Quad loops and other loop antennas

 




The graph above the graph above shows the gain that can be achieved by stacking dipole antennas. The gain depends primarily on the spacing between the dipoles. We should look at two such dipoles. If we do we see the maximum gain is about 4.5 dB over a single dipole and that occurs at about 3/4 wave spacing or just slightly less. To obtain this gain we must feed both dipoles in phase and that means connecting a feedline to both antennas. There are numerous ways to do this.  However a trick can be employed to use only one feedline. You can take a halfwave piece of wire ( upped dipole) and bend the ends down to meet the ends of the lower dipole that are bent up.This will insure that both dipoles are in phase. If the spacing between the dipoles is only a couple inches we have the common folded dipole. However if we increase the spacing to 1/4 wavelength and bend the ends of the upper dipole down and the ends of the lower dipole up enough to touch we have a full wave square or quad loop. Looking at the  graph above we see that the gain of two dipoles spaced at 1/4 wave is only about 1 dB over a single dipole. In the case of a bent dipole we lose gain by bending the ends down, so such a loop antenna can only have gain amounting to 1 dB over the bent antenna. Let’s assume that we did not lose any gain by bending the dipoles, in that case the square Quad loop would have at best 1 dB gain over a dipole. So logically the quad loop can only be 1 dB better than a dipole and probably less. 

No other configuration of a rectangular loop can have more gain than the square quad loop. If the sides are made longer to increase the spacing more than 1/4 wave then more of the ends will be bent down and the high current or center part of the dipoles will be shortened and gain goes down. The quad loop is the best compromise to afford maximum spacing without too much reduction in the individual dipole gain. Delta loops are just distorted shapes and are never as good as the square loop and likewise probably never as good as a straight dipole regardless of ham radio folk law! 

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