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.
Magnetic Long Wire Antenna Matcher (9:1) for SWL, 100 KHz-30 MHz
Palomar Engineers Magnetic long wire antenna matche has been redesigned for shortwave listeners who listen to a broad frequency range from VLF to HF and others needing to match a long wire antenna to 50/75 ohm coax feed line cable.
Specifications: Frequency: 100 KHz – 30 MHz. Input: 50/75 ohms for coaxial cable. Output: High impedance. Connector: SO-239 (for cable), wingnut/solder lug wire lead (for antenna). Mounting: Stainless steel eyebolt. Size: 2.5” diameter. Case: White PVC sealed weatherproof. External ground bolt with wing nut for external counterpoise or separate ground.
The long wire antenna often has a high impedance in the 500-1000 ohm range and the MLB-2 transforms this high impedance close to efficiently couple the received signal to the receiver. A long wire antenna with a matching balun such as the MLB-2 often has a 12 db (2 “S” units) better signal than a long wire without a matching balun.
Typical Installation Diagram
MLB-1/MLB-2 Typical installation diagram
For best results we recommend that the coax feed line be at least 1/4 wavelength on the lowest operating frequency since the coax braid is used as a counterpoise if you don’t use the external counterpoise terminal on the matching unit. We also recommend a feed line choke at the end of the coax feed line near the radio to prevent RFI common mode current (from neighbor noise sources) from interfering with the radio – check out the info below on feed line chokes.
Also available as a complete ready to hang SWL antenna. (see related products).