Build a Carolina Windom Antenna
The Carolina Windom is a refined evolution of the classic Windom antenna — an off-centre-fed wire that uses a deliberate imbalance at the feedpoint to allow a portion of the coax outer shield to act as a vertical radiator, adding low-angle radiation and genuine multi-band coverage without an ATU on several bands. Developed and popularised by K4EFW and the Carolina DX Association in the 1990s, the Carolina Windom differs from a plain OCFD by adding a carefully positioned RF choke on the coax feedline that controls how much of the coax radiates and isolates the shack from RF. The result is a true multi-band antenna covering 80m through 10m — directly into 50 Ω coax with an SWR below 3:1 on most bands — from a single wire requiring only two supports. This guide covers the theory, dimensions, RF choke construction, and complete installation for the standard 80m Carolina Windom.
From Classic Windom to Carolina Windom
The original Windom antenna (1929, Loren Windom W8GZ) was a single-wire-fed off-centre dipole — fed at the point where the antenna impedance happened to be a convenient value for a single-wire feedline. The modern OCFD (Off-Centre-Fed Dipole) replaced single-wire feed with coax via a 4:1 or 6:1 balun at the off-centre feedpoint. The Carolina Windom takes this further by intentionally allowing a controlled length of coax outer shield to act as a vertical radiating element:
Why the Off-Centre Feed Point Matters
Feeding a dipole at its centre produces a feedpoint impedance of approximately 73 Ω on the fundamental frequency. Moving the feedpoint off-centre changes this impedance — and, importantly, changes which harmonics the antenna presents a usable impedance on. The 1/3 : 2/3 feed ratio used in the Carolina Windom is specifically chosen because the resulting feedpoint impedance is manageable with a 4:1 balun on the most useful amateur HF bands:
The Two RF Chokes — What Each Does
The Carolina Windom uses two RF chokes — this is the key distinction from a plain OCFD. Each choke performs a different function:
- Upper choke (at the feedpoint balun): a 4:1 current balun wound on Mix 31 or Mix 43 ferrite. This is the impedance transformer — it converts the ~200 Ω feedpoint impedance to approximately 50 Ω for the coax. It also provides some common-mode suppression at the feedpoint itself. This choke is integral to the feedpoint assembly.
- Lower choke (on the coax, 25–30 ft below the feedpoint): a common-mode choke only — wound on Mix 31 ferrite, typically 8–12 turns of coax through or around a toroid core. This choke terminates the vertical radiating section of the coax outer shield. Everything above this choke (the hanging coax section) is part of the antenna; everything below is isolated feed cable to the shack.
- Vertical radiator length: the distance between the upper balun and the lower choke — typically 25–30 ft for an 80m Carolina Windom. This length is approximately a quarter-wave on 40m and contributes the vertical radiation component. Adjusting this length changes the antenna's radiation pattern and the SWR seen at the shack.
- Core material matters: Mix 31 (Fair-Rite) ferrite is preferred for both chokes on HF — it provides high choking impedance across 3–30 MHz. Mix 43 is acceptable but less effective on the lower HF bands (80m, 40m).
Carolina Windom vs Plain OCFD vs G5RV
Understanding where the Carolina Windom sits among common multi-band HF wire antennas helps set realistic expectations:
- vs plain OCFD: the Carolina Windom adds the vertical radiator element and lower choke. The plain OCFD with a 4:1 balun radiates only from the flat-top wire — the coax outer shield carries common-mode current that distorts the pattern unpredictably. The Carolina Windom intentionally controls this current to add a useful vertical radiation component. The Carolina Windom is the more engineered and more predictable design.
- vs G5RV: the G5RV uses a specific matching section and requires a coax ATU for most bands. The Carolina Windom connects directly to coax with no matching section and works on most bands without an ATU. The G5RV is simpler to build but less convenient to operate multi-band.
- vs doublet: the doublet with open-wire feedline and balanced ATU has lower feedline loss and covers more bands with better efficiency. The Carolina Windom trades some efficiency for installation simplicity — direct coax feed, no balanced ATU required, no open-wire routing challenges.
