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 Rated 1.5:1 CUBE™ Unun
Purpose. The CU-1.5-1500 (1500 watts PEP) or CU-1.5-5000 (5KW PEP) is used as a 1:1 unun(aka feed line choke, line isolator) for 50 ohm unbalanced coax cable input to 75 ohm unbalanced coax output at full amateur radio legal power (1500/5000-watts PEP) into a matched load 50 ohm load for 1.8-61 MHz (or a balanced output load for the CB-1.5-1500/5000). The unun (or balun) is designed to have a high RF impedance to common mode currents on the outside of the coax braid while passing RF currents on the inside of the coax braid to the antenna.
For maximum common mode current protection (and receiver noise floor reduction due to common mode current pickup by the coaxial braid), use one feed line choke such as the CU-1-1500 (for 50 ohms coax) or CU-1.5-1500/5000 (for 75 ohm coax) at the antenna and another at the radio shack equipment connection or input of antenna tuner.In some difficult RFI cases, additional common mode chokes are needed between equipment such as linear amps and antenna tuners, and transceivers.
This Unun or Balun designed to work with 75 ohm loads and the Unun/Balun will be damaged if you use at high impedances and high power such as driving a 40 meter antenna with an 80 meter transmitter signal where the load impedance on the antenna is very high.Best use is with antennas having a load impedance near 75 ohm range.