The 40-meter band (7 MHz) is a cornerstone of High-Frequency (HF) amateur radio, but a full-sized half-wave dipole for this band is about 66 feet (20 meters) long. For hams with limited space, this can be a serious challenge!. Enter the “Shorty Forty“—a classic solution that uses loading coils to achieve excellent performance in a much more compact footprint.
The Compact Design: How it Works
The image above details a specific, proven design for a shortened 40-meter dipole. The key to its reduced size lies in the center-fed loading coils.
A standard half-wave antenna is resonant because its physical length is half the wavelength of the signal. When you shorten the physical wire, the antenna becomes electrically “too short” and mainly capacitive. The loading coils—which are inductors—introduce the necessary inductance to cancel out the antenna’s capacitance, effectively “tricking” the radio waves into seeing a full-sized antenna. The key to its operation lies in the loading coilplaced near the center of each element, which compensates for the electrical length lost due to the shortened physical structure.
A loading coil is simply an inductor that is placed somewhere along the length of the antenna which makes a shorter antenna behave as a longer antenna.
Shorty Forty Specifications
The design shown dramatically cuts the overall length while maintaining resonance on 7 MHz:
- Total Antenna Length: The wire is split into two equal segments, each 12′ 6″ (381 cm) long. This results in an overall span of just 25 feet! This is less than half the length of a full dipole.
- Feed Point: The antenna is center-fed, typically with coaxial cable, right at the coil assembly.
The performance of the Shorty Forty hinges on the specifics of its loading coils. Building this component accurately is vital
Shorty Forty : Loading Coil Specifications
- Inductance: 36.191 µH
- Turns: 56 turns of 1.00 mm wire (AWG #18)
- Winding Spacing: 12.6 turns per inch
- Coil Form Diameter: 1.5 inches (38.1 mm)
- Coil Length: 4.4 inches (112.8 mm)
- L/D Ratio: 2.9
The specified turns density is 12.6 Turns Per Inch. This spacing is important to minimize inter-turn capacitance and achieve the precise inductance value required. While no shortened antenna is quite as efficient as a full-sized one, the Shorty Forty offers significant advantages for the space-constrained amateur:
- Space Savings: It’s the ultimate benefit—getting on the popular 40-meter band even in a small backyard, balcony, or attic.
- Ease of Construction: Once you’ve wound the coils, the rest of the antenna is simple wire and insulators.
- Performance: While there’s a small efficiency loss (typically a few dB) compared to a full dipole, this antenna will easily let you make contacts across continents on a good day.
Feed-Point Particulars (with coil tap)
Feed arrangement: center-fed but tapped at the loading coil (a center-loaded/tapped coil feed).
- Coax shield (ground) — connect to the exact physical centre turn of the loading coil. This is the mechanical and electrical ground reference.
- Coax centre conductor (hot) — connect to a tap 2–3 turns away from that centre turn on the coil (toward either one of the two legs as required by your build). That tap is the live feed point.
- Why a tap? The tap makes the feed effectively offset from the mechanical midpoint, allowing the antenna + coil combination to present a more usable impedance to the 50 Ω coax without excessively large matching networks. It’s a simple way to “move” the feedpoint electrically on a short, center-loaded dipole.
- Initial tap position: Start with 2 turns from centre toward the direction you prefer (toward either leg). If SWR is high, move the tap by ±1 turn and re-measure; 2–3 turns is a good starting point.
- Balun / choke: Still use a 1:1 current (choke) balun at the feed node if possible — wound on a small ferrite or use an air-wound choke on the coax near the coil — to reduce common-mode currents on the coax shield. If you use the shield-to-coil-centre method described above, common-mode reduction is helped, but a proper choke is recommended for best performance.
Performance and Installation Tips
- Mount the antenna at least 6–8 meters above ground level for best results.
- Use 50-ohm coaxial cable as the feedline, connected through a balun for improved balance.
- Tune the antenna by slightly adjusting wire length or coil spacing for a low SWR near 7.1 MHz.
- When installed properly, this antenna provides a good match and reliable communication range for both local and DX contacts.
If you’re looking to operate on 7 MHz but can’t hang a 66-foot wire, this Shorty Forty Antenna design is a fantastic project to tackle this weekend!
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