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.
The “Bullet” End Fed Antenna Counterpoise Kit
Th kit contains 1 pre-assembled counterpoise wire which easily attaches to the counterpoise connector on the BULLET matching unit. Improves performance on 1 or more bands. Black/Gray/Blue wire is standard.
Application Note: This kit is typically used with only with Bullet Antenna systems like the 41 foot, 71 foot, 92 foot, 111 foot or 155 foot when you DO NOT USE the coax braid as the counterpoise (such as when you coax is too short). The counterpoise radiates just like the other half of a dipole so DO NOT GROUND IT! It is used to fine tune particular bands (by varying the length to get the lowest SWR reading on as many bands as possible. Multiple length counterpoises can also be used if needed for a particular band) if your particular antenna geometry doesn't show a low enough SWR on a particular band.
Suggested counterpoise lengths are as follows:
41 foot antenna wire needs a 16 foot length (20 feet of wire supplied so you can adjust as needed by folding back on itself)
71 foot antenna wire needs a 42-50 foot length (50 feet of wire supplied so you can adjust as needed by folding back on itself)
92 foot antenna wire needs a 90 foot length (100 feet of wire supplied so you can adjust as needed by folding back on itself)
111 foot antenna wire needs a 37 foot length (50 feet of wire supplied so you can adjust as needed by folding back on itself)
155 foot antenna wire needs a 95 foot length (100 feet of wire supplied so you can adjust as needed by folding back on itself)
