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.
After many requests we have created a compact, special balun for OCF antennas with vertical radiators. The CB-4-1500ODCV is a CUBE™ enclosure containing a dual core, bifilar wound, voltage balun to force equal voltages on the unequal sides of the off center fed antenna and force current down to the vertical radiator so it acts like a vertical antenna adding to the radiation pattern at low angles for DX.
The vertical radiator is typically 10 feet of 50 ohm coax for the 40 meter long (67 feet at the top) and 22 feet long for the 135 foot OCF.At the bottom of the vertical radiator you should have a line isolator or 1:1 balun (such as the Palomar BA-8E or BA-58E sleeve balun, or CB-1-1500 cube balun) to stop the current on the outside braid of the coax from using the rest of your feedline back to the radio as a long vertical radiator causing RFI and other problems as well as skewing the radiation pattern. The balun can be used with any size.
The balun is rated for full 1500 watts PEP from 1.8Mhz – 61 Mhz and usable to over 150 MHZ! (see graph) The balun can be used with either a 50 or 75 ohm coax vertical radiator and feedline.
The matching line isolator is the MC-1-3000 which should be used at the end of the vertical radiator.
