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40m 2-element Phased Vertical Array

(Click on a thumbnail to see a larger photo.)

This antenna was developed from an idea of combining the simplicity of the 1/8th wavelength spaced Christman fed array with the simplicity of directional elevated radials. The antenna has 3dB gain over a single vertical. Importantly, for this DXpedition, it's light to transport, and easy to put up.

Close up of the feedpoint of one of the verticals. The grey pipe is a 55mm plastic drainpipe used to protect the fibre glass mast in transit. The feedpoint is a few inches above ground level and the radials go up from the coax socket to cane supports and then horizontally off at 1m above ground level.

The T-piece at the end of the 39-degree feeder section. The DK9SQ 10m poles are dropped into transport-protection tubes. These tubes are strapped to fence posts hammered into the ground.

During optimisation measurements were made with a MFJ-259B antenna analyser (tnx Marcus, G0IJZ). Each vertical and two radials were adjusted to resonance (X=0) at 7.050 MHz. The self impedances were measured at 38 Ohms.

The measured SWR of the finished array, with the phasing feedlines, (measured with an FT1000MP), was 1.2:1 at 7.000 MHz dropping to 1:1 at 7.100 MHz.

The feeders are coiled up to make choke baluns at the base of each vertical.

This antenna design is described on page 11-35 of "Low band DXing" (3rd Edition, 1999) by John Devoldere, ON4UN.

The dimensions for the 40m North facing implementation, with elevated radials.

The EZNEC wire model for the simulation. The quarter-wave verticals are supported by DK9SQ fibre-glass poles. Each vertical has 2 radials elevated 1m above ground. Each radial is angled at 30 degrees from the main direction of fire. Wire lengths are, both verticals = 11.07m, all four radials = 9.68m.

The 3D radiation pattern. The simulation models real copper and ground losses. Ground parameters 5 mS/m with a dielectric constant of 13, typical of real soil.

Elevation plot showing best gain at 25 degrees elevation.

Azimuth plot showing 126 degree beamwidth and good front-to-back ratio.

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