB over 100–2000 MHz. But the braid material, coverage and other construction details differ. Equal stated screening minima do not mean identical complete specifications, and that frequency range is not an HF screening curve.
Turn the Useful Check Into a Better Diagnosis
Receive-only investigation: prevent accidental transmission before changing antenna connections. Do not transmit into a partly connected plug or a small receive-test resistor. Keep protective earth, required bonding and lightning precautions intact; never lift safety earth to chase RF noise. Stop and have unexpected connector or chassis voltages investigated before handling connections; switching equipment off is not proof that an external cable is safe. Do not work on outdoor antennas during thunderstorms.
I would keep the original idea, but ask a separate question at each step:
| Comparison | What it helps answer | What it does not establish |
|---|---|---|
| A screened load directly at the receiver | The receiver-and-local-connection baseline with fixed settings | The behaviour of the outdoor cable |
| The same load at the far end, all coax connectors fully seated | Whether adding the cable introduces a distinguishable receiver response | Zero exterior current when no difference is visible |
| A calibrated RF current probe around the complete intact coax | Net current at that cross-section and frequency | The same current everywhere along the cable or the cable’s screening attenuation |
| A suitable choke added with the intended antenna connected | Whether changing the exterior-current path improves the installed receive result | An improvement merely because both signal and noise got quieter |
Repeat comparisons in A–B–B–A order when practical, keep geometry and settings steady, and follow a stable signal as well as the noise. For a real reception improvement, examine signal-to-noise ratio, not the quietness of the loudspeaker alone. If current is the concern, measure current as well.
Deeper engineering: a calibrated current probe has its own transfer impedance, Zprobe(f) = Vout/ICM, in Ω—not the cable’s Ω/m quantity. With the specified probe termination, ICM = Vout/Zprobe(f). Enclose the entire coax so the intended differential currents cancel. Check background pickup, probe loading and calibration, and measure more than one position: a current minimum at one point is not a current-free cable. Com-Power’s RF current-probe application note explains the probe’s voltage-to-current conversion.
The Useful Conclusion Is Stronger When It Is Precise
The club experiment makes a valuable point: a proper shield connection can make a dramatic difference to what a receiver detects. Keep that lesson. Keep the known termination, controlled comparison and explicit separation from the antenna itself.
Then add the missing foundation: shielding quality, conversion into the receiver and current on the cable exterior are three related questions, not three names for the same thing. Good shielding protects the inside path. Sound bonding and suitable mode control manage the outside path. Measuring each in the right way tells us which improvement the station actually needs.
The vanished signal is a useful result. The vanished common-mode current still needs to be demonstrated.
Sources and Further Engineering
The starting point is a club email forwarded to me about a terminated-coax receiving experiment. The linked Elecraft documentation, cable manufacturers’ specifications and IEC measurement-method descriptions let us connect that practical observation to the quantities each test actually measures.
Mini-FAQ
- Does a quiet terminated coax prove there is no common-mode current? No. It shows no distinguishable response from those broadcast signals above the displayed floor under those test conditions. Exterior current may still exist.
- Does a 50 Ω load terminate the outside of the coax? No. It terminates the intended centre-to-shield differential path. The exterior common-mode path has a different, installation-dependent impedance.
- Is the centre-pin-only comparison useless? No. It can reveal sensitivity to the shield connection, but it changes the circuit and does not directly measure exterior current or cable screening attenuation.
- Do cable shielding measurements require a half-wavelength-sized room? No. Standardised triaxial fixtures use a controlled coupling circuit. The appropriate metric and fixture limits depend on frequency and electrical length.
- What should be measured if exterior current is the question? Use a suitable calibrated RF current probe around the complete coax, with its specified termination, frequency response and sensitivity, and check more than one position.