Thursday, March 3, 2011

The W8NX trap dipole antenna


This antenna was designed for amateurs with limited space who also wanted to operate the low bands.It was first described in July 1992 QST by A. C. Buxton, W8NX, and features innovative coaxial-cable traps.

Fig  shows the antenna layout; it is resonant at 1.865, 3.825, and 7.225 MHz. The antenna is made of #14 stranded wire and two pairs of coaxial traps. Construction is conventional in most respects, except for the high inductance-to-capacitance (L/C) ratio that results from the unique trap construction.

The traps use two series-connected coil layers, wound in the same direction using RG-58 coaxial cable’s center conductor, together with the insulation over the center conductor. The black outer jacket from the cable is stripped and discarded. The shield braid is also removed from the cable (pushing is easier than pulling the shield off). No doubt you will want to save the braid for use in other projects. RG-58 with a stranded center conductor is best for this project.

The 7-MHz traps have 33 µH of inductance and 15 pF of capacitance, and the 3.8-MHz traps have 74 µH of inductance and 24 pF of capacitance. The trap Qs are over 170 at their design frequencies. These traps are suitable for high-power operation. Do not use RG-8X or any other foam-dielectric cable for making the traps. Breakdown voltage is less for foam dielectric, and the center conductor tends to migrate through the foam when there is a short turn radius. Loading caused by the traps causes a reduced bandwidth for any trap dipole compared to a half-wave dipole. This antenna covers 65 kHz of 160 m, 75 kHz of 80 m, and the entire 40-m band with less 2:1 SWR.

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