Terminated Loops, EFHWs and BBTDs: Follow the Watts
Terminated Loops, EFHWs and BBTDs: Follow the Watts
A flat SWR curve is easy to admire and easy to misunderstand. The first engineering question is where the accepted watts go.
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 flat SWR curve is easy to admire and easy to misunderstand. The first engineering question is where the accepted watts go.
Engineering position: Termination can buy stable impedance by deliberately dissipating power. An EFHW can buy resonant windows with a high-ratio network. Neither architecture is judged honestly until loss and pattern share the same reference plane.
Three Ways to Make a Transmitter Happy
A terminated loop or BBTD damps reflections with intentional resistance. An EFHW uses wire resonances and an impedance transformer. A tuner can transform any of these at the station. All can produce a manageable input impedance; none of those facts alone establishes useful radiation.
Intentional Loss Is Not Hidden Loss
The termination resistor's dissipation is part of the design and can be measured. That may be an acceptable price for wide operating range, reduced retuning or pattern stability. Transformer, feedline, conductor, ground and common-mode losses still remain separate terms.
The EFHW Trade Is Different
A multiband EFHW offers separated modal windows. The high-ratio transformer must cover their actual complex loads, and an 80–10 m request spans more than three octaves. A shunt capacitor can tune a region, but it cannot prove low loss or continuous coverage.
Compare the Use Case, Not the Brochure
For contesting or emergency operation, immediate band access may justify intentional loss. For a fixed weak-signal path, realised gain in that direction may dominate. State the objective, measure accepted power and component temperature, and verify the installed pattern.
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
- Does a terminated antenna waste power? It deliberately dissipates part of the accepted power; the amount and its trade against operating bandwidth must be measured.
- Does an EFHW avoid all deliberate loss? It avoids a termination resistor, but transformer, feedline, conductor, ground and common-mode losses can remain.
- Which has the best SWR? That is not a sufficient comparison; a good match can coexist with substantial dissipation.
- What is the fair performance metric? Complete-system realised gain or field strength in the required direction at the same accepted-power reference plane.