Loop antennas may be constructed in many forms including horizontal full wave loops in square, rectangle or triangle (delta) shapes. They can also be in the vertical plane and are most often in the same shapes with the delta being very popular as it has both vertical and horizontal polarization. The impedance at the loop resonant frequency is approximately 100 ohms but will very slightly on harmonics. Loops are “quiet” antennas compared to verticals and dipoles and are omni-directional. The also exhibit gain on harmonic bands. Once you try a loop antenna you will know why they are so popular with old timers but still a secret to newcomers. Shown below are some typical designs:
Full Wave Horizontal Loop Antenna (a.k.a Skyloop)
This antenna is horizontally polarized and should be mounted as high as possible but works well at low heights of 10-30 feet. They are quieter than a dipole or a vertical, have a broader bandwidth and will usually out perform a dipole antenna.To determine the approximate circumference in feet of a full wave loop antenna use the formula:
1005/Freq in Mhz = length in feet.
Here are the dimensions for various amateur radio frequencies. Divide circumference by 3 for equal sided delta or by 4 for equal side square to determine each side length (e.g. 50.25 Mhz square loop is 5 feet per side):
|Frequency (mhz)||Circumference (feet)|
The feed point impedance of a full wave loop antenna is theoretically in the vicinity of 100 ohms and requires a 2:1 impedance transformer to match with 50 ohm line or a 1.5:1 transformer to match with 75 ohm cable. These transformers are unbalanced (coax input) to balanced (loop output) (also called 2:1 baluns). You can connect the balun at any place on the horizontal loop but a connection point close to a support is the most convenient. a 2:1 balun will work also allow the antenna to work on harmonics with a manageable SWR.
Any shape loop will work – octagon, pentagon, square, triangle, circle, trapezoid, etc. The larger the area or aperture inside the loop the better the radiation/reception/gain. A circle has 1dbd gain over a square which has a 1db gain over a triangle. Most people use a square but if you only have 3 supports you can shape it like a triangle. Triangle loops are called Delta loops. If you use a delta make each leg an equal length as this gives the largest inside aperture or area.
Vertical Delta Loops
Vertical delta loops can be oriented several way but the most popular is to have the “pointy” end at the top (usually a single support) and the lower horizontal ends just out of reach of humans and animals. Best feed point is 1/4 wavelength (246/f(mhz)) from the top point down one side. Vertical delta loops use the same 2:1 baluns as the horizontal loops.
To reduce feed line loss for multi-band use, use ladder line feed line and a 4:1 ladder line to coax balun to connect to your antenna tuner.