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.
1500/3000 Watt 6:1 CUBE Balun™ – Model CB-6-1500/3000
Model CB-6-1500/3000 is a 6:1 (300:50) balun housed in a NEMA enclosure box (1.5KW in 4″ x 4″ x 2″, 3KW in 6" x 6" x 4") with two top studs capable of input power levels up to 1.5KW PEP watts when used with a matched load. The 6:1 ratio takes 50 ohms input and matches to 300 ohms balanced output from 1-61 MHz with SWR < 1.5:1 under matched load conditions. A great balun for windoms, OCF antennas >60 feet over ground, Hertz and folded dipole antennas, T2FD etc. 8 core, Use “EB” version with side eye bolts for wire attachments for OCF dipole use.
Typical SWR and impedance transformation from 1-61 MHZ shown below:
This transformer requires a feed line choke to isolate the coax feed line from the antenna. A coax noise filter is also recommended.
