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The full wave loop forms a second basic building block of antennas, along with the the half wave dipole. In fact, the two are similar in many ways: the full wave loop can be thought of two dipoles with the ends connected together, or a folded dipole opened up into a 2-dimensional shape.
Use this online calculator to determine the length of a full-wave loop antenna from the frequency. Both metric and English units of measurement are supported. Quarter-wave matching section lengths are also calculated.
The skywire loop is an excellent antenna - they can be a bit of work to get up in the air if you have a lot of obstacles (trees, xyl, etc.) but it is worth it. What's the right length? 574' feet for 160-15 meters (10 meters also, but with a short feedline).
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.
So, my "longer" loop is about 1.25 wavelengths on 80 meters (2.5 on 40m, 5 on 20m and 10 on 10m) and is installed between 30 to 40 feet in the air. Scaled-down versions, say 75% of a wavelength may also work fine if you don't have the room for a full-wave or longer antenna.
Full wave, closed loop antennas are broadband, low Q devices and exhibit a theoretical gain of approximately 2 dB over a half wave dipole. It's hard to believe says VE2BMC, but going from 1/4 wavelength vertical to a simple rectangular loop can lead to 3-4 "S" units improvement.
small transmitting loops, where efficiency is more important. full wave loops, (generally between 2/3 and 3/2 wavelengths) which have maximum radiation broadside to the loop. This is probably the most common loop for ham radio use.
The traditional formula for the length of a full wave loop antenna (in feet) is 1005 / MHz. (That is 306 / MHz in meters, although I haven’t found any reference that uses that number). However, this does not include any adjustment for the shape, wire size or insulation, height above ground, etc.
A full wave loop is also a standing wave antenna, 2 half-wave dipoles long, connected together so that they make a continuous, circuitous path. The current distribution is sinusoidal in each half of the loop.
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.