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Passive HF antenna

The best passive HF antenna design that we have found is described below. It necessitates no guy-wires and has no radial wires over ground. As of 2024, more than 800 of these antennas have been built in the Philippines, Hawaii and Mexico at 8, 12, 16 and 27 MHz, and resisted typhoons with no breaking. It scales with no modification from 4 MHz to 30 MHz, and has been used in 65 deployments.

Electrically, the antennas are normal-mode helical compact monopoles (Kraus, J.D., “The Helical Antenna”, Proc. I.R.E. 1949 pp. 263-272).

They consist of a vertical wire of length λ/4 wound over a mast of height λ/8 and diameter λ/300 to achieve a 50% height compression, a 3-loop tuning air-coil, and a network of 4 radials of length λ/4 on the ground or buried at shallow depth (1-2 cm) (λ is the electromagnetic wavelength). Radials and vertical wires are made of standard solid core AWG14 wire or PE-jacket antenna wire.

The air-coil ellipticity, hence the cross-area of the loop and the inductance, is adjusted to achieve resonance using a standard commercial VSWR meter. When tuning is complete, the loop is fastened to the mast with electrical tape and zip-ties.

The mast consists of a thick vertical tube, made of fiberglass, ABS drain pipe, PVC water pipe, solid wood and bamboo. The dimensions given in the sketch below are for 8 MHz; multiply by the ratio of frequencies for measurements at other frequencies.

The mast is planted in a hole in the ground, and adjusted/held vertically by wooden or plastic wedges; about 1/5 of the length should be underground, and 4/5 above ground, thus the total length of the mast is λ*5/32.

In rock, the hole can be drilled directly with a hammer drill and a carbide bit. In consolidated soil, the hole can be drilled with a mechanical auger; a fiberglass liner of a slightly larger diameter than the antenna will prevent collapse of the hole, the void being filled with self-leveling cement. In sand and gravel, a hole should be shoveled out, and a reinforced concrete base built with a fiberglass liner to hold the antenna.

The assembly consists of a RG-213 pig tail, the vertical being directly soldered to the center conductor, and the radials to the shield; the connections are then potted in Scotch-cast or similar electrical resin. Details of construction are found here.

Summary:

8 MHz as deployed in Aurora, Philippines

8 MHz antenna array in Dipaculao, Philippines