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 Watt 4:1/6:1/9:1 CUBE™ Balun – Model CB-4-9-1500
Model CB-4-9-1500 is a 4:1 and a 9:1 balanced output (balun) housed in a Nema enclosure box (4″ x 4″ x 2″) with a top eyebolt halyard hoist, two side studs for 4:1 and two additional side studs for 9:1 outputs, each capable of input power levels up to 1.5 KW PEP when used with a matched load. The 4:1 ratio takes 50 ohms input and matches to 200 ohms output and the 9:1 ratio takes 50 ohm input and matches to 450 ohms output between 1-61 MHz under 2:1 SWR under matched load conditions.
Now you don’t need to buy two separate baluns or guess which balun to use - you get the 4:1 and the the 9:1 in a single package! – just buy the CB-4-9-1500 and pick the output ratio that best matches your antenna tuner! (Note: you can only use 1 set of outputs at a time)
Application note: If you use 1 of the 4:1 and 1 of the 9:1 terminals on opposite sides, then the output wlll be 300 ohms or a 6:1 balun!
