High-Gain 5/8 Wave Mobile Antenna for 145 MHz

If you’re an FM operator on the 2-meter band and want a mobile antenna with real ground-wave punch, the classic 5/8 λ design is hard to beat. Based on an adaptation by OK1IKE (original Czech article here), this project gives you a compact, efficient whip with a low 15° radiation angle – perfect for mobile use.

In this post, I’ll break down the specifications, the coil details, and how to assemble this 3.5 dB gain antenna.

Why a 5/8 Wave for 2m FM?

A standard 1/4 wave whip is simple but radiates high (30-40°). A 5/8 wave antenna:

• Compresses the main lobe to ~15° above the horizon – excellent for ground-wave propagation.
• Provides theoretical gain of ~3.5 dB over a 1/4 wave.
• Keeps a omnidirectional horizontal pattern – no dead spots while driving.

The antenna described here is actually a shortened 3/4 λ design, using a base loading coil for both resonance and impedance matching to 50 Ω.

• 145 MHZ Delta Loop VHF Antenna

Parts You’ll Need

1. Radiator: 120 cm length of stiff 3 mm steel rod (or copper-welded steel).
2. Coil former: 16 mm OD plastic pipe (PVC, POM, or ABS).
3. Wire: 1–2 mm enameled copper wire (≈ 30 cm total).
4. Mount: PL-259/SO-239 based mount or a standard 3/8-24 mobile stud.
5. Weather protection: Heat shrink or coax seal.

Step-by-Step Build

1. Wind the Loading Coil

• Cut a 5–6 cm piece of 16 mm plastic pipe.
• Wind 10.5 turns of 1–2 mm copper wire neatly, close-spaced.
• Leave 3–4 cm of wire at each end for connections.
• Tap point: Count 4 turns from the radiator (top) side – solder a short wire here.

2. Prepare the Radiator

• Use a 120 cm long, 3 mm steel rod. Clean any paint or rust from the bottom 10 cm.
• Solder or clamp the top end of the coil to the bottom of the rod.
• The tap (turn 4 from radiator) will connect to the center pin of your feed connector.

3. Mount and Connect

• Fix the coil inside a suitable tube or cover (e.g., fiberglass tube or ABS pipe).
• The bottom of the coil (past turn 10.5) connects to the ground side (shield) of the coax.
• The tap (turn 4) connects to the center conductor of your 50 Ω coax.
• A 50 Ω match is achieved directly – no extra matching network needed.

4. Final Assembly

• Attach the assembly to your mobile mount (magmount, lip mount, or permanent).
• Seal all joints against moisture.
• Tune by slightly compressing/stretching the coil or adjusting the radiator length (cut 1 cm at a time if SWR is high).

Tuning Tips

• Use an antenna analyzer (e.g., NanoVNA) at 145 MHz.
• Aim for SWR ≤ 1.5:1 across the FM portion (144–146 MHz).
• If resonance is too low (frequency too high), add 1/2 turn to the coil or lengthen the radiator.
• If resonance is too high (frequency too low), reduce turns or shorten the radiator by 1 cm increments.

Whether you’re a seasoned ham or a keen SWL, building your own 2m 5/8 wave mobile antenna is a one-evening project with excellent results. The 10.5 turn coil with a tap at turn 4 is the heart of the 50 Ω match – get that right, and you’ll be working repeaters and simplex with ease.

Have you built a similar antenna? Share your photos or tuning results in the comments below.

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