Building a Quarter-Wavelength Coaxial Collinear Antenna for 2 Meters

For VHF enthusiasts, the Coaxial Collinear antenna remains one of the most practical, compact, and high-gain omnidirectional antennas. It is especially popular for the 2 Meter amateur band (144–148 MHz) because it combines ease of construction with impressive gain and low-angle radiation — ideal for local repeater work and FM simplex operation.

While most builders use half-wavelength sections of coax in traditional Coaxial Collinear antenna designs, the quarter-wavelength COCO offers an interesting alternative that simplifies matching and can be made more compact without compromising much performance. While less common in published literature for the COCO, using λ/4 sections connected with phase-reversing joins can still achieve the desired collinear phasing effect. This is often done by including an initial matching or phasing section that adjusts the impedance and phase relationship for the array.

Coaxial Collinear Antenna –Basic Concept

A collinear antenna means “elements in line.” In a Coaxial Collinear antenna design, the radiating elements are formed by alternating sections of coaxial cable, arranged vertically. Each section acts as a radiating element or phase inverter, depending on its electrical length. The alternating inner and outer conductors are connected in such a way that the current phase at each junction adds in phase along the vertical axis — producing vertical stacking gain.

For a quarter-wave COCO, each section of coax is λ/4 (quarter wavelength) long, and these sections are stacked end-to-end with alternating inner and outer conductors connected.

What is a Coaxial Collinear Antenna?

A Coaxial Collinear antenna  is essentially an array of multiple radiating elements stacked end-to-end, all aligned on a single axis (collinear). The key to its performance is the phasing mechanism—by carefully alternating the connections between the coaxial cable’s center conductor and braid for each section, the current in each radiating section is kept in-phase.

• In-Phase Radiation: When all sections radiate in the same phase, their signals combine constructively, which dramatically compresses the radiation pattern toward the horizon.
• High Gain: This horizontal compression results in a substantial gain increase compared to a single dipole or ground plane, making it excellent for local and repeater contacts.
• Omnidirectional: Being a vertical array, it maintains an omnidirectional (360-degree) pattern in the horizontal plane.

Why Quarter-Wavelength Sections?

Most Coaxial Collinear antenna use ½λ (half-wave) segments. But using ¼λ segments provides several benefits:

1. Shorter total antenna length – compact and rigid for portable or rooftop setups.
2. Better mechanical strength– since each segment is shorter.
3. Impedance transformation – quarter-wave sections act as impedance transformers, allowing smoother matching between sections.
4. Simpler construction – particularly when using mixed cable types (e.g., RG-58 and 75 Ω TV coax).

However, careful attention must be given to feed point design, as quarter-wave stacking changes phase differently than the half-wave COCO.

Quarter-Wave vs. Half-Wave Sections

Most classical COCO designs use half-wave (λ/2) segments. The reason is simple: a λ/2 transmission line section acts as a phase inverter — ensuring the current in the next element is correctly phased for additive radiation.

However, a variation exists using quarter-wave (λ/4) sections. While less common in published literature for the COCO, using λ/4 sections connected with phase-reversing joins can still achieve the desired collinear phasing effect. This is often done by including an initial matching or phasing section that adjusts the impedance and phase relationship for the array.

In these designs:

• The combination of a radiating and phasing section often totals ½ wavelength (λ/2).
• Proper alternation of inner-to-shield connections maintains the required current phase.
• Shorter λ/4 sections make the antenna mechanically more compact and easier to enclose in PVC tubing.

For simplicity and proven performance, the half-wave method remains the standard. However, the quarter-wave COCO provides excellent performance when designed carefully — especially for experimental and portable applications.

Design Parameters for 2 Meter Band

For 144 MHz, the free-space wavelength (λ) is:

Since the signal travels slower inside coaxial cable, we must correct for the velocity factor (VF) of the coax:

Therefore, if you’re using RG-58, cut each element to 34 cm (¼λ) of coax (center conductor length). If you mix cables, ensure each section is adjusted for its own velocity factor.

Coaxial Collinear Antenna –Construction Steps

 Materials

• Coax cable: RG-58 (50 Ω) or RG-59 (75 Ω)
• Main feed line: RG-58 (50 Ω preferred)
• Soldering tools and sharp cutter
• PVC pipe or fiberglass rod for support
• Heat-shrink tubing or tape for weatherproofing

Preparing the Sections

1. Cut 4–8 pieces of coax cable to ¼-wave electrical length.
2. Strip the outer jacket and braid at each end — leaving enough to solder.
3. For each junction, connect the inner conductor of one section to the braid of the next.
4. Continue stacking this alternating pattern. This ensures current reversal and in-phase radiation.

