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.
Your rotor cable and control box can cause RFI into your radio station and you need to suppress common mode RFI induced into the cable from your antennas. In many cases the rotor cable acts as an unintended counterpoise for your rotated antennas and can cause weird SWR issues since the control cables become part of the antenna,
The SOLUTION is simple: isolate the cable from RFI by adding the Palomar RFI kit for antenna rotors. Add 3 snap on filters near the rotor and another 3 snap on filters at the rotor control box and a ring filter for AC powered rotor control boxes to suppress RFI common mode currents that can affect the antenna SWR and radiation patterns. In addition, these filters may also help reduce station receiver noise since many rotors are connected to the station ground and the rotor cable acts as an additional receiving antenna.
Isolating the rotor cable works on the same principal as isolating coax cable - keep the TRANSMIT signal on the antenna by proving a feed line choke at the rotor itself and keep noise out of the station on RECEIVE by choking the cable at the rotor control box ( by suppressing noise pickup on the cable).
Kit includes 7 filters -six 1/2" snap on filters for the cable and a ring filter for the AC/DC power supply for the control box. Simple to install and cheap insurance to solve both the TRANSMIT and RECEIVE issues caused by rotor control systems.