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 RFi kit was created to eliminate RFI issues from the excellent Heil Parametric Receive Audio System which can highly tailor audio signal from your radio to a a powered speaker or multiple headphones. However when installed near a high powered HF antenna it can ave RFi issues. This kit was created out of necessity from a ham using high power antennas near the radio. The use of the ferrite filters completely eliminated the RFI issue allowing the ham to enjoy the use of the PRAS.
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. See product picture for suggested filter use.
What is included in the kit?
The RFI-PRAS kit contains a a ring filter for the AC power cord used on the DC supply cables to the PRAS and high powered speakerr and 3 additional snap-on filters for the both ends of the speaker cable and for the audio input (from the transceiver/audio source) to the PRAS..
You get 1 ring filter and 5 1/2" ID snap on filters and an installation manual.