- Best use case: the Carolina Windom excels as a permanent fixed-station all-band HF antenna where direct coax feed is strongly preferred, two end supports are available, and ATU-free operation on the major HF bands (80m, 40m, 20m, 10m) is desired.
| Version | Total wire | Short leg | Long leg | Vertical section | Bands (no ATU) | Min. span needed |
|---|---|---|---|---|---|---|
| 80m Carolina Windom | 133 ft | 44.3 ft | 88.6 ft | 25–30 ft coax | 80m, 40m, 20m, 10m | 140 ft |
| 40m Carolina Windom | 66 ft | 22 ft | 44 ft | 16–18 ft coax | 40m, 20m, 10m | 72 ft |
| 160m Carolina Windom | 267 ft | 89 ft | 178 ft | 50–60 ft coax | 160m, 80m, 40m, 20m, 10m | 275 ft |
Materials for the standard 80m Carolina Windom covering 80m through 10m
Building the 80m Carolina Windom
This guide builds the standard 80m Carolina Windom with a 133-ft flat-top wire, 4:1 balun at the off-centre feedpoint, a 25–30 ft vertical coax radiator section, and a lower RF choke. Build the chokes first — they are the most critical components and require care to get right.
Build or Select the Upper 4:1 Current Balun
The upper balun is the heart of the Carolina Windom feedpoint. It must be a current (common-mode choke) type wound as a 4:1 transformer, not a voltage balun. A voltage balun at this feedpoint allows common-mode current to flow on the coax shield in an uncontrolled way — the Carolina Windom's vertical radiator depends on controlled common-mode current below the upper balun, not random leakage through the balun itself.
Enclose the balun in a weatherproof ABS or polycarbonate box with sealed cable entries. Label the balanced (antenna wire) terminals and the unbalanced (coax) terminal clearly on the enclosure.
Build the Lower RF Choke
The lower choke is a simple common-mode choke — no impedance transformation, just maximum choking impedance across the HF spectrum. It terminates the vertical coax radiator section and prevents RF from reaching the shack equipment via the coax outer shield:
Cut and Prepare the Flat-Top Wire
Cut the short leg to 45 feet (44.3 ft nominal plus trimming allowance) and the long leg to 90 feet (88.6 ft nominal plus allowance). These lengths are not as critical as for a resonant single-band dipole — the Carolina Windom is a multi-band antenna and small variations in flat-top length shift the SWR curve on each band slightly rather than preventing operation. Cut long and trim if needed.
At the feedpoint end of each wire, form a soldered loop for connection to the 4:1 balun's balanced terminals. At each far end, form a loop through the end insulator and solder. Label the short leg and long leg clearly with different-coloured tape — connecting them reversed to the wrong balun terminal is a common installation error that shifts the antenna's band characteristics.
Assemble the Feedpoint
Connect the short leg wire to one balanced terminal of the 4:1 balun and the long leg wire to the other balanced terminal. Connect the 35-ft vertical radiator coax section to the balun's coax output (SO-239). The far end of the vertical radiator coax connects to the input of the lower choke. Attach the Dacron halyard to the strain-relief point on the balun enclosure.
Lay the complete antenna out on the ground before raising: flat-top wire in a line with the balun at the 1/3 point, vertical coax section hanging straight down from the balun, lower choke at 25–30 ft below the balun, and the shack coax run extending from the lower choke. Verify all connections are correct and all PL-259 connectors are fully tightened before raising.
Raise the Antenna
The Carolina Windom raises differently from a symmetric dipole — the feedpoint is not at the centre of the flat-top. Plan the support positions accordingly: the feedpoint balun (at the 1/3 point) should be positioned between the two end supports at the correct 1/3 distance, not at the midpoint. The feedpoint does not need to be at a support — it hangs at whatever height the end supports and wire catenary places it.
Raise both end supports to their full height, pulling the flat-top wire up between them. The feedpoint balun will rise to a height determined by the apex of the catenary formed between the two end supports. Aim for the feedpoint balun to be at least 25–30 ft high — this allows the vertical coax section to hang clear of the ground. The lower choke should be at 8–10 ft minimum above ground.
