The EFHW8010 Is Not a Broadband “Octave Antenna”
The EFHW8010 Is Not a Broadband “Octave Antenna”
An 80-to-10 m EFHW can offer several useful resonant bands. That does not make it continuously broadband, and the resonances do not belong to the wire alone.
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.
My objection to the phrase “broadband octave antenna” is simple. An EFHW 80/10 is a tuned multiband installation: it can present usable impedance windows around several wire modes, separated by frequencies where the match, loss or pattern may be unsuitable. Broadband coverage would require a stated performance criterion to be met continuously across a frequency interval.
Engineering principle: treat the radiator, matching transformer, compensation, counterpoise, coax exterior, choke and surroundings as one resonant system. Measure each desired band; do not infer continuous coverage or exact harmonic alignment from the lowest-frequency dip.
Multiband and Broadband Answer Different Questions
A multiband antenna meets its requirements in two or more separated frequency ranges. A broadband antenna meets declared requirements—such as impedance, realized gain, efficiency or pattern—throughout one continuous range. “It tunes on 80, 40, 20, 15 and 10 m” describes multiple windows. It says nothing about the frequencies between them.
The requirement also has to be named. A low SWR window does not prove transformer efficiency or a useful radiation pattern. The current IEEE Std 145-2025 keeps impedance, efficiency, directivity, gain and realized gain as distinct antenna quantities. I want to know which of those stays inside its limit, and across what interval.
End Effect Moves the First Resonance Down, Not Up
A thin, isolated mathematical wire exactly 0.5λ long is a useful starting model, not a finished antenna. The electric field does not stop abruptly at a physical tip. The terminal capacitance makes the wire electrically longer than its metal length, so a practical self-resonant half-wave is physically shorter than 0.5λ.
NBS Technical Note 1099 gives the sign explicitly: a cylindrical dipole must be made slightly shorter than λ/2 for zero reactance, with the required shortening dependent on length-to-diameter ratio. Therefore, for a fixed physical wire, the first self-resonance is lower than the naïve free-space value c/(2L). Insulation, conductor diameter, bends, nearby earth, supports and end hardware can move it again.
For fixed physical length L: ffirst, installed is not simply c/(2L)
End effect is not a universal instruction to shift every higher resonance upward. Each installed mode sees its own current distribution, terminal phase, feed discontinuity and environment. The ratios must be calculated for the installed geometry or measured.
Higher Modes Are Not an Exact Octave Ladder
A half-wave wire can support higher-order standing-current modes. In an idealized uniform wire those modes suggest integer multiples of a fundamental, which is why 80, 40, 20 and 10 m look attractive on paper. But the complete antenna has only two physical ends, one asymmetric feed region and mode-dependent current maxima and minima. End correction, wire diameter, insulation, bends, ground coupling and feed-system loading do not scale as one perfect constant.
fn ≈ n f1 is a starting estimate, not an installed guarantee
The resonance ratio can fall above or below the exact integer target. A transformer or compensation network can move the observed input-impedance minimum without moving every current mode by the same amount. Even “resonance” needs a reference: zero reactance at the wire terminal, minimum SWR at the coax input and maximum system current need not occur at one frequency.
This is why I call the antenna multiband. The separated modes can be genuinely useful, but every band has to survive the installed-system test. Thirty, 17 and 12 metres can lie near the third, fifth and seventh modal frequencies of a chosen 80 m resonance, but their allocations do not generally coincide with exact integer multiples. Any useful match there belongs to the complete installed system, not an octave rule.
The Transformer Is a Frequency-Dependent Network
The end of a half-wave mode is a high-impedance feed region, but its complex impedance is not one universal number. It changes with mode, height, wire shape, surroundings and return path. A “49:1” label is only a nominal impedance ratio derived from an ideal turns ratio. It neither fixes the antenna impedance nor guarantees a 50 Ω input.
A real ferrite transformer contains magnetizing inductance, leakage inductance, winding resistance, inter-winding capacitance, self-capacitance and core loss. Keysight’s transformer-measurement guidance models those parameters separately and shows why inductance and fixture capacitance create resonances of their own.
At the low end, insufficient magnetizing impedance shunts the transformed load and increases current and loss. Adding turns raises inductance approximately with N², but it also changes winding length, leakage and capacitance. At the high end, those parasitics and the winding’s transmission-line behaviour can dominate. “More primary inductance” is therefore not a free wideband improvement.
An 80–10 m label spans more than three octaves; 40–10 m still spans more than two. Common #43 and #52 material choices trade lower-edge magnetising impedance against upper-edge material and winding behaviour. Neither mix name certifies the completed transformer. The core dimensions, turns, winding, complex loads, power, duty cycle and temperature decide which edge becomes marginal.
Ferrite spread must be taken from the chosen part’s current datasheet. It is not a universal 20–30% law for every material and core. As one concrete example, the current Fair-Rite 5943001201 data specifies AL as 950 nH ±20%; other parts use different values and tolerances. Finished-transformer spread also includes turns placement, lead length, stacking, enclosure capacitance and assembly. Measure the unit instead of adding several unrelated tolerances into one folklore percentage.
