T2LT Antenna: Where the Coax Stops Feeding and Starts Radiating
T2LT Antenna: Where the Coax Stops Feeding and Starts Radiating
A T2LT can look like an end-fed wire, a dipole made from coax or simply a feedline with a coil in it. None of those descriptions is enough. The useful explanation begins by separating current inside the coax from current on its exterior, then measuring where the choke makes the lower radiating section end.
The argument around the T2LT usually gets trapped in names. Some call it an end-fed dipole; others call it a sleeve dipole, resonant-feedline dipole or Flower Pot. I am less interested in winning the naming contest than in drawing the conductors correctly. The coax carries the wanted transmission-line mode to the upper transition, while part of its outer shield surface is deliberately recruited as a radiator. That is the trick—and the choke boundary decides how much coax joins the antenna.
Follow the surface current. The centre conductor and shield interior carry the differential feed mode. The extended centre conductor or attached wire forms the upper radiating section. Shield-exterior current between the upper transition and the choke forms the lower section. Exterior current continuing below the intended choke means the installed antenna is larger and less controlled than the drawing suggests.
The T2LT Is Not a Transformer-Fed EFHW
“End-fed” is physically understandable: the coax approaches from the bottom and continues through the lower part of the antenna. Electrically, however, the energy reaches a transition between two radiating sections rather than a high-impedance end of an isolated half-wave wire. The upper conductor extends away from that transition; the exterior of the coax shield carries current in the other direction until the choke presents a boundary.
That makes the basic current distribution more dipole-like than EFHW-like, but it does not make the structure identical to a textbook, symmetrical, centre-fed two-wire dipole. The feedline occupies one radiating arm, the arm diameters and surroundings can differ, and the choke is a finite impedance rather than an open circuit. The physical arrangement can therefore convert energy between differential and common modes and can leave current on the continuing feedline.
No high-ratio transformer is implied by the T2LT topology. Nor does the topology guarantee a particular coax-side impedance. The measured input depends on radiator dimensions, conductor diameters, choke behaviour, cable route, mounting structure, height, ground and nearby objects.
One Coaxial Cable Carries Different Current Systems
Coaxial cable makes this antenna possible because the intended feed current is not the same current as the additional net current on the cable exterior.
- Inside the coax: the forward and return currents of the wanted transmission-line mode flow on the centre conductor and the inner shield surface. The external electromagnetic field is ideally small.
- Above the choke: additional current on the shield exterior is intentional. It is part of the lower radiating section.
- Below the choke: residual shield-exterior current is normally an uncontrolled extension of the antenna unless the installation deliberately includes it.
The shield is one continuous conductor, but RF surface-current paths can still be analysed separately. A current probe placed around the whole cable responds to the net exterior-current mode; it does not report the equal and opposite internal differential currents as though they were another radiating conductor.
The important distinction: exterior shield current is not automatically an error. In the intended lower section it is the radiator. The engineering failure is leaving its lower boundary undefined, then attributing changes caused by feedline routing to the nominal antenna alone.
The Choke Defines a Practical Boundary, Not a Perfect End
The choke raises the impedance seen by current trying to continue along the shield exterior. Its job is to make a current minimum near the intended end of the lower radiating section while allowing the internal coaxial mode to keep travelling toward the radio.
That boundary is frequency-dependent and never infinitely sharp. A coiled-coax choke, ferrite choke or other common-mode structure has resistance, inductance, capacitance, self-resonances and coupling to its surroundings. Its useful complex impedance can change when it is mounted against a mast, enclosed in a housing, wet, bent differently or connected to a different length of cable.
This is why a universal “number of turns”, ferrite recipe or fixed impedance target cannot define every T2LT. The familiar low-frequency approximation in which inductance rises roughly with turns squared stops being a reliable design rule when winding capacitance, transmission-line behaviour and core loss become important. Measure the finished choke across the required band, and verify it again as part of the complete antenna.
High common-mode impedance is useful, but its magnitude alone is not a power rating. The resistive and reactive parts determine current, voltage and dissipation differently. Cable dielectric, bend radius, connectors, weather sealing, ferrite temperature, duty cycle and the actual installed load all belong in the qualification.
Use Wavelength as a Starting Point
A simple single-band T2LT often begins with an upper section and an exterior-shield section each near a quarter wavelength. That is a starting geometry, not a cutting certificate. End effects, conductor diameter, insulation, support tube, choke geometry, mast coupling and the environment shift both the current distribution and the input impedance.
Do not apply the coax manufacturer's internal-mode velocity factor blindly to the shield-exterior radiator. That velocity factor describes propagation between the centre conductor and shield interior. The exterior path interacts with the jacket, air, support, ground and nearby materials, so it has a different effective propagation environment.
The two physical sections also need not finish at identical lengths. Their diameters and surroundings differ, and moving the choke changes both the lower radiator and the load presented at the transition. Start long, but treat upper-section length, choke position and choke construction as coupled variables. Change one variable at a time and restore the baseline between trials.
Free-space starting reference: λ = c / f
A quarter wavelength is λ/4, but the finished physical length must be established from the installed complex impedance and exterior-current distribution—not from the formula alone.
Input Impedance Is an Installed-System Result
The feed transition lies near the high-current region of the intended half-wave-like mode, so a moderate input impedance is plausible. It is not guaranteed to be 50–70 Ω. Arm asymmetry, choke leakage, conductor diameter, support coupling and feedline current can move both resistance and reactance.
