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Written by F6DED   
Tuesday, 19 August 2003 20:07

A homebrew W8JK antenna

w8jkface

 

Realization inspired by the F6BCU's work , published in Ref Radio January 1990 and November 1998,

 

Description

1) A W8JK of 10 meters scale :

It consists of 4 rods from 16.40 feets, 2 plasterboard rails of 2.50 m, a piece of bed frame, copper pipes, plumbing clamps, two handle brooms, to pieces of Plexiglas.
To avoid the use of an antenna rotor, the assembly is oriented from the ground by a nylon cord, the antenna rotates on a bearing drum washing machine.
The antenna is fed by an assymetric line build with two flexibles wires of 1.5 mm² and 5cm spaced with pieces of PVC electrician pipe's IRL16.


 

2) An 8-meter tower constructed with rafters :

Idea came from the "A Frame Mast" described in the American Handbook, it is performed by assembling four rafters of 4 meters. The ensembly has been operational for 9 months in 2002: it allowed me to make many DX continent with only 10/20 Watts in digital modes. I had to disassemble the tower to defuse a potential conflict with neighbors .... The uninitiated do not see the value of spin fishing rods to 8 meters from the ground, several miles from a point of water ......


Making of

1) A W8JK of 10 meters scale :

w8jkpresDimensions of the antenna: I have determined by synthesizing items of F6BCU work and recommendations of the antenna book published in August 2000. Dimensions are not critical, because this antenna is used with an antenna tuner. You just need to ensure that the length of the elements is at least a 1/4 wavelength of the lowest frequency (here 40m), the spacing is optimized to 1/8 wave for the band on which one wants to work the most (2.20 m here is a compromise), and the spacing between the 2 wires of the assymetric line is 5 cm (theoretical impedance of 500 ohms) if you want to go up in frequency. Spacing between elements: 2.20 meters boom length: 2.40 meters.

Length of each radiating elements is 8 meters.

Assymetric line : 2 wires of 1.5 mm² spaced of 5 centimeters, (theoretical impedance 500 ohms)

 

 

 

 

Boom mounting: two plasterboard rails bolted face to face enclose a square tube retrieved from a bed frame ... (to increase rigidity). Two pieces of wooden handle broom are uncertain, perpendicular at the ends for receiving the fishing rods.

The radiating elements: are made by inserting a wire of 1.5 mm² in fishing rods deployed. To have a radiating 4m, there should be a fishing rod of 5 meters where you remove the last piece. The last item is not hollow, it is impossible to fill the wire inside. To optimize the antenna, it is imperative to balance the electrical length of radiating wires. I advise you to prepare the four wires, before final assembly, as follows: Cut 4 lengths of 5.50 m millimeter, made a mark with a pen to 4 meters. You can then insert and paste wires in each cane deployed.

Final assembly: you can insert each canes on handle brooms, then screw each canes, leaving 5 centimeters between the tip of the cane (and therefore the output of the line) and the metal mass of the boom. An antenna of this type has elements that must be isolated from any metal mass. You can then solder the four strands radiating from the beginning of the assymetric line. Do not forget to cross one side to create the phase shift, and make sure that the length of each wire is the same. The easiest way is to check the length remaining, rather than measuring the length that is threaded.

The scaling down of assymetric line is made of solid copper: A capillary tube Recovering from a fridge . The capillary tube is very thin copper tube which is located between the condenser (the big black radiator behind the fridge) and the evaporator (cooling coil in the fridge). It is about 2 mm in diameter and is stiffer than an electric wire .Warning keep the ozone layer ! The fluid cooling circuit must have been extracted by a qualified technician before cutting the capillary tube. There are probably easier, but I took advantage of the tools and skills of my students in the refrigeration and air conditioning High School St. Joseph Lorraine .. The capillary tube is held in place by spacers made in falling Makrolon.

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This antenna must rotate 180 degrees, there should be a piece of flexible asymmetric line. If you want to put it simply, you can use a piece of twin lead semi-rigid trade and if you're handy, you can make the following way: get sheets of flexible plastic material, like plastic film projector. Cut into strips slightly wider than the desired spacing between your electrical lines. Make holes on the edges and your son go electric in the holes. It takes approximately 80 cm flexible supply line to achieve a 180 ° rotation.

 


2) An 8-meter tower constructed with rafters:
It consists of three treated rafters of 5x7cm for 4m length, purchased at a DIY store. I sawed one in half, for two sections of 2m. Then I assembled everything with screws 5x 70mm according to the schematic below. The result is a pole 10x7cm 8m high. For added security, I recommend two layers of three guys wires at 4m and 7.50 m.

In the future

It is nevertheless better to have a real rotor, even a simple geared down pylon, pylon true even of the type video light; aluminum elements nested, and a supply line 4 with 2 wires crafted lines 300 ohm flexible assembled side by side. The complexity of supply lines such ladders frog is a problem for the rotation. On or yagi antennas that are commonly used.

Leaves to use fishing rods, why not mount a cubical quad? I'm going back to work ....
F6DED, September 2003

 

You can consult excelent work of André F5AD about rotative levy and his antenna database.

 

Last Updated on Tuesday, 23 April 2013 09:40
 

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