Space is the final frontier for most ham radio operators when it comes to low band antenna systems. The shortest wire to easily put RF energy into is a 1/4 wave fed with unbalanced feed line (coax) and worked against an earth ground. This is the basic principal behind most commercial vertical antennas, that often become the solution to the SPACE problem for a reasonable antenna on 80 meters. If you have a small patch of land to provide an earth ground, and if you can get one end of a wire secured at about 25-30 feet off the ground, you can construct your own custom solution.
If you live on a farm and have lots of time and money to spend on your low band antennas, congratulations... For everyone else, an antenna on the 80 meter band will be a major compromise compared to what we might have for other bands like 10 meters. Fortunately, that fact tends to level the playing field for most hams on 80 meters. Any good CW or digital mode urban operator knows that 50 watts into a system to get 10 watts of effective radiated power will still provide satisfactory operation. If you can accept this reality, then you will soon be joining the fun on 80 meters... Here's how:
|Quarter Wave Sloper|
One common method of using a 1/4 wave end fed wire is to simply support it at one end to provide a 45 degree (or greater) angle above the earth. EZNEC modeling shows that this technique will provide an omni directional pattern, but with some "warping" in the direction of the wire slope. A small 3 dB "front to back" effect is seen, with slightly more gain in the direction opposite the slope. The effect is not very significant and the wire is basically a vertical antenna.
Another popular way to configure a 1/4 wave end fed wire is as an inverted L shape. This was the method I used and it had the advantage for me of raising the wire above the back yard to prevent interference with human activity. Also, by keeping the low end of the wire vertical, the pattern produced is more like that of a vertical antenna. I used a branch in the tree to support the first 16 feet and I tied the end of the antenna to another tree using nylon string.
For several years, before I added 80 meters to my NB6Zep Antenna, I operated that band with a center loaded wire antenna. Those were the days before I discovered antenna computer modeling as an alternative to the cut-n-try methods. I started by pounding a 8 foot ground rod into damp earth at a strategic spot about 5 feet from the base of a tree in my back yard. The limbs of the tree became supports for several experimental antennas used from the 160 to 30 meter bands. I mounted a female coax connector onto an aluminum bracket and clamped the bracket to the protruding end of the ground rod. I soldered my antenna wires to the connector and used tape and sealant around the coax. I had the best results with a straight 33 foot long wire on 40 meters and a similar length of wire center loaded on 80 meters. The center loaded wire I had for 160 meters required to much maintenance, so I did not repair it after it blew down the last time...
The physical length of an 80 meter 1/4 wave wire is approximately 66 feet, depending on what part of the band it is being cut to. To make this physically shorter, while keeping it electrically a 1/4 wave length, we add inductance to the wire. Traditionally this is accomplished with a coil near the bottom end of the wire, where current is highest (impedances is lowest). Bottom loading has the greatest effect on inductance and the coil can be made the smallest in size. It would be better to move the inductance coil away from where the antenna currents are highest, because this is the spot on the wire where most of the magnetic energy (RF) is produced. Moving the coil away from the end will require the inductance to be greater and therefor the coil to be larger. I determined that 2 coils placed in the bottom half of the wire would be more practical than 1 large coil.