Measure complex S11 or resistance and reactance at a declared calibration plane. A low SWR at the radio end describes the complete network at that point, including intervening coax loss and impedance transformation. It does not by itself prove that the radiating sections have the intended lengths, that the choke is effective or that most accepted power is radiated.
Resonance and best 50 Ω match are different questions. At a named plane, a zero-reactance crossing can identify an input resonance while the resistance remains far from 50 Ω. Conversely, mode conversion or a lossy choke can broaden an SWR curve without improving radiation.
Mounting Determines the Pattern You Actually Get
If the installed current distribution resembles a half-wave dipole, its broad pattern tendencies will also resemble that family. Orientation then matters: a vertical installation may be approximately omnidirectional in azimuth in open surroundings, while a horizontal installation often favours broadside directions. Those are starting expectations, not guaranteed plots.
Height above ground, soil, sloping terrain, cable descent, mast conductivity, support ropes, buildings and the residual current below the choke can change elevation and azimuth patterns. “Low angle”, “clean dipole pattern” and “excellent DX” require an installed pattern model or field evidence; they do not follow from the antenna name.
Route the cable and choose the support so that the intended lower radiator is not unknowingly coupled to a parallel conductor. There is no universal rule that the first quarter wavelength must remain clear or that every feedline must leave at right angles. Those arrangements can reduce coupling in some geometries, but a current map and an installed model answer the real question.
Commission the Antenna by Measuring Its Boundaries
- Draw every conductor. Include the upper wire, the whole coax, shield-exterior section, choke, connector bodies, mast, support and station-side cable route.
- Choose the design frequency and bandwidth. Record the intended operating segment rather than assuming a whole amateur band will fit inside the match.
- Build both radiating sections slightly long. Record physical dimensions and the materials surrounding each section.
- Characterise the choke. Measure common-mode complex impedance over the required frequency range and verify differential insertion loss where relevant.
- Calibrate at a declared plane. Save complex impedance or S11, not only minimum SWR.
- Map exterior current. Use a calibrated clamp-on RF current probe at repeatable positions above, through and below the intended choke boundary.
- Tune iteratively. Adjust the upper length, lower-section length or choke position one at a time; repeat A/B/A so cable movement is not mistaken for progress.
- Freeze the installation. Repeat the sweep and current map with the final mast, enclosure, cable route and weather protection in place.
- Qualify operating power separately. Remove the analyser, restore protection, then monitor voltage-sensitive points, connectors, cable and choke temperature at the intended power, waveform and duty cycle.
A second station-side choke may be helpful when another common-mode path exists, but it does not repair an undefined lower-radiator boundary. Measure current before adding it, after adding it and after restoring the original configuration. Otherwise the extra choke can simply move a resonance and create a different antenna.
What a T2LT Result Can Honestly Claim
A repeatable record should include section dimensions, coax type, choke construction and measured complex impedance, mounting photographs, cable route, mast and ground description, analyser calibration plane, complex sweeps and an exterior-current profile. That evidence can show whether the intended radiator boundary is credible and whether the feedline below it is comparatively quiet.
Efficiency requires accepted-power and loss or radiation measurements. Pattern requires a validated full-geometry model or calibrated field measurements. Power handling requires electrical and thermal qualification. Receive noise requires a controlled comparison in which pattern, feedline coupling and receiver conditions are held constant. SWR alone cannot establish any of those conclusions.
Primary technical references
- John Bishop, VK2ZOI — original Flower Pot antenna articles and construction development
- IEEE 145-2025 — antenna and antenna-system terminology
- IEEE 149-2021 — recommended practice for antenna measurements
- Bockelman and Eisenstadt — combined differential- and common-mode scattering parameters
- Keysight — S-parameter design, reflection and reference planes
- ARRL — common-mode current and common-mode chokes
- Com-Power — calibrated RF current monitoring probes
- ICNIRP — RF exposure guidelines from 100 kHz to 300 GHz
Joeri's bottom line: the T2LT is elegant because one cable performs two jobs, not because the current path disappears. Treat the upper conductor and shield exterior as the antenna, treat the choke as a measured boundary, and do not award the feedline below it a free pass. Sometimes looking balanced is good disguise; the real trick is knowing where the current stops.
Mini-FAQ
- Is a T2LT just an EFHW without a transformer? No. Its intended feed transition is between an upper conductor and a lower shield-exterior radiator, not at the high-impedance end of an isolated half-wave wire.
- Is current on the coax shield always unwanted? No. Shield-exterior current above the intended choke is part of this antenna. Current continuing below that boundary is an uncontrolled extension unless deliberately designed in.
- Must the choke be exactly a quarter wavelength below the top transition? No. A quarter-wave lower section is a useful starting geometry, but insulation, choke behaviour, supports and surroundings shift the installed current boundary.
- How much choke impedance is enough? There is no universal number. Measure complex common-mode impedance, current above and below the choke, differential loss and operating temperature across the required band.
- Will a T2LT automatically present 50 Ω? No. Input impedance depends on both radiator sections, conductor diameters, choke, feedline route, mounting, ground and nearby objects.
- Does low SWR prove the antenna has a clean dipole pattern? No. SWR describes reflection at one plane. Pattern needs a complete-geometry model or calibrated field measurement, with residual feedline and mast current included.