Off-Center Fed HF Dipole Antenna at W5JGV
April 19, 2006
Because hurricane Katrina damaged all of the HF antennas at my QTH, I was faced with the problem of getting some sort of antenna in the air so that I could once again operate on the HF bands. Whatever I used, it had to be inexpensive, easy to construct, and easy to replace if this years hurricane season once again proved to be as damaging as the last one was.
Because I am planning to relocate my QTH, I did not want to erect an antenna that would cost very much or require too much work to take it back down again. I decided that I would install one of my favorite antennas, a simple wire dipole. My design criteria were simple - put as much wire up in the air as I could and make it as high as possible.
I still had the lower 18 feet of my tower - slightly bent, of course, but I figured that I could use it for the feedpoint of the new antenna. One of the 100 foot long 9913 feedlines that ran to the old TH3-jr beam was still available to feed the dipole at that point. I calculated that if I ran the dipole in a zigzag fashion limited by the boundaries of my property and where I could fasten the ends of the wire, I could manage to hang about 141 feet of wire. The feedpoint for the dipole would be slightly off-center, with one leg of the dipole being 57 feet long, and the other leg 84 feet in length.
I usually construct my wire antennas from 14 gauge THHN insulated copper house wire, but in this case, the weight of the wire itself was a consideration because I was planning on using simple unguyed support masts at several places to hold up the antenna. I decided that the best wire to use for this antenna was aluminum electric fence wire. I had recently obtained a mile of this wire in 17 gauge, so I was all set as far as the wire was concerned. The end and center insulators were fashioned from some thin Lexan plastic scrap that was left over from another project.
Because the average height of the antenna would be only 20 feet, I expected that most of my HF operation would be in the NVIS mode. As it turned out, the antenna works surprisingly well for mid-range DX as well as NVIS for close-in contacts from 0 to 300 miles from my QTH.
My initial attempt at feeding the antenna used a 1:1 Balun between the 9913 coax and the antenna. This resulted in a fairly good load between my Yaesu FT-747GX transceiver and the antenna, but I noticed that the reception on several of the bands seemed rather anemic. A session with EZNEC showed that the feedpoint impedance of the antenna was not a good match for the 1:1 Balun. It looked as though a 1:9 Balun might work better, although performance at frequencies below 5 MHz would suffer somewhat.
The final result after adding the 1:9 Balun is an antenna that exhibits a VSWR match ranging from 1.2:1 to 2.5:1 on the bands between 7 MHz and 30 MHz. On 160 and 75 Meters, the VSWR is worse, but I can still make contacts on those bands with the antenna without using a separate antenna tuner.
The remains of my tower which was damaged in hurricane Katrina now supports the feed point for my new off-center fed dipole antenna. The 1:9 Balun is mounted inside the plastic soft drink bottle seen above the antenna feed point.
The 1:9 Balun is constructed of two parallel lengths of RG-58 coaxial cable which have had their outer jackets removed. These cables are threaded 5 times through an FT-240-77 ferrite core. The high permeability of the 77 material core allows the use of a very small number of turns on the Balun with good low frequency performance. The small number of turns gives good high frequency performance.
The Balun was placed inside a plastic soft drink bottle. The side of the bottle was slit with a knife to allow inserting the Balun in the bottle. The bottle was them mounted with the slit facing down to keep water out of the assembly. Using the bottle as a weather shield eliminated the need to weatherproof the Balun itself.
The Balun and bottle assembly was fastened to a length of ABS plastic pipe that I had on hand. The 9913 feedline was fed through the neck of the bottle and connected to the coax connector of the Balun, then the assembly was wiggled through the slit and into the bottle. The wires from the Balun to the antenna pass downwards through the slit in the bottle. Note the very minimalistic plastic insulator used for the feedpoint of the antenna.
When the sun angle is just right, you can see the entire length of one side of the dipole. Most of the time, the wire is almost invisible from just a few feet away from it.
The 9913 coax cable passes over the top of what remains of the tower and along the ABS pipe that holds the Balun in a Bottle. The ABS pipe is attached to the tower by using some lengths of 14 gauge THHN insulated wire. The wire is simply twisted around the tower structure and the pipe. It lasts forever, never works loose, and is reusable.
To support the other side of the dipole, I grabbed some of the salvaged Dacron guy line from my old tower and lashed a 25 foot length of thin wall aluminum tube to a corner fence post. The aluminum tube was some salvage material from the 1984 World's Fair which was held in New Orleans. It was used as stair railing. If you were a visitor to the 1984 World's Fair, perhaps your hands have touched this very pipe!
I installed a rope and pulley system to haul a lead line up the mast so I could adjust the height of the antenna wire to clear some small branches from the Oak tree. This pipe supports the center of the antenna. This side of the dipole makes a right angle bend across my lot and ends at the other corner where another mast is installed.
This mast supports the far end of the antenna, and at 18 feet in length, is slightly shorter than the other mast. This mast is also is made from some salvaged 1984 World's Fair railing tubing. No guy lines are used for either of these masts, since the pull from the aluminum wire used for the antenna is very gentle. The antenna wire is visible in this picture; can you find it?
The end of the dipole is fastened to the mast with a simple plastic insulator.
As seen - or not - from about 25 feet away, the antenna is almost invisible. After a few days outside, the shiny finish of the aluminum wire weathers off, and is replaced by a dull gray color, rendering it almost invisible against the sky. So far, results using this inexpensive and quick to install antenna have been good.
73, Ralph W5JGV
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