Vertical Phased Array Antenna System

This document specifies a four-element vertical phased array antenna system for HF beyond-line-of-sight (BLOS) communication. The array provides electronic beam steering across 270° azimuth with selectable elevation angles for both long-range skywave and near-vertical incidence skywave (NVIS) propagation modes.

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1. System Overview

The four-element square array configuration offers an excellent balance between complexity and capability for shore-based HF operations. Elements V1–V4 are vertical monopoles with individual phase control (φ₁–φ₄), managed by a central Phase Control Unit (PCU).

Figure 1

Four-element square array configuration. Elements V1–V4 are vertical monopoles with individual phase control (φ₁–φ₄). PCU = Phase Control Unit. Dashed circles indicate ground radial extent.

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2. Vertical Element Design

Each element is a quarter-wave vertical monopole optimized for broadband operation across the HF spectrum.

Figure 2

Single vertical monopole element showing structural components, feed arrangement, and ground system interface. Height optimized for quarter-wave resonance at 3.75 MHz with broadband matching network.

Element Specifications

Component	Specification
Height	20 meters
Diameter	150mm
Material	6061-T6 Aluminum or Hot-dip Galvanized Steel
Base	Insulated mount with matching network
Guy wires	Non-metallic (Kevlar or fiberglass)
Top loading	Optional 2m diameter hat for extended low-frequency response

Feed Point Detail

The feed point includes a broadband matching network that transforms the antenna impedance to 50Ω for connection to the Phase Control Unit via coaxial cable.

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3. Electrical Specifications

Parameter		Specification	Notes
Frequency	Operating Range	2.0 – 30.0 MHz	Full HF band coverage
	Optimum Range	3.0 – 18.0 MHz	Best VSWR and pattern
	NVIS Mode	2.0 – 10.0 MHz	Near-vertical incidence
Array Gain	Broadside (4 elements)	10 – 12 dBi	At beam peak
	Single Element	0 – 2 dBi	Over perfect ground
	Array Factor	+6 dB	4-element coherent sum
Beam Steering	Azimuth Range	270° continuous	Seaward coverage
	Elevation Range	15° – 90°	Long-range to NVIS
Beamwidth	Azimuth (−3 dB)	45° – 60°	Frequency dependent
	Elevation (−3 dB)	20° – 40°	Frequency dependent
Impedance	Element Feed	50Ω nominal	After matching network
	Array Port	50Ω	Single combined feed
VSWR	Optimum Band	≤ 1.5:1	3–18 MHz
	Extended Band	≤ 2.5:1	2–30 MHz
Power Rating	Continuous	10 kW PEP	Per element, 40 kW array

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4. Phasing Network Architecture

The Phase Control Unit (PCU) is the heart of the array, providing individual phase and amplitude control for each element.

Figure 3

Complete signal path from antenna elements through phase/amplitude control to combined RF port. Control bus provides digital interface for beam steering commands.

Signal Path Components

1. Broadband Matching Networks — Transform antenna impedance to 50Ω
2. Variable Phase Shifters — 0°–360° range with ≤1° resolution
3. Amplitude Control — 0–100% for pattern shaping and null steering
4. 4:1 Wilkinson Combiner — Combines all four channels to single port
5. T/R Switch — PIN diode or relay for transmit/receive isolation
6. PA (Transmit) — 1–10 kW power amplifier
7. LNA (Receive) — Low noise amplifier for receive sensitivity

Phase Steering Equations

For a planar array with elements at positions (x_n, y_n), the required phase shift to steer the beam to azimuth θ and elevation ψ is:

φ_n = −(2π/λ) · [x_n sin(θ) cos(ψ) + y_n cos(θ) cos(ψ)] — Eq. (1)

For the square 4-element array with spacing d:

φ₁ = −(πd/λ) · [sin(θ) + cos(θ)] cos(ψ) — Eq. (2a)

φ₂ = −(πd/λ) · [sin(θ) − cos(θ)] cos(ψ) — Eq. (2b)

φ₃ = −(πd/λ) · [−sin(θ) − cos(θ)] cos(ψ) — Eq. (2c)

φ₄ = −(πd/λ) · [−sin(θ) + cos(θ)] cos(ψ) — Eq. (2d)

Implementation Note: Phase shifters must accommodate the full 0°–360° range with resolution ≤1° to maintain beam pointing accuracy within ±2° across the operating band. For frequencies above 15 MHz, 0.1° resolution is recommended.

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5. Ground System Requirements

An inadequate ground system is the most common cause of poor vertical antenna performance.

Figure 4

Ground radial layout for single vertical element. 120 radials at 3° angular spacing, each 0.4λ long at lowest operating frequency. Radials buried 50–100mm below grade.

Ground System Specifications

Parameter	Specification	Performance Impact
Number of radials	120 per element	Diminishing returns above 120; 60 minimum acceptable
Radial length	0.4λ @ fmin (≈30m)	Shorter radials increase ground loss 1–3 dB
Radial material	#14 AWG bare copper	Phosphor bronze for coastal (corrosion)
Burial depth	50–100mm	Surface-laid acceptable but less durable
Center ground mesh	3m × 3m copper mesh	Reduces feed point ground resistance
Soil conductivity (desired)	>10 mS/m	Coastal sites typically 20–30 mS/m

Critical Design Consideration: A vertical with 16 radials over poor soil may exhibit 6 dB higher ground loss compared to 120 radials over good soil. This 6 dB difference is equivalent to reducing transmitter power from 10 kW to 2.5 kW. Ground system investment yields the highest return per dollar in any vertical antenna installation.

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6. Site Layout

A complete installation requires approximately 4.5 acres (150m × 120m minimum).

Figure 5

Recommended site layout showing antenna array, equipment shelter, backup power, and access. Array positioned for seaward coverage with 30m minimum setback from boundaries.

Site Components

Component	Specification
Antenna Array	75m × 75m for four elements with radial systems
Equipment Shelter	Housing for PCU, power amplifiers, and SDR equipment
Backup Power	50 kVA generator for off-grid operation
Utility Connection	3-phase power
Setbacks	30m minimum from site boundaries
Access	All-weather road to equipment shelter

The array should be oriented with the primary coverage sector (typically 270°) facing seaward or toward the intended coverage area.

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7. References

1. Kraus, J.D. and Marhefka, R.J., Antennas for All Applications, 3rd ed., McGraw-Hill, 2002.
2. ARRL, The ARRL Antenna Book, 24th ed., American Radio Relay League, 2019.
3. Sevick, J., Transmission Line Transformers, 4th ed., Noble Publishing, 2001.
4. MIL-STD-188-141D, Interoperability and Performance Standards for Medium and High Frequency Radio Systems, 2017.
5. ITU-R P.533, Method for the Prediction of the Performance of HF Circuits, 2019.