EFOC and T2LT: Why They Look Similar but Are Not the Same Antenna
EFOC and T2LT: Why They Look Similar but Are Not the Same Antenna
A wire, a length of coax and a choke can make two antennas look closely related. To understand the EFOC and the T2LT, follow where the feedline delivers power, which surfaces radiate, and how the current distribution changes with frequency.
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 reader asked whether the EFOC could be described as a multiband version of the T2LT. I understand the comparison: both can use a deliberately bounded section of coax exterior as part of the antenna. But I would not describe the EFOC that way. The difference includes the feed arrangement, the unequal radiating branches, the impedance transformation and the modes used on different bands. Counting bands misses those differences.
The resemblance has a physical basis
In a basic T2LT, the upper radiating section is an exposed center conductor or an attached wire. The exterior of the coax shield below that transition forms the other radiating section. A choke farther down the coax is intended to limit how much exterior surface participates.
In the coax-return EFOC arrangement, the long wire is one radiating branch. A defined section of shield exterior between the feed unit and the choke is the shorter branch. Looking at the installation from a distance, it is easy to see “wire above, coax below, choke at the bottom” in both.
That shared use of a conductor surface is real. It is also incomplete as a description of either antenna. The positions of the feed transition and choke, the electrical lengths of both branches, and the network between the feedline and those branches determine the antenna that you actually have.
Separate the current inside the coax from the current outside
The coax supports the wanted differential transmission-line mode between its center conductor and the shield's inner surface. With a common direction convention, those currents are approximately equal and opposite:
Icenter + Ishield,inner ≈ 0
Inet,whole cable ≈ Ishield,outer
The second expression describes the additional exterior mode in the usual well-shielded coax approximation. A current probe encircling the complete cable responds to this net current; the internal differential pair largely cancels. The shield is one continuous conductor, but at RF its inner and outer surface-current systems can behave differently.
This is why a length of coax can feed an antenna internally while its outside surface is part of that same antenna. It is also why “the coax is the counterpoise” needs a defined length and boundary. The return current on the shield's inside surface is not automatically the radiating current on its outside.
Both designs exploit this distinction. It explains their resemblance without making their dimensions, feed impedances or operating modes interchangeable.
The basic T2LT feeds a half-wave-like radiator near its middle
In the conventional simple T2LT at its design frequency, the upper section and the shield-exterior section are each approximately a quarter wavelength electrically. Together they form a roughly half-wave radiating structure. The differential feedline carries power through the lower section to the transition between those two radiating sections.
The coax entering from the bottom can make the antenna look end-fed mechanically. Electrically, the intended excitation is near the central, high-current region of that half-wave-like mode. There is no requirement for a 4:1 impedance transformer in the basic construction.
The two physical lengths need not be identical. The upper wire and lower coax have different diameters and dielectric surroundings, and the choke is a finite impedance. A useful fundamental-mode match is an installed-system result; calling the antenna a T2LT does not guarantee 50 Ω.
This construction is explained in more detail in my T2LT article. John Bishop, VK2ZOI's half-wave Flower Pot construction is also a useful primary example of an exposed upper conductor, a coaxial lower section and a choke boundary. The related names cover variants; I am comparing the basic current arrangement here, not declaring every Flower Pot, sleeve dipole and T2LT identical.
The EFOC deliberately uses an asymmetric feed arrangement
The EFOC arrangement in this comparison has a long main wire and a substantially shorter return branch. A 4:1 UNUN interfaces that asymmetric antenna load to the coaxial feed system. In the coax-return version, the outside of a selected coax section supplies the short branch.
The feed unit is therefore at the end of the visible long wire, but it is not at the end of the complete radiating current system. The other branch continues along the coax exterior to its intended choke boundary. This is the useful meaning of the off-center part of the description.
A concrete example is the EFOC17. Its coax-return documentation specifies a 16.8 m main-wire starting dimension and a first return/choke boundary approximately 3.7 m from the feed unit. Those visibly unequal branches are a different design choice from the two near-quarter-wave sections of a basic T2LT. They are installation starting dimensions, not a universal formula for every EFOC or a prediction of the final electrical lengths.
Neither this asymmetry nor the transformer creates a new law of radiation. Both antennas still belong within the broader physics of fed conductor systems with dipole-like modes. The distinction is the particular geometry, excitation and impedance-management strategy. Saying that the EFOC is simply a T2LT with more bands hides precisely those choices.
