Follow the Current Path.
A reactive antenna, a high SWR reading, a noisy waterfall and a struggling repeater can each tempt us toward a quick explanation. This month’s articles ask us to take one more step: identify the current path, the measurement plane and the mechanism before naming the cure.
We begin at the coax shield, follow reflected waves back toward the transmitter, visit two crowded 70 cm sites, and look at what a receiver’s noise display actually includes. Along the way, a reader adds an important correction to the history of W2DU’s laboratory.
Reactance changes the phase between voltage and current. It does not, by itself, make the outgoing and return currents in a pure differential feedline mode unequal. An installed antenna can certainly excite current on the outside of the coax—but that requires a coupling mechanism and another path through the surrounding system.
A low SWR can coexist with substantial outside-shield current; a reactive mismatch can coexist with very little. Match and balance answer different questions.
Read: Does Antenna Reactance Create Common-Mode Current?
“ELI the ICE man” describes the phase of current relative to voltage in an inductor or capacitor. It does not let current enter one terminal of an isolated load and simply fail to leave the other. At RF, the full accounting can include displacement current and coupling to the coax exterior, mast, chassis or environment.
This follow-up works through Kirchhoff’s Current Law, current-direction conventions and what a whole-cable current probe actually measures. It also explains why a common-mode choke acts on the external mode—and why its losses cannot be inferred from reflected power alone.
Read: KCL, Reactance and Common-Mode Current
A 4:1 impedance ratio states a circuit relationship; it does not settle every question about balance, common-mode behaviour or power handling. Core material, winding geometry, insulation, enclosure and operating conditions all affect the finished transformer.
This design guide opens up the engineering behind RF.Guru’s Ruthroff transformers and explains how to choose a rating for the job the device will actually do.
Read: Inside the RF.Guru 4:1 Voltage Balun
Yes. A mismatch produces a measurable reverse-travelling electromagnetic wave. But a directional coupler’s forward and reverse readings are only part of the story. Standing waves, net power flow, line loss and transmitter behaviour need their own accounting.
Read: Does Reflected Power Really Flow Back?
No automatic second bounce is guaranteed. What happens when the reverse wave reaches the source depends on the source-side boundary and the wave’s phase. In a continuously driven system, the forward and reverse waves must be added as complex amplitudes; real loss does not vanish because a story about “recycled watts” sounds reassuring.
These two articles make a useful pair: first establish that the reverse wave is real, then ask what the complete system does with it.
Read: Reflected Power Does Not Automatically Re-Reflect
Two repeaters can meet their individual specifications and still interfere when installed together. A strong nearby transmitter can block a receiver, contribute noise at its receive frequency, drive mixing products or expose a nonlinear connection elsewhere on the site.
Antenna separation and cavities help only to the extent that measured isolation and filtering address the mechanism involved. The article builds a site test around the unwanted level at the victim receiver and the resulting loss of wanted-signal performance.
Read: Repeater Desense at a Shared 70 cm Site
Channel spacing is not a promise of coexistence. Where the coverage of adjacent-channel repeaters overlaps, receiver selectivity, transmitter emissions, site geometry and propagation determine whether the arrangement works. An external cavity cannot provide unlimited rejection just 12.5 kHz from the wanted analogue-FM channel without affecting that channel too.
The practical starting point is coordination backed by measured equipment performance—not a distance rule or a filter specification taken out of context.
Read: 12.5 kHz Apart, 50 km Too Close
A double-stub network can add useful band-selective isolation in a multi-transmitter station. The promised 20–25 dB has to be understood at defined measurement planes, alongside notch bandwidth, interaction with other filters, power handling and safe switching.
Read: How a 20–25 dB Coaxial Stub Adds Isolation at 1.5 kW
A waterfall trace is not a single universal noise floor. Thermal noise, weather, human-made interference, antenna temperature, feedline loss, receiver noise and measurement bandwidth all contribute. Before comparing two antennas or receivers, establish what dominates in the band and location being tested.
Read: Radio Noise Floor Explained: Johnson Noise, Weather & RFI
A broken loop began RF.Guru’s receive-antenna project. The continuing question was whether each revision kept more useful signal intact through the antenna, feedline and receiver. This is the story behind the changes and the long-term A/B testing used to judge them.
Read: Will It Make Any Difference?
Conductivity describes a material property. Antenna efficiency, ground-wave attenuation and received field strength describe complete electromagnetic problems. A map can supply valuable model inputs, but converting its conductivity colours to a logarithmic scale does not by itself calculate “dB left on the table.”
Read: A Soil Conductivity Map Is Not an RF Performance Map
A centre-fed full-wave dipole and a full-wave loop present different feed impedances and patterns. Neither can be judged by wire length alone. The useful questions concern geometry, feed position, current distribution, matching and loss—not whether the radiator happens to be longer than a half wavelength.
Read: Full-Wave Antennas Aren’t Bad
Heaviside gave engineers a much cleaner field notation and moved potentials out of the foreground. He did not remove a verified collection of electromagnetic effects. Radial and longitudinal components in antenna near fields are real; that does not make them a separate long-range “scalar wave.”
This article follows the history and the physics, including why near-field coupling can be strong where a far-field antenna pattern predicts a null.
Read: Did Heaviside Delete Maxwell?
RF history is a chain of practical problems: noise, fading, impedance, polarization and coverage. This account follows RCA’s work and its links to amateur radio, from early wireless systems to spacecraft antennas.
Read: From Marconi to the Moon
After reading my article on W2DU, his son Rick Maxwell, W8KHK, wrote with a specific correction. The account of couplers, bridges and oscilloscopes described Walt’s early work, but left out the vector instruments that entered his laboratory later. Rick’s letter adds those missing years—and improves the record for everyone who reads the technical discussion that follows.
Read: W2DU’s Son Read My Article—Then Corrected the Historical Record
For the next antenna, choke or site comparison, record the operating frequency, configuration, reference plane and measured quantity. If you change a tuner, move a coax run or add a filter, measure the effect on the problem you meant to solve: outside-shield current, accepted power, wanted-signal performance or interference at the receiver.
A good explanation should identify the path and survive a measurement of that path.
Thanks for reading, testing and sending corrections. Rick’s letter is a reminder that engineering history, like engineering itself, gets better when someone brings new evidence to the bench.
Keep building. Keep measuring. Keep following the current path.