Rockloop HF antenna for 10,15,20 metre ham bands
The Rockloop HF Antenna ‘s a tuned loop antenna measuring 33 feet square (10m x 10m) designed for 15m, 20m, and 30m bands.This rock loop antenna tunes to 15 meter band with 15 to 20pf,20 meters with about 30pf, and 30 meters with about 60pf, much suitable for apartment dwellers and QRP Portable operations.
The key feature is the variable capacitor tuning system with a T-50-2 toroidal transformer providing impedance matching to 50Ω coax. The loop uses #14 house wiring wire and requires insulators at each corner.
The Rockloop HF Antenna represents a fascinating approach to compact multiband HF antenna design that emerged from the need to achieve reasonable performance on multiple bands within space-constrained installations. Developed by C.F. Rockey (W9SCH), this antenna demonstrates how loop antennas can be effectively implemented for amateur radio applications while maintaining manageable physical dimensions.
The fundamental design philosophy behind the Rockloop centers on creating a resonant loop antenna that can be efficiently tuned across multiple amateur bands through the use of variable capacitance. The 33-foot square configuration creates a total loop circumference of 132 feet, which provides effective operation on 15, 20, and 30 meters. This circumference represents approximately one wavelength at the design frequency, making it what antenna engineers classify as a “full-wave loop” on the primary operating frequency.
Construction and Technical Details
The mechanical construction utilizes standard #14 AWG “house wiring” wire, which provides an excellent balance between conductivity, durability, and cost-effectiveness. The choice of this wire gauge ensures adequate current-carrying capacity while remaining manageable for installation and maintenance. The diamond or square configuration requires four corner insulators, each capable of supporting the wire tension while providing electrical isolation from the support structure.
The tuning system represents the most sophisticated aspect of the Rockloop HF Antenna design. The variable air capacitor, with a maximum capacity of 100 picofarads, allows continuous tuning across the intended frequency ranges. This capacitor must be of high quality with low loss characteristics, as it will handle significant RF currents when the antenna is operating at resonance. The capacitor’s positioning within the loop affects both the impedance characteristics and the radiation pattern, making its placement critical for optimal performance.
The impedance matching network employs a T-50-2 toroidal core wound with a 20:5 turn ratio (4:1 turns ratio), creating approximately a 16:1 impedance transformation. This transformer serves dual purposes: it provides the necessary impedance transformation from the high-impedance loop to the 50-ohm coaxial feed line, and it helps isolate the antenna from common-mode currents that might otherwise flow on the coax shield.