W8JK by ZR6TXA
The W8JK is a famous and effective DX antenna
The W8JK Introduction The W8JK is a famous and effective DX antenna, first built by John Kraus, W8JK, in 1937. After seeing a paper by George H Brown with theoretical calculations about the performance of closely-spaced dipoles, Kraus built the first W8JK beam antenna with two parallel dipoles driven with opposite phase, with the unprecedented close spacing of an eighth of a wavelength. The beam design I will describe was given by Kraus in QST magazine, in June 1982. Some of the characteristics are as follows :
• It is a compact, 6-band antenna, only 7.3m long, and covering the 20, 17, 15, 12, 10 and 6m bands • It operates over a continuous frequency range of more than 3:1 • It requires no loading coils or traps in the antenna
• No antenna or feed system dimensions are critical • It can be operated in the horizontal or vertical position • It is ideal for finding open round-the-world communication paths • It can be fed with cheap, low-loss twin line. Compared to the very popular tri-bander trapped beams, the W8JK has some advantages and disadvantages :
• Having no traps to wind and adjust makes it a realistic project for the home constructor. Construction can be of aluminium tubing with some insulators. • It has gain over a continuous range of frequencies, not just on three bands. This means it can be used in the WARC bands with equal performance to the staple 10, 15, 20, bands
• It continues to work up to 50MHz, with fairly good gain, allowing operation there immediately, without a second antenna. • It has lower gain than the tri-bander, especially on 10m. At 6m, a 3-element beam has more gain than the W8JK.
• It fires both ways, so has no Front to Back ratio. However, unwanted signals can still be nulled out on the sides of the beam. • It does not present a 50Ω input impedance, so it requires an ATU. Kraus does give a transmission-line matching method for this antenna, but I will not discuss it here.
Construction The antenna is made with two parallel dipole elements, 7.3m long, and 2.6m apart. The dipoles are fed in the centre, on the boom, and are fed out of phase. A pair of wires connect the antennas together, the left-hand side of one antenna being connected to the right-hand side of the other, and the same on the other side.
The feed line is connected to the two wires, in the centre of the antenna, and runs down the mast from there. A neat method of construction would be to make the “boom” out of a 2.8m long, 60mm diameter thick-walled aluminium tube.
This is mounted to your tower and rotator with a cross-over plate. Four half-elements are constructed of tapered aluminium tubing, each one having a total length of about 3.55m. The elements are then pushed onto a fibreglass tube, or plastic or well-varnished wooden rod, to keep them insulated from the boom and each other. The insulators are clamped to both ends of the boom with cross-over plates.
The elements can be guyed to the boom for improved stability, with plastic rope from about 2m into each element, across to the other end of the boom. Use a dark coloured polypropylene rope, which should last 5-10 years in the sun, rather than Nylon, which will perish in a year or two. For flexibility in take-off angle, and polarisation, the antenna can be designed to rotate into the vertical position, around the central boom shaft. To achieve this, the boom can be placed inside a larger diameter steel or aluminium pipe, which is clamped to the rotator. An arrangement of ropes will allow the antenna to be rotated.
Round-the-world paths The bi-directional nature of this antenna makes it possible to discover open round-the-world paths, something not possible with a normal beam antenna. The technique used by Kraus is to rotate the beam slowly, sending short Morse code dots, with a full-break-in or QSK transceiver. The delay time for the signal to return is about one seventh of a second, so there is plenty of time for your transceiver to switch to receive mode. When you have found and peaked an open round-the-world path, call CQ, and you may be rewarded with DX anywhere along the path. Also, the question of Long-path and Short-path does not arise – you are transmitting on both paths at once, giving you a greater chance of catching the other station’s beam direction…. See More:
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