A 17M Delta Loop Antenna
I have been a fan of loop antennas ever since I built my first one; a 40M square full-wave loop. Full wave loops radiate broadside to the plane of the loop, and are often mounted vertically. They can be configured for vertical or horizontal polarization depending on where the antenna is fed. A square full-wave loop will have an impedance of around 100 ohms, which can easily be matched to 50 ohm cable by means of a quarter wave coaxial transformer made of 75 ohm cable. However, if fed with 300 or 450 ohm balanced line, they can be operated on harmonic frequencies. At higher frequencies, multiple lobes will develop.
A Delta Loop is a variation on the square loop that is configured in the shape of a triangle. The included angle can vary, but the more you "squash" the loop, the smaller the enclosed area and hence, the lower the gain. I opted for a delta in the shape of an isosceles triangle so I could hang it from a single high support using a tree branch. I fed it 1/4 wavelength from one of the lower corners, which results in vertical polarization. The loop length is typically 1005/f (MHz) or in this case, about 55-1/2 feet. I used #12 THHN insulated stranded wire from Lowe's, so the loop ended up wanting to be a few inches shorter to resonate due to the velocity factor resulting from the insulation. I used a Guanella type current balun, which I recommend for any balanced, resonant antenna.
I was only able to get the top of the loop about 25 feet high, which meant that the bottom corners were only about 6 feet off the ground. I discovered that this was not ideal, and the antenna did not perform significantly different or better than the ground mounted vertical. It also lacked the benefit of the vertical antenna's ground radial system, which reduces losses in the ground near the antenna. Finally, the match was not that good even after I inserted the standard 1/4-wave coaxial impedance transformer. The best I got was around 2:1.
Next, I decided to try horizontal polarization. You can feed the antenna at one of two points for this; either at the top (the apex of the triangle) or else in the center of the bottom leg. I connected mine there because it was easiest. As it turned out, horizontal polarization was not as good on the stations I heard. The problem with using horizontal polarization for a low loop is that the maximum radiation goes up at a pretty high angle, which is not very useful for communication on this band.
After reading some additional articles on delta loop antennas, one statement stuck in my mind: If you can put up a delta loop, put up a horizontal dipole instead because the radiation center will be higher. For this type of antenna, higher is definitely preferred. That led to my next antenna project, a 17M dipole.
A Delta Loop is a variation on the square loop that is configured in the shape of a triangle. The included angle can vary, but the more you "squash" the loop, the smaller the enclosed area and hence, the lower the gain. I opted for a delta in the shape of an isosceles triangle so I could hang it from a single high support using a tree branch. I fed it 1/4 wavelength from one of the lower corners, which results in vertical polarization. The loop length is typically 1005/f (MHz) or in this case, about 55-1/2 feet. I used #12 THHN insulated stranded wire from Lowe's, so the loop ended up wanting to be a few inches shorter to resonate due to the velocity factor resulting from the insulation. I used a Guanella type current balun, which I recommend for any balanced, resonant antenna.
I was only able to get the top of the loop about 25 feet high, which meant that the bottom corners were only about 6 feet off the ground. I discovered that this was not ideal, and the antenna did not perform significantly different or better than the ground mounted vertical. It also lacked the benefit of the vertical antenna's ground radial system, which reduces losses in the ground near the antenna. Finally, the match was not that good even after I inserted the standard 1/4-wave coaxial impedance transformer. The best I got was around 2:1.
Next, I decided to try horizontal polarization. You can feed the antenna at one of two points for this; either at the top (the apex of the triangle) or else in the center of the bottom leg. I connected mine there because it was easiest. As it turned out, horizontal polarization was not as good on the stations I heard. The problem with using horizontal polarization for a low loop is that the maximum radiation goes up at a pretty high angle, which is not very useful for communication on this band.
After reading some additional articles on delta loop antennas, one statement stuck in my mind: If you can put up a delta loop, put up a horizontal dipole instead because the radiation center will be higher. For this type of antenna, higher is definitely preferred. That led to my next antenna project, a 17M dipole.