433MHz 5/8 Wavelength Antenna

🛠 Bill of Materials (BOM)

Durability is the priority in this project.

• Antenna Elements: 10mm Diameter Aluminum Pipe (Standard hardware store tubing).
• Coil Wire: 1.5mm thick solid core copper wire (NYA).
• Connector: SO239 Chassis Mount Connector + a piece of RG174 Coax.
• Screws: M2.5 Screws (For assembly).
• 3D Printed Parts: 5x Pipe Caps and the Main Body.
• Insulation: Liquid Gasket and Self-Amalgamating Tape.

📏 Critical Dimensions (The Golden Ratios)

These measurements are our starting point for resonance.

• Radials (GND Rods): 4 pieces x 142 mm. (These are fixed, do not cut them).
• Active Radiator (Top Element): Start with 430 mm. (Will be trimmed during tuning).
• Coil: 4 Turns total on an 18mm diameter form.

⚙️ Step-by-Step Assembly & Coil Logic

The heart of this antenna is the coil connection. We will use the "Tapped Coil" method to match the impedance to 50 Ohms.

1. Mechanical Assembly:
   • Insert the aluminum pipes into the 3D printed parts.
   • Drill pilot holes using a 2mm drill bit to secure them.
   • Since aluminum is soft, drive the M2.5 screws directly into these holes to cut their own threads. This ensures a rock-solid mechanical connection.
2. Coil & Connections (The Trick):
   • Wind 4 full turns of the 1.5mm solid wire.
   • Ground Connection: The bottom end of the coil must connect to the 4 radials (GND).
   • Live Tip Connection (Tap Point): Solder the center core of the RG174 cable exactly to the 1.5th turn counting from the bottom. (The remaining 2.5 turns continue upwards to the active radiator).

🎯 Tuning (SWR Adjustment)

When you first assemble the antenna, the frequency will likely be around 420 MHz (too low).

1. Connect your NanoVNA.
2. Start trimming the Active Radiator (the 430mm pipe) millimeter by millimeter or use a file/sandpaper.
3. Stop immediately when you see the SWR "dip" point sitting perfectly at 433 MHz.

💧 Final Touches (Waterproofing)

• Seal all joints, screw heads, and gaps on the antenna body with Liquid Gasket.
• Wrap the coaxial connector connection with Self-Amalgamating Tape (Self-fusing tape) for a watertight seal.

73, TA2DMX cc: deeplabstudio.com