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.
SNAP On Feed Line Choke
Each end fed antenna needs a feed line choke at the radio end of the coax to stop the antenna common mode current (on the coax braid) from getting into the radio when transmitting and also to stop RFI common mode noise when receiving. A
simple easy to adjust in different places along the coax used as a counterpoise is the snap on ferrite core with a 1" ID (hole) installed as shown in the picture. (coax not included, but you can use RG-8X size for 5 turns and RG-8X size for 3 turns).
Add the Feed line choke (part # SOFLC) to your end fed antenna today today and stop the pesky RFI from getting into your radio and causing RFI, audio distortion or unexpected operation!
NOTE: Coax shown in picture is NOT INCLUDED and is shown as an installation example only. If you want a pre-wound choke check part# JC-1-1500-3 or JC-1-1500-6.
For non-resonant end fed antennas placement of the choke from 10-50 feet away from the antenna feed point to use the coax braid as the counterpoise (since these antennas are really off center fed dipoles with the short side using the coax braid as the count5erpoise. Wind 8-10 turns thru center for an effective 3-30 Mhz choke.
For End fed Half Wave antennas place 3-10 feet from antenna feed point to act as short counterpoise for these voltage fed antennas. Use 8-10 turns.
TECH NOTE: SNAP On Chokes for 80 and 160 meter end fed antennas
If you use RG-8X size cable, use 2 SOFLC if you have 5-6 turns thru the center of each.See picture on left as an example of how to wind the cable.
Measure 40 feet from the matching unun and then wind 6 turns thru the center of the SOFLC.Close it up and then wind an additional 6 turns thru another SOFLC placed adjacent to the first one and on the radio station side of the coax.See picture to left.
If you use LMR-400 or 1/2" coax cable you will only get 3 turns thru the snap on choke and you will need an additional choke (with an additional 3 turns) in series to provide enough choking on 40 and 80 meters. Part# SOFLC.
In lieu of the 2 chokes shown above, you may also use a F400-31 ring (3” ID) with 6 or more turns which provides a much higher common mode suppression and better antenna tuning on 80 meters. See picture for winding example.
