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.
Ladder Line to coax transformers
Ladder Line baluns are used to match the impedance seen at the end of the ladder line to a coaxial cable, usually 50 ohms, but a ladder line balun can also be used to match to 75 ohm coax like RG-6/U used for cable and satellite installations. While RG-6 doesn’t have the same power rating as larger cable, it is very useful for beverage antennas or receiving antennas which don’t carry a lot of power.
The impedance seen at the end of the ladder line will not be the same as the characteristic impedance of the ladder line (typically 300 or 450 ohms). Good ladder line antenna designs try to have the impedance at the end of the ladder line attachment to the balun close to 200 ohms so that a 4:1 balun can be used to match to 50 ohm coax.
A typical ladder line antenna is shown in the picture below:
Palomar Engineers has ladder line to coax 4:1 baluns in both kit form and also assembled in a variety of 1 watt to 10KW power levels to suit the needs of many customers. The ladder line baluns are designed for balanced output and are generally used at the end of a run of ladder line from the antenna. The baluns connect to the ladder line and then transition to coax cable to you antenna tuner or radio station.
