The 2 Element Christman Method Vertical Array

The first kit we released with Performance Z Arrays was the 2-element vertical phased array kit. It is based on the Christman method of phasing. Although not the most efficient method of phasing two vertical antennas, this design lends itself to being agnostic in the band of operation. What I mean by this, with any given properly constructed pair of vertical antennas, and phasing cables this kit can be used on any band. You can have a set of verticals and phasing cables for 20 meters, once set for 40meters and another for 10meters and the same controller and relay switching network can be used for operations.

What is the Christman method of phasing? Developed by Alan Christman, K3LC it allows the use of 2 vertical elements to produce 2 end fire patterns and one broadside pattern. (See Figure 1). What Christman did was compute the point where the voltage magnitude of the feedlines was equal and thus could provide the proper power in the correct timing to produce a 90deg phase shift between the elements. With the elements spaced 1/4 wavelength apart this creates a constructive interference pattern in one direction and a destructive interference patten in the opposite direction. This produces a cardioid pattern in the direction of the constructive interference and a null in the direction of the deconstructive interference. When compared to a single vertical element, the end fire pattern will produce up to +3dB gain with as much as 20dB in front to back in one of two directions. The design of the kit allows for the shorting of the delay line, thus feeding both elements in phase. This creates a somewhat omni directional patter with a +1dB to +1.5dB gain broadside to the array.  

The array design consists of two (2) equal length quarter wave vertical elements and a radial field for each element.  The elements can be wires that are supported by nonconductive masts or metallic vertical elements.  The system can be ground mounted with a radial field or could be elevated with tuned counterpoises.  The two elements are fed with 84deg feedlines and a delay line of 71 degrees used to delay signaling to one of the elements. 

The design was popularized in the Jon Devoldere, ON4UN's book "Low Band Dx'ing"

Figure 1 - The three available radiation patterns for the 2-element array.

Taken from Jon Devoldere, ON4UN's book "Low Band DX'ing"