A 4:1 UNUN transforms the load; it does not create the bands
For an ideal transformer with an impedance ratio of four from antenna side to coax side:
Zcoax(f) = Zantenna(f) / 4
If Zantenna = 200 + j80 Ω, then Zcoax = 50 + j20 Ω.
The example shows both the benefit and the limitation. The resistance has moved to 50 Ω, but the reactance remains. A transformer scales the complex impedance; it does not make an arbitrary wire resonant, erase reactance, or manufacture additional radiating modes. The example is ideal arithmetic, not a measured EFOC load.
Real transformers also have frequency-dependent magnetizing impedance, leakage, capacitance and loss. The selected ratio has to suit the load presented by the complete antenna on the intended bands. A broadband transformer means that the network can be useful over a frequency range; it does not imply a broadband 50 Ω antenna load.
Adding a 4:1 transformer to a working T2LT without changing its radiating system can therefore make the match worse. If the antenna already presents a load near 50 Ω, an ideal 4:1 impedance step-down would present approximately 12.5 Ω. That would be 4:1 VSWR on a 50 Ω line for a purely resistive load. A ratio is a design response to an impedance, not an upgrade by itself.
Off-center excitation changes which modes are accessible
The relationship between feed position and multiband operation is easier to understand using an idealized straight, thin wire of total length L. A simplified current shape for its nth standing-wave mode is:
In(s) ≈ In,max sin(nπs/L), 0 ≤ s ≤ L
At the center, s = L/2: In(L/2) ≈ In,max sin(nπ/2).
The center is a current maximum for the fundamental mode in this model. For the even-order modes, it is a current node. Feeding near a current node is associated with a high driving-point impedance and a difficult direct match. Moving the feed away from the center changes its coupling to the modes and can give more useful impedances on several of them.
The relationship between feed position and harmonic current maxima is also illustrated in W8JI’s modeled OCF examples. This is a reason for choosing an off-center arrangement. It does not mean every off-center position works on every harmonic. A new feed position can still lie near a node of another mode, and real antennas have shifted resonances, unequal conductor diameters, bends, ground coupling and finite choke impedance.
The equation is an explanatory wire-mode approximation, not an EFOC simulation. You cannot insert the physical coax length into it and obtain a certified band plan. The actual multiband design combines conductor geometry, modal feed impedances, the transformation network and a usable exterior-current boundary on each intended band.
The current EFOC17 documentation identifies 40, 20, 15 and 10 m as primary bands, with 30, 17 and 12 m potentially requiring a tuner depending on deployment. That is a defined product application; it does not imply continuous matching between those bands or identical current distributions on them.
For the coax-exterior branch, the cable manufacturer’s velocity factor describes the internal transmission-line mode; it is not automatically the propagation factor for exterior current. The jacket, air, ground and nearby structures load that exterior path. Similar-looking lengths of coax do not establish equal electrical radiator lengths.
The choke has a related job in both antennas
In the basic T2LT, the choke helps establish the end of the lower shield-exterior radiating section at the design frequency. In the coax-return EFOC, it helps establish the end of the intended short exterior branch across the operating bands. In both cases, moving the choke changes the radiating system.
The choke presents a common-mode impedance ZCM(f) = RCM(f) + jXCM(f). It has to be assessed in the complete exterior-current path. It is not an infinite open circuit, and its impedance at one frequency does not establish adequate isolation on another band.
A resonant air-wound coax choke can be useful over a selected range. A suitable ferrite implementation can offer a different useful impedance range. Neither “air wound” nor “ferrite” proves the installed boundary, and copying a T2LT coil into an HF multiband installation does not transfer its performance.
Transformation and choking are separate jobs. The EFOC's 4:1 UNUN addresses the differential load; a separately specified 1:1 current choke addresses exterior current. One function cannot be inferred from the other. A low SWR at the radio does not establish that current has stopped at the intended boundary.
The two-wire EFOC version makes the distinction especially clear
The EFOC17 also has a documented two-wire off-center-fed installation. In that arrangement, a dedicated shorter wire provides the second radiator leg. The hard 1:1 current choke belongs directly at the coax side of the 4:1 feed unit, so the downstream cable is suppressed as an unintended additional branch.
The reason is physical: the second wire now supplies the branch that the coax exterior supplied in the other arrangement. Leaving the choke farther down the cable would allow a different current system, involving both the intended wires and additional shield exterior.
