Ron Haltermon WA4HWN For Ham Radio Community Chapter
There is nothing mysterious about the Sky Loop Antenna. The main draw back is the necessity of having sufficient real estate, particularly for 160 meters, and the means of supporting this configuration. The advantages include reduced background noise (in the horizontal configuration), gain over a reference dipole and low cost to acquire, build and maintain such an antenna. Feeding the antenna is extremely simple, although somewhat controversial.
The sky loop is a full wave wire antenna using #14 or #12 wire. I have also used, for the higher bands, (10 and 17 meters) co-axial cable, such as RG-58 or RG-8 mini-foam. If the antenna is cut for 160 meters, then operation on the harmonic bancs (80, 40, 20 and 10 meters should be possible. With a tuner, (highly desireable) operation on any band from 160 through 2 meters is possible.
The formula for determining the length of the loop is 1005 divided by the frequency in Mhz. The result is the length in feet. For example, if you want the antenna to be resonant on the 160 meter band at 1.9 Mhz, the length is: 1005/1.9 or 528.94736 feet. The actual length is not particularly critical. (Mine is 530 feet.) For those of you who wish to build a multi-element 40-10 meter quad antenna (rotatable), the formula for the reflector is 1030 divided by the frequency in Mhz and the director is 975 divided by the frequency in Mhz.
Anyhow, back to the sky loop. Ideally, the antenna should be at least 40 feet off the ground. Remember the usual rule is an antenna should be located at least 1/4 wave length above ground at the lowest operating frequency. Higher is always better! However, at 30 feet, or ever lower, in height, the ground wave out some 500 miles, or so, will be noticeably strong. Beyond that the antenna will act much like a dipole.
Generally the gain of a loop antenna is directly proportional to the area enclosed by the loop. In other words, the best configuration would be a circle. This would provide slightly more gain than a square shape. The square shape is slightly greater, in gain, than that of a triangle (delta). The input impedance of a circular antenna is approximately 160 ohms. This antenna will provide a gain of approximately 1.18 db over a reference dipole. A square configuration gives .9 db gain and an input impedance of about 120 ohms. The triangle is .6 db gain and a 100 ohm feed point impedance (***See footnote). The highest gain results in vertically orienting the antenna but bandwidth is poor and the input impedance is low. Obviously, the noise level is best with the horizontal configuration.
Most sky loops are in the square configuration due to the physical requirements in erecting a circular supporting system. If you have property with a lot of trees, such as I do, then the circular pattern is more easily approached. Rectangular configurations are also good but are best suspended vertically. I have a 10 meters sky loop constructed of co-ax that is vertically suspended at a very low height with the bottom being only 10 feet off the ground. It works extremely well. The swr is less than 1.4 to 1 across the entire 10 meter band with about 260 Khz in the middle of the band being absolutely flat.
Some recommend the use of stripped, or bare wire due to the added weight of insulation. Others recommend the use of insulated wire. In my case, I am using trees to support the wire and therefor am using insulated #14 wire. The insulation is a pretty heavy, good quality rubber insulation. Teflon coated, stranded, wire is excellent, too.
The feed point can be anywhere along the wire, but is normally made near one of the supports simply to lend stability to the antenna system. A loop is a loop, so anywhere you "break" it for the feed point, it's still a loop! The use of a balun is controversial. Some argue that the feed point impedance must be compensated for through the installation of a balun. Others argue that the installation of a balun will result in the radiation of undesirable harmonics and the loss of power. Tests that we ran way back when I was in college at the antenna laboratory at O.S.U. resulted in the determination that directly feeding the loop with 50 ohm co-ax was most desireable. A quarter wave length of 75 ohm co-ax may be inserted in the feed line for a closer match if you desire.
Directly feeding the loop antenna is obviously the simplest, however, you be your own judge and feel free to try it both ways. All 3 of my loops are directly fed with 50 ohm co-ax. My 17 and 10 meter loops are both vertically suspended and are relatively in the clear but low to the ground.
My 160 is definitely not in the clear but is horizontally suspended using various trees. The feed point is only about 18 feet high. The antenna varies from that point, in heights ranging from 20 to 44 feet with the average being an estimated 25 feet. It surely does work. My 40 meter loop is in the clear and is suspended at aproximately 40 feet up. The reports that I have been getting are extremely encouraging both in the 100 to 1,000 mile range as well as dx. I run only 100 to 120 watts, even on 160 meters.
The antenna is supported by a plexiglass or plastic panel at the feed point on the 160 meter antenna and I made an interface out of pvc pipe and caps for the 40 meter loop. I purchased a clear plastic paint paddle which all ready had some small holes in it. I merely drilled 4 extra holes (see drawing) for the antenna ends and the co-ax. I soldered the co-ax shield to one end of the antenna and the center conductor to the other end. I drilled a hole in the top of the panel to fasten a rope through for hoisting up into the trees. If you use poles or masts to support your system, feed the antenna near one of the poles and run your feedline down the support rather that supporting it from above. Keep the feed point (and the entire antenna, for that matter) away from metal structures!
You can use these antennas in place of mass produced antennas which may cost you more money than you would want to spend. I found that building your own antennas beside saving you money even work better than ones you would buy retail. I have also designed some other types of antennas in my homebrew projects. I have designed a new way to put a small efficient stubby antenna on you hand held transciever, works better than the best on the market today. If you are interested in knowing how to do this just try this High Efficiency Stubby Antenna for seeing a written demonstration.
73 de Ron WA4HWN, and KD4SAI
** These figures were taken from Radio Handbook- Wm. Orr W6SAI - 22nd Edition.