Carolina Windom (RadioWorks / Homebrew)
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| Frequency | 80m: 3.5-4.0 MHz, 40m: 7.0-7.3 MHz, 20m: 14.0-14.35 MHz, 17m: 18.068-18.168 MHz, 15m: 21.0-21.45 MHz, 12m: 24.89-24.99 MHz, 10m: 28.0-29.7 MHz |
| Bands | 80m, 40m, 20m, 17m, 15m, 12m, 10m |
| Modes | SSB, CW, AM, FM, digital |
| Connector | coax through the matching unit; 50-ohm feed below the line isolator |
| feedpoint | 4:1 voltage balun (~50:200 ohm) at the off-center feedpoint; 1:1 line isolator at the bottom of the vertical coax section |
Estimated (unverified): design year (1980s); homebrew/commercial price ranges (vary by builder and dealer)
One wire, many bands, and a low-band DX component: on 80 and 40 m the deliberately radiating vertical coax section adds low-angle signal that a plain dipole at the same height cannot make. The off-center feed lands near a usable impedance on several harmonically related bands through the 4:1 balun, and the design hangs as a flat-top, inverted-V, or sloper. Cheap to build, and the commercial hardware (matching unit and line isolator) is still easy to buy.
The feature is also the failure mode: the antenna works by controlled common-mode current, so everything depends on the 1:1 line isolator being present, effective, and in the right place. Get the choke wrong and the 'vertical radiator' becomes your whole feedline and station - RFI in the shack and a skewed pattern. Published leg lengths disagree between versions, SWR still needs a tuner on some bands, and the off-center feedpoint plus hanging coax section make for a fussier installation than a center-fed wire.
An off-center-fed wire with both strong advocates and real critics. The radiating vertical section gives it a low-angle component most wires lack, but the same mechanism makes it unforgiving of sloppy choking. Built exactly to recipe, it earns its following; built casually, it earns the criticism.
In September 1929, Loren Windom described a deceptively simple antenna in QST (he cheerfully credited the underlying work to others, but the name stuck to him): feed a wire off center with a single wire drop, and the feedpoint impedance lands somewhere usable on several harmonically related bands. The off-center-fed dipole has carried his name ever since. The Carolina Windom (a trademark of RadioWorks, and one of the most copied wire designs in ham radio) is the OCFD with a deliberate twist: instead of fighting the feedline radiation that off-center feed invites, it harnesses it.
The recipe matters. At the off-center feedpoint of a roughly 133-foot wire sits a 4:1 voltage balun, a voltage type specifically, because a current balun would choke off the very thing this antenna wants. Common-mode current flows down the outside of the coax for about 22 feet (10 to 18 feet, depending on the build, on the shorter 40 m version), and that length of coax shield is a vertical radiator, fed from the top. At its bottom end, a 1:1 line isolator slams the door: below the choke, the coax goes back to being ordinary feedline. The result is a dipole and a top-fed vertical sharing one wire run, and on 80 and 40 m, where the common-mode current is strongest, the vertical part contributes low-angle radiation that a flat wire at typical back-yard height does not produce. On the higher harmonics the effect largely fades, and patient side-by-side testing has questioned how much it buys in practice, a fight that keeps the antenna forums warm.
The catch is that the antenna is exactly as good as its choke. Leave the line isolator out, place it wrong, or let it degrade, and the controlled 22-foot radiator becomes an uncontrolled one the full length of your feedline, with RF in the shack and a pattern skewed toward whatever your coax run happens to do. The published dimensions also vary more than newcomers expect (builds document off-center splits from about 44/89 feet to 50/83 feet depending on version and vintage) and like any OCFD it still wants a tuner on some bands. None of this is hidden; the design's own literature is unusually frank that the magic lives in the matching unit and isolator, which is why those two components are what the commercial versions are really selling.
If the idea of feedline-as-radiator sounds like more moving parts than you want, the conventional alternatives are a parallel dipole like the Alpha Delta DX-CC, which buys its bands with separate wires and no common-mode tricks, or an end-fed like the EFHW-4010. The Carolina Windom remains the contrarian's multiband wire: it takes the one thing every antenna manual warns against (a radiating feedline) and turns it into the feature on the box.