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.
This sub-woofer RFi kit is specifically designed to suppress audio interference triggered by Radio Frequency Interference problems from AM, FM, CB and Ham radio stations AND to minimize the interference to nearby electronics caused by a sub-woofer without magnetic shielding (Velodyne, Hartley, Hsu, etc). The ferrite filters are selected to suppress RFI from 1 to 300 MHz. The filters are easy to install, don’t require modification of the protected equipment and work in almost all cases.
What is included in the kit?
The RFI-HTS-SW kit contains a a ring filter for the AC power cord to the sub-woofer and 4 snap-on filters for the speaker wires (either for audio pass through to main speakers or direct line out to the sub-woofer).
Tech Note: This kit is INCLUDED in the full home theater RFI Kit (part #RFI-HTS) and you do not have to purchase a separate RFI kit for the sub-woofer.