Sunday, May 26, 2013

Multiband wire vertical

Multiband wire vertical

The first antenna everybody probably makes is a dipole and the second a quarter wave vertical. The third antenna will probably be a multiband version of these antennas (or yagi). This is the third antenna..

Below a small description of an easy to make and cheap multiband wire vertical. The antenna has like the fan dipole parallel wires for the bands for the bands it has to work on. All antenna wires are all attached to one side of the balun and attached to the center pin of the coax cable. All radials are all attached together to the other side of the balun and attached to the shield of the coax cable. Some distance is needed between the wires to make the antenna easy to adjust.See the picture of the first version for 10, 15 and 20 meters.

The antenna is:

Easy to make
No traps
Light in weight
An easy antenna project
Great for field day use and as ‘extra’ antenna at home for additional bands (or contests).

Needed materials are:

Fishing rod (of better) with length equal to lowest band ¼ wavelength
Bread cutting board (broodplank) (from Hema, IKEA etc.)
Some 5/8 or ¾ inch electricity plastic
Some tyraps
Feed point with 1:1 balun
Some cheap wire as antenna and radial wire
Some gear for cutting and drilling

The pictures give an idea how I made the first example. It took me about 2 hours to make it.

Mast and wire support
There are several supports used for the wires on the sides of the mast. The center wire for the lowest band is attached alongside to the center mast. The other wires are supported using PVC electricity tubing. I like the gray ‘heavy duty’ tubing.

The PVC tubes are attached with tyraps to a strip of plastic cut from a bread cutting board. This board is again attached with tyraps to the mast. See the pictures below.

The distance from the center mast to the end of the side wires is 25 cm (10 inch). Not very critical but it was the total amount of tubing I had available which gave me this length. Don’t make the distance to the center mast very short (like 5 to 10 cm). Of course this can be done (like the Cushcraft R8 or Hy-Gain AV-620 / AV-640) but will make adjusting the lengths very critical.

When 3 bands are used a center pole and 2 wires on the sides are used. The center wire will always be the lowest band and to the sides the other two or four bands. When 5 bands in total are used the 4 wires will be around the center wire each on 90 degrees. When more than 3 bands are used the 2nd and 3rdband are on the same spreaders and also the last two bands to use a minimum of spreaders.

In the spreader tubes holes are drilled to pass the wires through. The wire will only be fixed on the bottom spreader tube and the top spreader tube. The wire is allowed to slip through the holes of all additional support spreaders in between. To not have slipping the wires through the holes in the spreader tubes I bent the wires a bit. See pictures below. A probably better way is to use a kind of thickening of the wire (using a luster or screw terminal over the wire). The top spreader tube will be about 10 cm under the end of the wire. The top of the wire can be fixed the same way on the top spreader as on the bottom spreader. When the center tube is very flexible it is probably wise to use some king of flexible material so the wires can slide through the holes in the spreader tubes and only fixed to the spreaders at one point. In this case the wire should have a flexible connection to the top or bottom spreader tube and fixed to the other top or bottom spreader tube.

Front side


As PVC is used no additional insulators are needed with low to medium power. The top of the wires will have the highest voltage and might need better insulation with high power.

Feed point and Balun
This was the first test and no balun was available… Of course the antenna needs a real 1:1 balun/choke to isolate the antenna from the coax cable. The antenna wires going up are all attached to one side of the balun which has to be the middle pin of the coax cable. All radials are attached together to the other side of the balun and attached to the shield of the coax cable.

The balun used is of the type W1JR using RG303 Teflon coax on a RK4 ferrite core bought from the DARC Verlag. This ferrite core is cheap and has good specs. Below two versions of the baluns I use.

First try feed point.. 

Balun with connectors on the sides. 

Balun with N-connector in and out.

Wire length
The length of the wires can be easily calculated using the formula for a quarter wave dipole which is 75 / frequency. Most ften we can take of 5 percent making the formaula (75 * .95) / frequency = 71.25/freq (per side of the antenna. Starting a bit longer will never hurt. Always start with the longest wire (lowest frequency) and then go back (from lowest frequency to the highest frequency). The wires are closely spaced and will interact. So some tweaking is needed to get it working for all bands. The wire I use is 0.8 mm aluminum wire without insulation (cheap buy).

The mast length used has to be at least half to one meter longer than the longest wire length as a minimum but longer is always OK.

The radials have (about) the same length of the radiator wire for that band. Each band uses one radial (as a minimum). The easiest is to place the mast on a pole and slope the radials to all sides to ground poles/pins (of course isolated to ground). Also the radials need one or two spreaders.

The mast should not be to flexible. As it bends, the centre mast will not change length much. The wires on the sides sticking out of the centre mast will become longer or shorter when the mast bends. So guy the mast or make it rigid.

Alternative antenna using double height.
An alternative is to double the mast length and also use spreaders for the radials below the feed point. Using this setup the coax cable has to be directed away from the feed point at a 90 degree angle. An extra PVC pipe could be used to have the coax extended out of the mast for 50 cm (or more) and then slope the coax to ground at a 45 degree angle.

Example Antennas

For 20 and 15 meter at 4.95 and 3.28 for the ends of the wires and at 20 cm. For 10 and 6 meter at 2.43 and 1.32 for the ends of the wires and at 20 cm. Plus two radial spreaders.

The 17 and 15 meter wires are using the same spreaders. The 12 and 10 meter spreaders also use the same spreaders.

For 40 and 20 meter at 10.0 and 4.95 for the ends of the wires and at 20 cm. For 15 and 10 meter at 3.28 and 2.43 for the ends of the wires and at 20 cm. Plus two radial spreaders.

The 80 meter wire will be used as inverted L and will extend from the top of the mast at around 11 meters away from the mast for the additional length of the wire. An insulator at the end should be used. As the mast now will probably bend to the direction of the 80 meter wire 1 (or better 2) wires could be used to keep the mast straight. With a length of 11 meters for the mast one or more additional guy wire could be used.

Many more antenna configurations are of course possible.

Below a picture of the antenna on the back of our barn. It’s on a pipe and the radials are spread out over the flat roof.

1 comment:

Anonymous said...

Gray PVC pipe is made with graphite, and graphite absorbs RF. Not a good choice.
will it work yes.

73 Shawn, N3TEE