8-Element Grounded Folded Dipole Array
915 MHz ISM Band Omnidirectional Base Station Antenna
DC-Grounded Elements with Broadband Phasing Harness
Based on Decibel Products DB-224/DB-448 Design Practice
BY Gary, NZ5V
1. Design Overview
This design uses grounded folded dipole elements, where the center of each dipole is DC-grounded to the mast. This provides two critical advantages over standard folded dipoles:
- Lightning Protection: Direct DC path to ground prevents ESD damage to the phasing harness and connected equipment.
- Out-of-Band Signal Rejection: Symmetric grounding causes signals outside the resonant frequency to be shunted to ground rather than passed to the feedline.
1.1 Key Specifications
| Parameter | Value |
| Design Frequency | 915 MHz (902-928 MHz ISM Band) |
| Number of Elements | 8 grounded folded dipoles |
| Element Type | Center-grounded folded dipole (~150Ω) |
| Estimated Gain | 10-11 dBd (12-13 dBi) |
| Pattern | Omnidirectional (horizontal plane) |
| Polarization | Vertical |
| Feed Impedance | 50Ω (unbalanced) |
| Harness Cable Types | 35Ω + 75Ω (both 65.9% VF) |
| Lightning Protection | DC ground at each element |
| Approximate Height | 2.1 meters (83 inches) |
2. Grounded Folded Dipole Theory
A standard folded dipole fed at center presents approximately 300Ω impedance. In a grounded folded dipole, the center of one conductor is bonded to ground (the mast), and the feed point is offset to one side. This configuration:
- Reduces feedpoint impedance to approximately 150Ω (varies with feed offset position)
- Maintains balanced radiation pattern
- Provides DC ground at the antenna element
- Rejects common-mode currents and out-of-band signals
2.2 DB-224 Style Harness Analysis
Based on the actual Decibel Products harness using 35Ω (VB83) and 75Ω (VB11) cables with 65.9% velocity factor:
| Stage | Cable Z | Transformation | Result |
| Element Feed | 35Ω | 150Ω → ? | ~8Ω per element |
| Pair Combine | 75Ω | 2×8Ω parallel → ? | ~140Ω per pair |
| Quad Combine | 35Ω | 2×140Ω parallel → ? | ~27Ω per quad |
| Final Combine | 75Ω | 2×27Ω parallel → ? | ~50Ω output |
3. Grounded Folded Dipole Element Design
3.1 Element Configuration Diagram
GROUNDED FOLDED DIPOLE ELEMENT – TOP VIEW
←————— 155 mm (6.1″) —————→
┌═══════════════════════════════════════════════════┐
║ ┃ ║
║ TOP CONDUCTOR ┃ GROUND ║ ↑
║ (3/8″ aluminum) ┃ TO MAST ║ │
║ ↓ ║ │ 28mm
║ ║ │ (1.1″)
║ ●━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━● ║ │
║ ↑ ↑ ↑ ║ ↓
║ FEED FEED GAP FEED ║
║ POINT 1 ~100mm offset POINT 2║
║ (to coax shield) (to coax center)
└═══════════════════════════════════════════════════┘
│
↓
┌─────────────────┐
│ 35Ω λ/4 COAX │
│ (54mm long) │
│ VF = 65.9% │
└────────┬────────┘
│
↓
To Phasing Harness
NOTES:
• Center of top conductor is GROUNDED to mast (DC path)
• Feed gap is OFFSET from center for ~150Ω impedance
• Both ends shorted together (folded dipole configuration)
• Material: 6061-T4 Aluminum tubing or Copper tubing, 3/8″ OD × 0.035″ wall
3.2 Element Dimensions
| Dimension | Metric | Imperial |
| Total length (horizontal) | 155 mm | 6.1 inches |
| Conductor spacing | 28 mm | 1.1 inches |
| Tubing OD | 9.5 mm | 3/8 inch |
| Tubing wall | 0.9 mm | 0.035 inch |
| Feed gap width | 12 mm | 0.5 inch |
| Feed gap offset from center | ~50 mm | ~2 inches |
| Ground bond location | Center | Center |
| Feedpoint impedance | ~150Ω | ~150Ω |
Critical Construction Notes: The center ground bond must be a solid electrical connection (brazed or bolted with star washers). The feed gap is offset from center to achieve the desired ~150Ω impedance. Both feed points connect to the phasing harness – shield to one side, center conductor to the other.
