DX Engineering & Comtek Verticals for phased arrays

Q:  Are DX Engineering monoband verticals the right choice for a transmit four-square or other phased arrays? What details should I know about four-square array construction?

A: Yes, DX Engineering and COMTEK mono-band verticals, available in rope guyed and self-supporting models, are excellent for use in arrays with DX Engineering and COMTEK or other array phasing units.

When setting up a four-square array, here are the most important details to know to get the best results:

1. The array elements must be in the clear, away from metal structures. No structure should be in the middle of the array.

2. Shunt fed towers cannot be used. Each vertical must be insulated from ground with a ground-mount that places the bottom of the radiator less than 12 inches above the ground. Each vertical radiator requires a complete ground radial system consisting of a minimum of 40 and a maximum of 120 wire radials that are minimum 1/8 to a maximum 1/4 wavelength, with the intersecting radials terminated to one of four copper straps crossing the radial systems as shown on pages 18 and 19 of the Transmit Four-Square manual linked below.

3. The spacing of the verticals is determined using the targeted array center frequency and the one-quarter wavelength free space formula. For example, the typical array for 80 meters is centered at 3650 kHz for operations from 3500 kHz to 3800 kHz, which is possible with the COMTEK and the DX Engineering hybrids. So, on the side of this example four-square, the spacing between each of the verticals is 246/3.65 MHz = 67.4 feet.

4. RG-11U Foam 75 ohm coaxial cable phasing feedlines, with a velocity factor (VF) higher than 0.80 are required from the relay unit centered in the array and the antenna feedpoints. These cables should be electronically tuned to one quarter-wavelength at the array center frequency, which is 3650 kHz in this example. These are available as custom assemblies, precision cut, electronically tuned to frequency by DX Engineering, array center frequency specified at time of order. These RG-11U foam feedlines, that connect the array controller relay unit to each vertical feedpoint, should be buried under the radial system. DX Engineering’s 75 ohm foam cable, DXE-11U, has a waterproof, direct bury jacket. Do not use coiled coax chokes, inline toroidally-wound baluns or base loading or matching coils, as these devices can change the phase of the antenna and ruin an array pattern.  Do not add a length of cable with beads to any phasing feedline. However, beads installed over the feedpoint end of an electronically tuned one-quarter wavelength feedline, before connectors are installed, will insure that RF feedback, cable shield detuning and current imbalances will not alter the array phasing or performance.

5. Each vertical radiator must be tuned independently, with the other verticals lying on the ground with their feedlines disconnected from their feedpoints. Each radiator is tuned to resonance, which is minimum reactance (NOT minimum SWR), at the frequency that is 4% lower than the array target center frequency. So, for the example array with a center frequency set for 3650 kHz, each radiator is tuned to about 3500 kHz, with a length that approximates the quarter-wave antenna formula; 234/3.50 MHz = about 66.86 ft., for suspended wire or tubing elements. Alternatively, to achieve resonance on shortened vertical elements, the only loading allowed in a phased array is top loading using wires and/or tubing, but each radiator must be identical to the other. Due to capacitive effects to ground, insulated base tower section verticals require a considerably greater physical length for resonance. Resonance tuning must be performed with the analyzer at the vertical element feedpoint. Each radiator should be tuned as nearly as possible to the ideal vertical, with a feedpoint impedance of R 36 +/- j0. Again, use no coiled coax, toroid baluns or loading or matching coils. Phased array elements should not to be tuned to 50 ohms or for minimum SWR for use in phased arrays.

6. Attention to all of these details and the use of the appropriate tubing and hardware anti-oxidants, cable connector dielectric grease and sealing tapes will provide successful operations on your long-lasting, high performance phased array.

See the manual for the DX Engineering Transmit Four Squares for additional array construction information.

To see the PDF manual, Please click here: DX Engineering Transmit Four Square System