Basic Principles of Phased Array Antenna Construction in Amateur Radio
A phased array antenna uses two or more radiating elements driven by the same transmitter. By carefully controlling the timing (phase) of the RF signal reaching each element, the overlapping electromagnetic waves create constructive and destructive interference. This allows you to electrically steer the signal in a specific direction without physically rotating the antenna — a massive advantage on low bands where a rotary Yagi is impractical.
The Three Core Variables
To build a functional array, you are manipulating three primary parameters:
- Physical Spacing: The physical distance between elements, measured in fractions of a wavelength (λ). For example, a classic two-element directional array often uses λ/4 spacing.
- Electrical Phasing: The time delay introduced between the elements. A 90° phase shift means the RF signal reaches the second element exactly one-quarter of an RF cycle later than the first.
- Current Magnitude: The amount of power delivered to each element. Most basic arrays drive elements equally, but advanced arrays (like a 4-Square) use unequal power distribution to improve the front-to-back ratio and suppress side lobes.
Key insight: Broadside arrays (elements fed in-phase, 0°) radiate perpendicular to the line of elements. End-fire arrays (e.g., 90° phase, λ/4 spacing) fire along the line of the elements, away from the delayed element.
Construction Principles
When homebrewing a phased array for HF, the system breaks down into three main physical components:
1. The Elements
For low-band arrays, these are typically λ/4 vertical elements. Because verticals require an excellent ground system to be efficient, each element needs its own robust radial field. If the radial fields of adjacent elements overlap, that’s perfectly fine — bonding the ground system together at the intersections improves overall efficiency.
2. Phasing Lines (Delay Lines)
You create the phase delay by cutting specific lengths of coaxial cable. Since RF travels slower through coax than in a vacuum, you must factor in the cable’s velocity factor (VF).
The physical length of a phasing line is calculated as:
L = (300/f) x (θ/360) x VF
Where L is length in meters, f is frequency in MHz, θ is the desired phase shift in degrees, and VF is the velocity factor of your coax (e.g., 0.66 for standard solid polyethylene RG-213).
3. The Switching Network
To steer the beam, you need a controller that switches different phasing lines in and out of the circuit to change which element receives the delayed signal.
The AI generated diagram shows a classic Christman feed system for two elements. Instead of complex hybrid couplers, it uses precisely cut coaxial lines (typically 71° and 84° electrical lengths) and a relay matrix. By toggling relays to switch which element gets the delayed signal, you can flip the directivity 180 degrees instantly.
For a 4-Square array, the network becomes more complex, often using a quadrature hybrid coupler (sometimes wound on large toroids) to split the power into three phases (0°, -90°, and -180°) while dumping any reflected imbalance power into a 50-ohm dummy load to maintain a perfect match to the transmitter.