4. Array Configuration
| Parameter | Value |
| Element center-to-center spacing | 280 mm (11.0″) = 0.855λ |
| Total span (8 elements) | 1,960 mm (77.2″) |
| Overall antenna height | ~2,100 mm (83″) with elements |
5. Phasing Harness Design
The harness uses the same cable types as the Decibel Products DB-224: 35Ω and 75Ω coax, both with 65.9% velocity factor. All quarter-wave sections are the same physical length.
5.1 Cable Specifications
| Cable | Impedance | Vel. Factor | λ/4 Length | Construction |
| VB83 equivalent | 35Ω | 65.9% | 162 mm | 2× 75Ω parallel |
| VB11 / RG-59 | 75Ω | 65.9% | 162 mm | Single cable |
5.2 Quarter-Wave Length Calculation
At 915 MHz with VF = 0.659:
λ = c / f = 299,792,458 / 915,000,000 = 327.6 mm
λ/4 (free space) = 81.9 mm
λ/4 (in cable) = 81.9 × 0.659 = 54.0 mm (2.13 inches)
5.3 Complete Harness Wiring Diagram
8-ELEMENT PHASING HARNESS – SCHEMATIC
══════════════════════════════════════
[FD1]──35Ω──┬──75Ω──┐
150Ω │ │
[FD2]──35Ω──┘ ├──35Ω──┐
150Ω │ │
[FD3]──35Ω──┬──75Ω──┘ │
150Ω │ ├──75Ω─────[50Ω OUT]
[FD4]──35Ω──┘ │
150Ω │
│
[FD5]──35Ω──┬──75Ω──┐ │
150Ω │ │ │
[FD6]──35Ω──┘ ├──35Ω──┘
150Ω │
[FD7]──35Ω──┬──75Ω──┘
150Ω │
[FD8]──35Ω──┘
150Ω
ALL quarter-wave sections: 54 mm (2.13″) with VF=65.9%
CABLE SUMMARY:
───────────────
35Ω sections (2×75Ω parallel): 8 element feeds + 2 quad feeds = 10 pcs
75Ω sections (single RG-59): 4 pair combines + 1 final = 5 pcs
T-connectors: 7 total
5.4 Harness Component List
| Component | Qty | Cable Z | Length | Function |
| Element feed transformers | 8 | 35Ω | 162 mm | 150Ω→~8Ω |
| Pair combiner transformers | 4 | 75Ω | 162 mm | 16Ω→~140Ω |
| Quad feed transformers | 2 | 35Ω | 162 mm | 70Ω→~27Ω |
| Final output transformer | 1 | 75Ω | 162 mm | 13.5Ω→50Ω |
| T-connectors | 7 | — | — | Power combining |
6. Construction Guidelines
6.1 Making 35Ω Cable
35Ω cable is made by connecting two lengths of 75Ω cable in parallel:
- Cut two identical lengths of 75Ω cable (RG-59 with 65.9% VF or equivalent)
- At each end, join the two center conductors together
- At each end, join the two shields together
- Result: 75Ω ∥ 75Ω = 37.5Ω ≈ 35Ω
6.2 Critical Notes
- Velocity Factor: The 65.9% VF is critical. Standard RG-59 (66% VF) is close enough. Verify with VNA if possible.
- Cable Lengths: All cables at each tier must be exactly the same length for proper phasing.
- Ground Connections: Every element center must bond solidly to the mast. Use stainless hardware.
- Weatherproofing: All connections must be sealed with self-amalgamating tape and heat shrink.
7. Expected Performance
| Parameter | Expected Value |
| Gain | 10-11 dBd (12-13 dBi) |
| Horizontal pattern | Omnidirectional (±1 dB) |
| Vertical beamwidth (-3dB) | 12-15° |
| VSWR (902-928 MHz) | < 1.5:1 |
| Bandwidth (VSWR < 2:1) | > 40 MHz |
| Harness loss | ~1 dB |
| Power handling | 500W continuous |
8. Bill of Materials
| Item | Qty | Specification |
| Aluminum or Copper tubing (elements) | 5 m | 3/8″ OD × 0.035″ wall, 6061-T4 |
| Aluminum mast | 2.5 m | 1.5″ OD structural tube |
| 75Ω coax (RG-59, 66% VF) | 3 m | For 35Ω pairs and 75Ω sections |
| F-type T-connectors | 7 | Weatherproof, 75Ω rated |
| F-type to N-type adapter | 1 | For 50Ω output |
| Stainless steel hardware | 1 set | For ground bonds and mounting |
| Self-amalgamating tape | 1 roll | Weatherproofing |
| Heat shrink tubing | 1 m | Various sizes |