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.
Model CB-9-5000 is a 9:1 multi-core, balanced output balun housed in a NEMA enclosure box (4″ x 4″ x 2″) with top studs (TS) used to translate a 50 ohm input up to 450 ohms at RF power levels up to 5000 watts PEP when used with a matched load. With matched 450 ohm load the SWR over the frequency range 1 to 31 MHz is less than 2:1 on the coax feed side.
All power rating are PEP, 100%, continuous digital modes are 30% duty factor rating of PEP, FT-8 digital mode rated at 1500 watts.
Application note: If using 450 ohm ladder line on the output of the balun top studs, make sure the impedance seen at the balun end of the ladder line is near 450 ohms since the ladder line acts as an impedance transformer, the impedance at the end of the ladder line may be different than the characteristic 450 ohms of the ladder line. For example a 1/4 wavelength of ladder line will transform a high impedance at the antenna feed point to a very low impedance at the other end connected to the balun thus placing a very low impedance load on a balun expecting 450 ohms and such a load may damage the balun. The impedance seen at the balun output should be near 450 ohms for full power rating.