Compensation Is Part of the Design, Not Free Bandwidth
A capacitor across the transformer primary changes the transformer’s high-frequency impedance. ARRL’s EFHW kit instructions, for example, offer a primary capacitor to compensate secondary capacitance on the upper bands. That is one specified network for one construction. The required value and result depend on leakage, winding capacitance, load and fixture; a copied capacitor can improve one build and over-correct another.
A series compensation or loading coil in the antenna wire does something different. Its reactance and its position in each mode’s current distribution change the electrical phase and current on the sections either side. It may move selected upper-band modes, isolate an extension on higher bands or lengthen the low-band path. The result is mode- and placement-dependent, not a generic “upper resonances always move down” rule.
ARRL’s 80 m EFHW extension instructions make the practical interaction visible: adding the coil and extension can shift upper-band resonances, and the complete antenna must be rechecked after installation at operating height. Once a capacitor or coil is present, include it in the schematic, model, loss budget and thermal test.
The Return Path Is One of the Conductors
An end-fed antenna is not a one-terminal circuit. Current leaving the transformer’s antenna terminal returns through the other terminal by some combination of an intentional counterpoise, capacitance to the environment, coax-shield exterior, station bonds and connected wiring. ARRL’s generic EFHW construction includes an explicit counterpoise connection; installations that omit a separate wire still have a distributed return path.
If the coax exterior carries current, changing feedline length, routing, grounding or operator position can move the input impedance and pattern. ITU-T K.136 defines converted common-mode current as current produced by cable or network unbalance and requires unwanted current on a coaxial exterior to be controlled in RF measurements.
A common-mode choke does not make return current disappear. It moves the boundary and encourages current to use the intended path on the antenna side. Choke position and impedance therefore become tuning variables. Declare the counterpoise and choke geometry before comparing wire lengths or transformer samples.
Ten Metres Changes the Pattern as Well as the Match
A wire near one half-wave on 80 m is several wavelengths long on 10 m. Its upper-mode current has multiple maxima and reversals, so the far field develops more lobes and nulls. Height, slope, bends, orientation, ground and common-mode feedline current decide where they point.
An acceptable 10 m SWR therefore does not make the installed antenna a general-purpose broadside radiator. NTIA’s HF antenna-selection report makes the general long-wire boundary explicit: the number of pattern lobes increases with electrical length and the pattern changes with frequency. Model and verify the azimuth and elevation pattern for the paths that matter.
A Measurement Sequence That Separates the Variables
- Define the bands and criteria. State the allowed SWR or complex impedance, transformer loss, temperature rise, common-mode current and desired pattern for each band.
- Characterize the transformer. Measure complex input and transfer behaviour with calibrated fixtures and representative high-impedance complex loads; record core part, turns, winding geometry, enclosure and compensation.
- Fix the return path. Install the intended counterpoise and common-mode choke before trimming the wire. Record coax length, route and bonds.
- Measure the final geometry. Sweep complex S11 at a declared reference plane with the antenna at operating height. A handheld analyser at the transformer and one in the shack are different measurements unless cable effects are removed.
- Map exterior current. Use a characterised clamp current probe around the complete coax at several positions and on every operating band.
- Add compensation deliberately. Change one part at a time, then remeasure every band, transformer loss, current distribution and temperature.
- Verify the pattern. Use a model that includes the real wire shape and return conductors, then check important azimuths with repeated field or reciprocal receive measurements.
Keysight’s network-analyser calibration note explains why known standards and declared reference planes are essential when fixture and adapter effects could be mistaken for device performance.
My Practical Name for the Antenna
I call an EFHW8010 a tuned multiband end-fed system. That name gives the antenna full credit for what it can do without pretending that several resonant islands form one continuous octave-spanning passband.
The wire supplies useful modes. The transformer makes their high terminal impedances more manageable. Compensation can align selected windows. The return path completes the circuit. The installed geometry supplies the pattern. Leave any one of those out, and the word “octave” hides more than it explains.
Mini-FAQ
- Is an EFHW 80/10 a broadband antenna? No. It is a tuned multiband system with separated operating windows. Broadband would require stated impedance, loss and pattern criteria to be met continuously across one frequency range.
- Which way does capacitive end effect move the first resonance? It makes the physical wire required for resonance shorter than 0.5λ. For a fixed physical wire, the first resonance is therefore lower than the naïve c/(2L) estimate.
- Are the higher modes exact multiples of the first resonance? Not in an installed antenna. Mode-dependent end phase, wire construction, environment, feed network and return path can move each resonance, so every band must be measured.
- Does a nominal 49:1 transformer create the wire resonances? No. It transforms the terminal impedance imperfectly and adds its own magnetizing, leakage, capacitance and loss behaviour. Its input match is part of the complete system response.
- Can one compensation value be copied to every EFHW? No. Primary capacitance and wire inductance interact with the particular transformer, load, modal current and installation. Change one variable and remeasure all bands, loss and current.