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.
The feed point impedance of a full wave loop antenna is theoretically in the vicinity of 120 ohms and requires a 2:1 impedance transformer (for single band loops or a 4:1 balun to match on multiple bands) with 50 ohm line. You will also need a feed line choke or better yet, just get the Hybrid 4:1+1:1 in a single box.
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 or 4:1 for multi-band operation.
SABRE 50:100 (2:1) CUBE™ Balun (1.8-31 MHz), 2KW Top Studs Output
SPECIAL ORDER SABRE 2:1 CUBE Balun™ – (1.8-30 MHz)
(Original product part # was 770013S - circa 1999)
Model SABRE-2-2000 is a 2:1 (100:50) quad core balun used to translate a 50 ohm input to 100 ohms at RF power levels up to 2000 watts PEP when used with a matched load. This assembled balun features a quad core configuration for increased power handling and flatter frequency response to 31 MHz. With a matched 100 ohm load the SWR over the frequency range 1.8 to 30 MHz is shown in the following table measured with an AIM 4170 network analyzer. Notice that the conversion from 100 to 50 ohms is very consistent from 1.8-31 Mhz and SWR is less than 1.5:1 over the range allowing easy tuning by most radio internal antenna tuners or an external tuner. Graph is for a 2:1 balun often used with delta loop, log periodic, horizontal loop antennas and HF Radar.
Input connector is 50 ohm type N (SO-239 also available - inquire).SABRE-2-2000 is in a NEMA 4x4x2 weatherproof box with mounting flanges at bottom to attach to fixed location (mast, wall, beam antenna, etc.).
Eye bolts and studs are all stainless steel for long life.