Seven-Band EFHW vs Terminated Loop: A Flat SWR Is Not the Verdict
Seven-Band EFHW vs Terminated Loop: A Flat SWR Is Not the Verdict
One antenna hides gaps between resonant windows; the other openly pays for impedance stability in a resistor. The honest comparison puts both costs on the same ledger.
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.
One antenna hides gaps between resonant windows; the other openly pays for impedance stability in a resistor. The honest comparison puts both costs on the same ledger.
Engineering position: Do not compare a known termination loss with an unknown transformer loss and call the unknown one free. Measure both complete systems, including pattern and return current.
The Coverage Mechanisms Are Not the Same
A seven-band EFHW relies on several wire modes transformed through one high-ratio network. Between those modes, impedance or pattern may be unsuitable. A terminated loop uses resistance to damp standing waves and broaden the input response, accepting intentional dissipation.
The Shunt Capacitor Is Part of the EFHW Cost
A capacitor across declared transformer nodes changes admittance. It may improve an upper-band match, but its ESR, ESL, self-resonance, voltage and RF-current limits join the system. It can neither remove ferrite loss nor make separated resonances continuous.
The Resistor Is Not the Whole Terminated-Antenna Cost
Termination dissipation is measurable, but transformer, wire, ground, feedline and exterior-current losses also matter. Conversely, the resistor may buy an operational advantage through broad impedance behaviour. Efficiency alone does not describe every mission.
Use One Measurement Ledger
| Term | Measure |
|---|---|
| Match | Complex impedance at the same reference plane |
| Dissipation | Termination, transformer and feedline loss separately |
| Stress | Voltage, current and equilibrium temperature at stated duty |
| Radiation | Installed realised gain/pattern in required directions |
| Return current | Exterior-coax and other unintended-conductor current map |
Engineering references used for this cluster
- IEEE Std 145-2025 for antenna, gain, efficiency and pattern terminology.
- Current Fair-Rite complex-permeability/material records for frequency-dependent ferrite behaviour.
- Mini-Circuits and Keysight transformer, impedance and calibrated network-measurement guidance.
- ITU-T K.136 for converted common-mode current and cable-current measurement boundaries.
- Current RF.Guru product pages only for the five owner-identified model names and intended band groupings linked above.
Mini-FAQ
- Is a seven-band EFHW broadband? No. It normally provides separated operating windows rather than continuous compliant coverage.
- Does a shunt capacitor improve efficiency? Not automatically. It changes the network match and must be evaluated for loss and RF stress.
- Is termination loss always unacceptable? No. It can be a deliberate trade for operating bandwidth or impedance stability when quantified.
- How should the two antennas be compared? At the same accepted-power plane, geometry and direction, with transformer, termination, feedline and common-mode losses separated.