Optimise the Vertical Section Length
With the antenna raised, connect the NanoVNA to the shack end of the coax (after the lower choke) and sweep each band. Record the SWR at the centre of each band — 3.650 MHz, 7.150 MHz, 14.150 MHz, 21.200 MHz, and 28.500 MHz. Then reposition the lower choke by 2–3 feet and re-measure. Repeat across the range of 20–32 ft vertical section lengths:
Once the best vertical section length is found, secure the lower choke at that position with a UV-resistant cable tie to a wooden post or non-conductive support if the coax does not hang straight. The lower choke should be weatherproofed and the coax run from the lower choke to the shack should be routed along the ground or along the building without additional RF chokes.
Verify Band Coverage and Document Results
With the vertical section optimised, make a final sweep of all HF bands and record the SWR at the band centres. Document the results for future reference — these SWR values are the baseline for this installation and any significant future changes indicate a fault (corroded connection, damaged coax, shifted lower choke position).
40m Carolina Windom — Compact Version
The 40m Carolina Windom uses a 66-ft flat-top wire with the feedpoint at the 22-ft / 44-ft split, and a 16–18 ft vertical coax section. It covers 40m through 10m with the same ATU-free capability on 40m, 20m, and 10m as the full-size 80m version:
- Span required: approximately 70 ft between end supports — fits most suburban lots
- Minimum apex height: 25 ft — allows a 16 ft vertical section to hang with clearance above ground
- 80m capability: the 40m version is usable on 80m with an ATU but is electrically short — efficiency on 80m is limited. If 80m is a priority, build the full-size 133-ft version.
- Vertical section: 16–18 ft — shorter than the 80m version's 25–28 ft, which means the lower choke position is more critical. Optimise in 1-ft increments.
- Best use case: suburban installation where 140 ft of horizontal span is not available but 70 ft is, and 40m through 10m is the primary operating focus.
Installation Orientation for Maximum Performance
Unlike a symmetric dipole, the Carolina Windom's radiation pattern is asymmetric — the off-centre feed and vertical radiator section combine to create a pattern that is not a simple figure-eight. Some orientation guidance:
- Flat-top orientation: orient the flat-top wire broadside toward your primary DX direction — the flat-top contributes a broadside horizontal radiation component similar to a dipole, strongest perpendicular to the wire.
- Vertical section orientation: the vertical coax radiator hangs below the feedpoint and radiates somewhat omnidirectionally at low angles — this component is less sensitive to flat-top orientation.
- Height over orientation: getting the flat-top as high as possible matters more than precise orientation for DX performance. At low heights (under 30 ft) the radiation angle is high regardless of orientation — maximise height first, then optimise direction.
- Inverted-V configuration: if only one high support is available, the Carolina Windom can be installed in an inverted-V configuration with the feedpoint at the apex and the two legs drooping toward ground anchors. The vertical section still hangs from the feedpoint. Performance is slightly degraded on 80m DX compared to a flat-top installation, but the configuration is practical and widely used.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| High SWR on all bands — no usable match | Open circuit in flat-top wire or balun connection failure | Check DC continuity from each balun balanced terminal to the wire end; check coax connection at balun output | Re-solder wire connections at balun terminals; inspect balun internally for open winding; replace balun if faulty |
| SWR high on 80m but acceptable on 20m | Short leg and long leg connected to wrong balun terminals | Swap the two wire connections at the balun balanced terminals; re-measure SWR on 80m | Reconnect short leg and long leg to correct terminals (short leg to one terminal, long leg to other); re-verify all bands |
| RF feedback in shack — RF on equipment chassis | Lower choke insufficient or positioned too high — vertical section too long | Touch equipment chassis with RF meter or feel for tingle; check lower choke position and construction | Verify lower choke has at least 8 turns through 2× FT-240-31 cores; reposition lower choke closer to feedpoint (shorten vertical section) |
| SWR good on first install, degraded after weeks | Moisture ingress at upper balun or lower choke connections | Inspect all outdoor connections for green oxidation or water inside connector bodies | Disassemble, dry, re-solder, and re-weatherproof all outdoor connections; add second layer of self-amalgamating tape |
| SWR varies with weather or time of day | Lower choke position shifting due to wind or thermal expansion of coax | Observe whether SWR change correlates with wind or temperature | Secure lower choke and vertical coax section against movement with additional support or cable ties to a non-conductive post |
| Cannot achieve SWR below 3:1 on 40m | Vertical section length not optimised; or balun is voltage type not current type | Verify balun type — current choke type required; try vertical section lengths from 20 to 32 ft in 2-ft increments | Replace voltage balun with 4:1 current balun; re-optimise vertical section length systematically |
| Antenna works on 80m and 20m but not 40m | 4:1 balun ratio may not suit this specific flat-top length on 40m | Try a 6:1 balun at the feedpoint in place of the 4:1; measure SWR on all bands | Some 133-ft OCFD installations match better with a 6:1 balun on 40m; experiment with both ratios |
Is a 4:1 or 6:1 balun correct for the Carolina Windom?
