- The KC5LDO Vertical Antenna - |
Here's a simple yet rugged six band 1/4 wave vertical antenna system perfect for field day or home use. The only tool required for assembly is a screwdriver which is used to adjust the vertical element. The rest is assembled using wingnuts and carriage bolts. Assembly can be done by one person including sixteen radials in about thirty minutes. The majority of the project was built with aluminum materials salvaged from the scrape pile at work. Aluminum makes the antenna light weight and easy to transport.
The dimensions given are for my antenna and slight variances should not matter. I like to be creative with the materials I have on hand. The base is 1/4 inch thick aluminum plate 10 1/4 by 11 1/4 inches. I used a dinner plate to mark a circle on the base plate. I then drilled sixteen holes around the circle to accommodate one inch long 8/32 screws and nuts. These use wing nuts to attach the radials. Use a minimum of four radials per band. I used eight radials for 20m., four for 10m. and four for 15m. Of course you could use sixteen radials on one band or even more if desired.
The four aluminum legs that attach to the base are 1 1/2 in. square tubing, 54 in. long. They are attached to the base with 5/16 in. carriage bolts and wing nuts. Hole spacing is six inches. The leveling system at the end of each leg is quite simple and allows the radiating element to be kept vertical. I used 7/16 inch all thread rod which comes in three foot sections from most hardware stores. This was cut into four pieces. For ground contact, a large washer is attached to one of the all thread. The ends of the legs are then tapped to accept the all thread. This will allow each leg to be adjusted independently. NOTE OF CAUTION: Not shown in the photos are protective coverings over the ends of these legs. Cover the ends of each rod with something such as a doorknob or any similar, large, blunt covering. The purpose would be to prevent injury to a person by a possible fall onto these leveling rods.
The vertical element is attached to the base plate by using an aluminum L bracket. The L bracket dimensions are as follows, 1/4 inch thick, 14 inches long by 4 inches wide. This was bent 90 degrees, five inches from one end using a hydraulic press to form a L. Four bolts attach it to the base. Two 2 3/4 inch antenna mast clamps purchased from Radio Shack are used to secure the vertical element to the L bracket. The clamps also use wing nuts. A small aluminum L bracket is used to mount the SO239 connector. This bracket should have a good ground connection to the base. I used 1/8 inch wide wire braid to connect the center of the SO239 to the vertical element.
The vertical element is made using three 8 ft. sections of aluminum tubing. Use three diameters that will telescope into each other. My sizes are 1 in., 7/8 in. and 3/4 inch. These sizes are strong enough not to require guying. The two lower sections must have one end slotted. A hack saw works great and only two slots are needed on each section. This will allow you to adjust the antenna length to be resonant to a band of choice. A hose clamp is then used to secure the sections together. A seven inch piece of 1 inch I.D. PVC pipe was used for the insulator. This must be slotted along its length to allow it to squeeze tightly around the tubing. A 6/32 screw and nut was installed at the bottom of the tubing for connection to the SO239. A wing nut was also used here. To determine the proper element and radial length use the following formula: L=234/freq. L is the length of the element or radial and freq. is the frequency in MHz.
The antenna project took about fifteen hours to build and cash outlay was about $30. Most of this was spent on aluminum tubing. Square tubing is not cheap and angle iron could be substituted for the legs. The wire for my radials came from a surplus wiring harness that was given to me. It looks to be about 20 ga. As an after thought, I put eyelets on the ends of the radials and made L pieces out of coat hanger wire. These are pushed into the ground and the radials kept in a line.
I found that using the formula to calculate element length will get your SWR very
close to perfect. Near by objects and ground conductivity may make it necessary to
vary your length a few inches either way. I had to make adjustments 1/4 in. at a time.
The tubing diameters used give an acceptable band width on bands 20 thur 6 meters.
Till then, 73 and good DX! James Tobola - KC5LDO Articles written by James Tobola - KC5LDO
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