This is why I insist on using the manual for the chosen configuration. A choke position suitable for the coax-return version cannot simply be copied to the two-wire version. See the EFOC17 two-wire OCF guide for that arrangement. The two versions share a design platform but require different control of the feedline exterior.
Single-band versus multiband is not the defining distinction
The simple T2LT is commonly built around one fundamental operating band. That does not make single-band operation a law of the entire coaxial-dipole family. VK2ZOI documents a dual-band Flower Pot modification using an added sleeve to control operation near the third harmonic.
That specific modification demonstrates why band count alone is insufficient. It does not turn a Flower Pot into an EFOC, and it does not establish that an unmodified T2LT will work properly on arbitrary other bands. Higher-frequency operation changes the current distribution, choke behavior, feed impedance and radiation pattern.
The same care applies to EFOC multiband use. A match on several bands does not imply that the same useful lobe points in the same direction on all of them. As electrical length increases, current reversals and additional pattern lobes become possible. Matching, efficiency and useful directional gain remain different quantities.
Compare the actual structures
| Question | Basic T2LT at its fundamental | Coax-return EFOC discussed here |
|---|---|---|
| Where is power delivered? | At the transition between two roughly quarter-wave radiating sections | Through a 4:1 feed unit between a long main wire and a shorter return branch |
| What radiates below the transition? | The defined shield-exterior section | The defined shield-exterior return branch |
| What sets the intended exterior boundary? | A choke qualified for the selected operating range | A choke boundary qualified on the intended bands |
| What establishes useful matching? | Both radiator sections, their surroundings and the choke | Asymmetric branch geometry, modal impedance, transformer and choke; tuner where required |
| Does the outline establish the pattern? | No; the complete installed current distribution does | No; the complete installed current distribution does |
What would demonstrate electrical equivalence?
If two installations are claimed to be the same antenna electrically, I want the comparison made at the same frequency with their complete conductor geometry and feed networks included. Record complex input impedance at a declared reference plane, normalize to the same accepted power, and compare current amplitude and phase along the radiating conductors and the continuing feedline.
An amplitude-only clamp-probe survey is useful for finding unintended exterior participation. It does not, by itself, recover the phase relationships that determine the full radiation pattern. A validated electromagnetic model or suitable calibrated measurements are needed for that stronger comparison.
Two low-SWR traces do not establish equivalence. Neither does a shared choke position in a photograph. If the branch lengths, excitation or phase distribution differ, the antennas can have different patterns and matching behavior despite their similar appearance.
My answer to the original question
I would not call the EFOC a multiband T2LT. I would say that both can deliberately use part of the coax exterior as a radiating conductor, which explains the resemblance. The basic T2LT uses a near-central feed transition in a half-wave-like structure. The EFOC discussed here uses a transformer-fed, asymmetric arrangement with a defined return branch and a multiband impedance strategy.
That description gives the similarity its proper place while preserving the engineering differences. It also leads to the right installation decisions: choose the intended branches first, use the matching network appropriate to their load, and place the choke according to the current system you are building.
Mini-FAQ
- Is the EFOC a multiband T2LT? That description is incomplete. Both can use a defined coax-exterior radiating branch, but the EFOC discussed here uses unequal branches and a 4:1 feed unit with a multiband impedance strategy. The basic T2LT feeds a half-wave-like structure near its middle.
- Why do they look similar? In both coax-based arrangements, a visible upper wire and a shield-exterior section participate in radiation, with a choke intended to limit the exterior current farther down the cable.
- Is the T2LT electrically fed at its bottom? The feedline approaches from the bottom, but its internal differential mode delivers power to the transition between the upper conductor and lower shield-exterior radiator.
- Does the 4:1 UNUN make the EFOC multiband? The UNUN transforms the complex load. Useful multiband operation also requires appropriate branch geometry, modal impedances and choke behavior. A transformer does not create additional radiating modes or remove arbitrary reactance.
- Must every T2LT-related coaxial antenna be single-band? No. The basic design is commonly optimized for one band, but deliberately modified relatives exist, including VK2ZOI’s dual-band Flower Pot. This does not establish arbitrary multiband operation for an unmodified T2LT.
- Can I copy the choke position between EFOC versions? No. The coax-return version uses a defined exterior coax section as a branch. The documented two-wire EFOC17 version uses a separate short radiator leg and places the current choke directly at the coax side of the feed unit.
- Do similar SWR curves prove the antennas are equivalent? No. Compare the complete geometry, feed networks, current amplitudes and phases, losses and radiation patterns at the same frequency and accepted-power reference.