The standard 80m Carolina Windom uses a 4:1 current balun at the feedpoint. This matches the theoretical feedpoint impedance of approximately 200 Ω (at the 1/3 point of a full-wave flat-top on 80m) to 50 Ω coax. Some builders report better results on 40m with a 6:1 balun, which more closely matches the higher impedance seen at the feedpoint on that band. If 40m is your most critical band, try both ratios — build or obtain a 6:1 balun and compare SWR on 40m. For most operators the 4:1 is the correct starting point and works well across all primary bands.
What power level can the Carolina Windom handle?
The flat-top wire and coax feedline handle legal-limit power (1500W) without concern. The limiting factor is the 4:1 upper balun — commercial units from DX Engineering, Balun Designs, and similar quality suppliers are rated 1.5–3 kW. At 100W any quality commercial 4:1 balun handles the load easily. At 500W and above, verify the balun's power rating and ensure it is a current-choke type on the correct ferrite mix — a voltage balun or a balun on the wrong ferrite mix will overheat under sustained carrier (digital modes, RTTY) at moderate power levels.
Do I need an ATU for the Carolina Windom?
For the primary bands — 80m, 40m, 20m, and 10m — a well-built Carolina Windom with optimised vertical section length presents SWR below 2.5:1 and most transceivers will load directly without an ATU. For the WARC bands (30m, 17m, 12m) and 15m, SWR is typically higher (3:1–5:1) and an ATU is needed. A wide-range internal rig ATU handles these bands in most cases. A small outboard ATU (LDG Z-11, MFJ-941) completes the coverage on bands the rig's internal ATU struggles with. The Carolina Windom is designed for ATU-free operation on its primary bands — that convenience is one of its main appeals over the doublet or G5RV.
How does the Carolina Windom compare to the EFHW for multi-band use?
Both are popular multi-band HF wire antennas fed with coax, but they work differently. The EFHW requires a 49:1 or 64:1 impedance transformer at the high-impedance wire end and covers its fundamental band and harmonics. The Carolina Windom uses a 4:1 balun at an off-centre feedpoint and benefits from the additional vertical radiator. For 80m coverage in particular the Carolina Windom is the more practical choice — an EFHW for 80m requires a 130-ft wire and a very high-performance 49:1 transformer. For 40m and above only, the EFHW is simpler to build and equally effective. For a fixed station needing genuine 80m performance and coverage to 10m from a single feedpoint, the Carolina Windom is the better choice.
Can I install the Carolina Windom in a small garden?
The full-size 80m Carolina Windom needs approximately 140 ft of horizontal span — not suitable for a small garden. The 40m version (66-ft flat-top) needs only 70 ft and fits many suburban lots. If even that is too large, a compromise is to bend the ends of the flat-top downward — an end-bent Carolina Windom loses some performance but can fit in a significantly smaller space. The vertical section is unchanged regardless of flat-top configuration. For very small gardens (under 40 ft available span) a magnetic loop, loaded vertical, or attic antenna is a more practical approach than any version of the Carolina Windom.
Why does the vertical coax section radiate — isn't coax shielded?
Coaxial cable shields the inner signal from external fields very effectively — but only if no net current flows on the outer surface of the shield. When common-mode current is present (current flowing on the outside of the shield in the same direction, not as part of the differential coax signal), the outer shield surface radiates just like any other conductor carrying RF current. In the Carolina Windom, the 4:1 current balun at the feedpoint allows a controlled amount of common-mode current to flow on the outer surface of the vertical coax section. This is intentional — the common-mode current on the outer coax shield is what makes the vertical section radiate. The lower choke then stops this common-mode current from continuing down the coax to the shack.