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.
CMNF-1500LF for transmitter/receiver coax lines up to 1500 watts PEP - 100 KHz - 12 MHz, Up to -38 dB Common Mode Noise Suppression!
For coax feed lines with receive and transmit signals, you will love the small size, wide frequency range and excellent attenuation of common mode noise interference
- Reduces broadband common mode receiver noise* picked up on the outside of the antenna coax feed line.
- Wide frequency range works on all brands of radios and coax fed antennas from 200 KHz to 7 MHz
- Common mode noise rejection up to 20-36 DB (3-6 "S" units) depending on frequency
- Simple plug & play installation: plug a short jumper from the CMNF-1500LF to your amplifier output and plug the feed line (or a jumper to your antenna tuner) into the other SO-239 CMNF-1500LF input. Either connector can be input or output. Rated at 1500 watts PEP.
- will work with Bias-T antenna switches and will pass DC voltage from input to output with no attenuation
* not effective for noise from AC power lines or noise received directly by antenna
Installation Note: For best results, and to reduce co-interference between choked and unchoked coax feed lines with multiple antennas, ALL antenna feed lines entering the radio room need to have a separate feed line choke particularly if they are at a different frequency then HF, like VHF or UHF.
If you use a antenna switch for multiple antennas in the .2-7 MHz range, place the CMNF-1500LF in the "common" line of the antenna coax switch and the antenna tuner/amplifier.
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