SD3-75
VHF High Gain Stacked Dipole Array
| Model | Product Description | Frequency | Gain | Download | Enquiry | |
|---|---|---|---|---|---|---|
| SD3-75 | VHF High Gain Stacked Dipole Array | 66 - 88 MHz | 3 dBd. |  |  |  |
The SD3-75 design focuses on its high-performance reliability in the VHF Lower Band (66–88 MHz). By emphasizing its stacked dipole array design, we position it as the go-to solution for PMR, Repeater, and Paging sites that require a balance of high gain and field-adjustable radiation patterns.
Logistics: Modular Design for Rapid Deployment
The SD3-75 is engineered to streamline the deployment of high-performance VHF infrastructure. While the array reaches an overall length of 6 meters, the central aluminum support pipe is cleverly shipped in two 3-meter sections. This modular approach significantly reduces shipping costs and simplifies handling during tower climbs. The system is supplied with two robust mounting brackets and 4.5-foot horizontal side arms, ensuring the antenna can be securely side-mounted on any standard tower or mast while maintaining optimal RF clearance.
Durability: Heavy-Duty Construction for Extreme Weather
Built for 24/7 mission-critical operations, the SD3-75 utilizes 6063T6 military-grade aluminum alloy for both the support pipe and the folded dipoles. The entire system is rated to withstand wind speeds of up to 200 Km/Hr. To ensure long-term electrical stability, the phasing harness is sealed with high-quality heat-shrink tubing for total waterproofing. Safety is prioritized through a direct D.C. ground design, providing built-in lightning protection for sensitive base station equipment. All mounting hardware is crafted from marine-grade stainless steel to prevent corrosion in high-humidity or saline environments.
Efficiency: Flexible Radiation Patterns & High Gain
The SD3-75 offers unmatched versatility in the field. By arranging the two folded dipoles 180 degrees apart around the mast, it delivers a consistent 3dBd (5.15dBi) Omni-directional pattern. If the mission requires targeted sector coverage, the dipoles can be aligned collinearly to achieve a 6dBd (8.15dBi) Offset pattern.
With a 36-degree vertical beamwidth and integrated null-filling characteristics, the SD3-75 ensures high signal density at the horizon while providing reliable close-in coverage. Capable of handling up to 500 Watts of RF power, it is the ideal choice for high-population radio sites requiring long-haul coverage.
Technical Features
- VHF Lower Band Optimization: Specifically tuned for the 66–88 MHz frequency range with a 6 MHz bandwidth.
- Dual-Pattern Flexibility: Field-adjustable configuration for 3dBd Omni-directional or 6dBd Offset radiation patterns.
- High-Power Capacity: Safely manages up to 500 Watts continuous RF power, perfect for heavy-traffic repeaters.
- Modular Mast System: 6-meter mast splits into two 3-meter sections for easy shipping and handling.
- Integrated Lightning Protection: Operates at D.C. ground to safeguard connected electronics from surge strikes.
- Extreme Wind Resistance: Structurally reinforced for a 200 Km/Hr (125 MPH) wind rating.
- Precision Phasing Harness: Heat-shrink sealed cables ensure smooth VSWR (< 1.5:1) and total weatherproofing.
- Side-Mount Ready: Includes two 1.35-meter (4.5 ft) horizontal arms for optimal tower side-mounting clearance.
- Superior Materials: Constructed from 6063T6 Aluminum Alloy and marine-grade stainless steel for maximum service life.
- Low Noise Performance: Engineered for high-density radio sites to minimize interference and provide long-haul coverage.
Field Configuration Guide: SD3-75 Pattern Optimization
The SD3-75 is a versatile 2-element array that allows for rapid field adjustments. By changing the physical orientation of the two folded dipoles on the 6-meter mast, you can shift the signal from a broad broadcast to a concentrated sector beam.
1. Omni-Directional Setup (3 dBd Gain)
To achieve a 360-degree horizontal pattern, the two dipoles must be mounted 180 degrees apart (facing opposite directions) around the central mast.
- Tactical Use: Ideal for repeaters located in the center of a service area or on a hilltop where coverage is needed in all directions.
- The Result: This balances the radiation pattern to provide a consistent 5.15 dBi (3 dBd) signal for mobile units circling the tower.
2. Offset/Directional Setup (6 dBd Gain)
To focus the signal in one specific direction, mount both dipoles collinearly (stacked vertically on the same side of the mast, facing the same direction).
- Tactical Use: Best for sites at the edge of a coverage zone, such as a coastal station or a repeater at the mouth of a long valley.
- The Result: By aligning the elements, you narrow the horizontal beam and increase the forward gain to 8.15 dBi (6 dBd). This effectively doubles your power in the target direction, allowing the signal to reach significantly further.
3. Vertical Beamwidth & Null-Fill
The SD3-75 features a 36-degree vertical beamwidth. Because it uses two stacked elements fed in-phase, the signal is compressed toward the horizon.
- Close-in Coverage: Unlike high-gain "pencil beam" antennas that might overshoot nearby users, the SD3-75 is designed with null-fill characteristics. This ensures that users located closer to the base of the tower still receive a reliable signal even while the main lobe handles the long-distance traffic.
4. Side-Mounting Clearances
The system includes two 4.5-foot (1.35-meter) horizontal side arms.
- Tower Interference: At 66–88 MHz, the wavelength is relatively long. If the dipoles are mounted too close to a metal tower, the tower will act as an unintended reflector, distorting your pattern and increasing VSWR.
- Installation Tip: Always use the provided side arms to maintain the 1.35-meter separation. This ensures the antenna operates within its 1.5:1 VSWR specification and maintains the integrity of your chosen radiation pattern.
Step-by-Step Field Adjustment
- Loosen U-Bolts: Using a standard wrench, loosen the stainless steel U-bolts holding the dipole mounting blocks to the mast.
- Rotate Elements: For Omni, rotate the bottom dipole 180° relative to the top. For Offset, align them perfectly in the same plane.
- Check Phasing Harness: Ensure the phasing harness cables are not under tension and that the heat-shrink seals remain intact.
- Secure & Level: Tighten the U-bolts and use a spirit level to ensure the 6-meter mast is perfectly vertical. A tilt of even a few degrees can significantly move your 36° vertical beam off-target at the horizon.
Design Features: The SD3-75 high performance VHF stacked dipole array is designed for use with highly populated radio sites requiring long haul omni-directional coverage. The SD3-75 is heavy duty stacked dipole array features, wide bandwidth, high gain, high power handling capacity, low VSWR, low noise performance and null filling coverage with omni directional characteristics. This stacked dipole array is highly suitable as base station antennas for Repeater and Paging sites.
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