My 160 Meter Loop Antenna
Mick Lindley, KB4UPI
I had wished to put up a loop antenna for 160 meters for many years but I never had the room. I finally moved to a location with 2.5 acres so I decided to take the plunge. I hope my experience will help you out if you decide to do it. The antenna takes up a lot of real estate. If you made it a perfect circle the diameter would be 167 feet. This explains why the antenna is not in use much by radio amateurs.
This antenna is simple. One end of the loop connects to the center conductor and the other to the shield if you use coax. You should use a 4 to 1 balun for a better match. The loop will connect to each terminal on the top of the balun. It connects to each side of ladder line if you decide to use ladder line. It is a direct short at DC, but remember RF is not DC, RF is AC.
Using the formula 1005/freq in Mhz you get 529 feet total length for a freq of 1.9 Mhz. Use stranded or solid copper wire of 12 or 10 gauge preferably black or a dark color. Look for wire at garage or estate sales. I found a 500 ft roll of #10 solid insulated copper wire at an estate sale for $10. I had heard for years that if using insulated wire you must shorten your wire length by 3 to 5 %. I always thought this was foolish but I was proved wrong. After getting my antenna up it was resonate at 1.77 Mhz. I ended up cutting it off to 509 feet. Now it is resonant at 1.92 Mhz. SWR on the bottom of 160 is 1.8 and at the top is is 1.7. In the middle it is 1.2. This gives a good match and avoids having to use a tuner. I like resonant antennas so it was important to me to get the length right. Other factors can also affect the resonance of the antenna like height above ground or nearby metal objects. The antenna does not seem to be affected by trees.
The picture below represents a general layout of my antenna. The small circles represent trees. The shape is not all that important. Use whatever trees or other supports are there. You should try to encompass as much area as possible with your circle, oval, square, rectangle, etc.. You should also get it up as high as reasonably possible. Mine is 15 to 25 feet and works very well with good SWR. I used ultraviolet resistant rope at the tie points and made a ring from a 8 inch length of wire. After tying this onto the end of my insulators I ran the antenna wire through it. This allows the antenna wire to slide through it when the trees sway in the wind. The other end of the insulators gets the rope. The rope continues to the trees.
Since I don't like ladder line I decided to feed the antenna with 50 ohm coax. It is a terrible mismatch with the antenna being approximately 100 ohms. I obtained a 4 to 1 balun and placed it at the tie point and all is OK. My feedline is 21 feet long.
I hope this has been helpful. 73 de Mick, KB4UPI