An 80 Meter Horizontally Polarized Full Wave Loop Antenna
Where I live, there are many trees that often get in the way of antennas and antenna towers. However, they can also be very useful as antenna supports. Branches are one of the main problems to be dealt with. I have found that late fall (after leaves have dropped) and early spring (before they have reappeared) are the easiest times of the year to put up wire antennas hung from tree branches, because you can more easily see where to route them.
One of this summer's projects was to put up a full-wave 80 meter horizontal loop antenna, sometimes called a "loop skywire". This type of antenna radiates most of its signal at very high angles, which is perfect for using Near Vertical Incidence Skywave (NVIS) propagation to contact stations up to about 500 miles away on the 80 meter band. My preferred wire for this type of project is #14 insulated stranded wire. The insulation is great for maintaining antenna performance if your antenna happens to contact a tree branch. It also prevents oxidation of the wire. However, you could use just about any kind of wire as long as it is strong enough to hold up, but I would not go much smaller than #14 because you are going to have some pretty long spans with this antenna. DX Engineering sells good quality #14 stranded wire, and so does The Wireman ("Certified Quality"), but the biggest bargain is a 500 foot spool of #14 THHN insulated stranded from either Lowe's or Home Depot, which runs around $33 currently. You are going to need around 266 feet; the exact length is not terribly critical (insulation makes the wire appear electrically longer). Some authors criticize the use of THHN wire because it has a thin clear plastic coating that eventually peels off, but I have not found this to be a problem even when it happens. So if you want to save money, just buy THHN and don't worry about it. If it breaks or gets damaged, it's inexpensive to replace.
You can feed this antenna directly with coax if you add a quarter wave section of 75 ohm line (e.g. RG-59 coax) at the antenna to transform the feedpoint impedance from ~100 ohms down to 50, but I don't recommend this because if you use balanced line, you can operate this antenna on just about any higher band. I used 300 ohm "window" line from DX Engineering for mine. The line can be almost any length that is convenient. You have to be careful not to bend it sharply or run it near metal objects. I mounted a good quality 1:1 ATU balun from Balun Designs just outside the shack and ran a short length of Belden #9913 coax inside to my tuner. You should disconnect the antenna from the balun whenever it is not in use and particularly when any storms are approaching, to prevent damage to your house and equipment. The Standing Wave Ratio (SWR) on the 300 ohm line and the short coax run could be very high, but it will not cause much additional loss because the matched loss of these lines is already very low. The only potential issue is the situation where your antenna tuner is just not able to cope with the impedance it sees. In this case, either add or subtract some length of 300 ohm line or else use a different tuner.
Ideally you should mount this antenna about 1/4 wavelength or 20 meters (66 feet) above ground for maximum gain, but I was unable to do this and had to settle for just 15-20 feet. Nevertheless, it works fine for my purposes. I am probably losing a few dB gain due to ground losses due to the low height. The ground under my antenna is sandy, so it has very poor conductivity. If you have good ground, you basically have a 2 element beam pointed straight up with this antenna.
You can configure this type of antenna in just about any available shape-- round, square, rectangular, triangular, or what have you. It has the most gain when the enclosed area is maximized. Some authors prefer a triangular shape fed at one of the corners because according to computer models, it appears to have a better omnidirectional pattern at higher frequencies. In my case, the most convenient shape was a rectangle fed at approximately the middle of one of the long sides. Not optimum, perhaps, but it works. If you have a choice, it is recommended to feed at a corner, which also saves you one support point.
One of the best things about this antenna is that as you go higher in frequency, the peak radiation angle drops, making it useful for long distance communications. This is exactly what you normally want. I have tested mine successfully on 40, 20, and 17 meters so far and had no problems. I would not expect this low antenna to be competitive against a Yagi at 70 feet, but it definitely gets out.
This is a fun antenna to build and if you have all of the materials you could easily put it up in an hour or two. I estimate the cost to be under $50 if you already have a suitable balun; otherwise you'll need to factor that into the budget.
One of this summer's projects was to put up a full-wave 80 meter horizontal loop antenna, sometimes called a "loop skywire". This type of antenna radiates most of its signal at very high angles, which is perfect for using Near Vertical Incidence Skywave (NVIS) propagation to contact stations up to about 500 miles away on the 80 meter band. My preferred wire for this type of project is #14 insulated stranded wire. The insulation is great for maintaining antenna performance if your antenna happens to contact a tree branch. It also prevents oxidation of the wire. However, you could use just about any kind of wire as long as it is strong enough to hold up, but I would not go much smaller than #14 because you are going to have some pretty long spans with this antenna. DX Engineering sells good quality #14 stranded wire, and so does The Wireman ("Certified Quality"), but the biggest bargain is a 500 foot spool of #14 THHN insulated stranded from either Lowe's or Home Depot, which runs around $33 currently. You are going to need around 266 feet; the exact length is not terribly critical (insulation makes the wire appear electrically longer). Some authors criticize the use of THHN wire because it has a thin clear plastic coating that eventually peels off, but I have not found this to be a problem even when it happens. So if you want to save money, just buy THHN and don't worry about it. If it breaks or gets damaged, it's inexpensive to replace.
You can feed this antenna directly with coax if you add a quarter wave section of 75 ohm line (e.g. RG-59 coax) at the antenna to transform the feedpoint impedance from ~100 ohms down to 50, but I don't recommend this because if you use balanced line, you can operate this antenna on just about any higher band. I used 300 ohm "window" line from DX Engineering for mine. The line can be almost any length that is convenient. You have to be careful not to bend it sharply or run it near metal objects. I mounted a good quality 1:1 ATU balun from Balun Designs just outside the shack and ran a short length of Belden #9913 coax inside to my tuner. You should disconnect the antenna from the balun whenever it is not in use and particularly when any storms are approaching, to prevent damage to your house and equipment. The Standing Wave Ratio (SWR) on the 300 ohm line and the short coax run could be very high, but it will not cause much additional loss because the matched loss of these lines is already very low. The only potential issue is the situation where your antenna tuner is just not able to cope with the impedance it sees. In this case, either add or subtract some length of 300 ohm line or else use a different tuner.
Ideally you should mount this antenna about 1/4 wavelength or 20 meters (66 feet) above ground for maximum gain, but I was unable to do this and had to settle for just 15-20 feet. Nevertheless, it works fine for my purposes. I am probably losing a few dB gain due to ground losses due to the low height. The ground under my antenna is sandy, so it has very poor conductivity. If you have good ground, you basically have a 2 element beam pointed straight up with this antenna.
You can configure this type of antenna in just about any available shape-- round, square, rectangular, triangular, or what have you. It has the most gain when the enclosed area is maximized. Some authors prefer a triangular shape fed at one of the corners because according to computer models, it appears to have a better omnidirectional pattern at higher frequencies. In my case, the most convenient shape was a rectangle fed at approximately the middle of one of the long sides. Not optimum, perhaps, but it works. If you have a choice, it is recommended to feed at a corner, which also saves you one support point.
One of the best things about this antenna is that as you go higher in frequency, the peak radiation angle drops, making it useful for long distance communications. This is exactly what you normally want. I have tested mine successfully on 40, 20, and 17 meters so far and had no problems. I would not expect this low antenna to be competitive against a Yagi at 70 feet, but it definitely gets out.
This is a fun antenna to build and if you have all of the materials you could easily put it up in an hour or two. I estimate the cost to be under $50 if you already have a suitable balun; otherwise you'll need to factor that into the budget.