 Feed Point

• The bottom end can be directly connected to your RG-58 feed line.
• For best results, include a ¼-wave matching stub or choke balun (a few turns of RG-58 near the feed) to minimize feedline radiation.
• The top end is left open (insulated).

Phasing and Matching

Each junction between inner and braid inverts current and maintains correct phase stacking. The result is several radiating sections, each adding its field vertically in phase — yielding a gain of ~3 dB per doubling of sections (theoretically).

Typical total gains:

Because each section is ¼-wave, the current maxima occur near the junctions — giving a strong low-angle radiation pattern, perfect for VHF line-of-sight coverage.

Testing and Tuning

After assembly:

1. Check continuity with a multimeter — there should be no shorts between center and shield except at the intentional connections.
2. Mount the antenna vertically, at least 1 m above ground.
3. Test with an SWR meter or antenna analyzer.
   • Expect a near 1.3–1.5 : 1 SWR across the 144–146 MHz band.
   • Small trimming (1–2 mm per segment) can fine-tune resonance.

Mounting & Protection

• Mount the antenna inside a PVC pipe (20–25 mm dia.) for rigidity.
• Seal all joints with silicone or self-amalgamating tape.
• Use a U-clamp or pipe holder to attach it to a mast.

For permanent installations, ensure the top is sealed against moisture ingress — water trapped in coax severely detunes performance.

Performance

When properly tuned, the ¼-wave COCO can offer:

• Gain: 5–8 dBi depending on number of sections
• Radiation Pattern: Omnidirectional, low elevation angle
• Bandwidth: 3–5 MHz
• Feed Impedance: 50 Ω (close match to RG-58)

On-air results often show 2–3 S-unit improvement over a simple ½-wave vertical or ground plane.

Feed Point and Decoupling

In a CoCo collinear coaxial antenna, the feed point is a critical area since the array inherently exhibits high impedance. One of the most common challenges here is common-mode current flowing along the feedline, which can distort the radiation pattern and reduce efficiency.

λ/4 Shorted Stub for Impedance Matching

To address this, a quarter-wave (λ/4) shorted stub is often placed near the feed point or at the end of the last element. This stub, made from a piece of coaxial line shorted at one end, acts as a parallel resonant circuit at the operating frequency. Its high impedance helps block unwanted currents and aids in matching the feedline to the antenna’s impedance.

Common-Mode Choke or Balun

Additionally, a common-mode choke—often implemented as an “ugly balun” (a few tight turns of the coax feedline wound into a coil) or by adding ferrite beads around the coax—plays an important role. It suppresses feedline radiation, ensuring that only the antenna elements radiate, maintaining a clean omnidirectional pattern and stable SWR.

Variations & Experiments

• Use alternating 75 Ω and 50 Ω coax sections to improve impedance balance.
• Try foam dielectric RG-6 for lighter weight and higher VF.
• Add a ¼-wave sleeve or ground skirt at the base for improved pattern symmetry.

The quarter-wavelength Coaxial Collinear antenna is an elegant experimenter’s project — blending RF theory and practical design. With simple coax, a few cuts, and careful soldering, you can build a rugged, high-gain vertical for the 2 Meter band.

Whether you’re calling a distant repeater or experimenting with homemade antennas, the COCO design proves that great performance doesn’t always require complex parts — just coax, patience, and curiosity.

Coaxial Collinear Antenna  –Further Reading and References

• Electronics Notes: Coaxial Collinear Antennas — A good overview covering the basics, how they work as phased arrays, and their use in VHF/UHF communications.
• Collinear and Coparallel Principles in Antenna Design (PDF) — A more academic paper discussing the evolution of collinear arrays, including coaxial and microstrip versions.
• Build A 9 dB, 70cm, Collinear Antenna From Coax — A step-by-step DIY guide for building a high-gain CoCo antenna, including element length calculation and matching techniques.
• Coax collinear Antenna dimension calculator — A helpful resource for calculating the correct segment lengths based on your desired frequency and the coaxial cable’s velocity factor.
• “How the Coaxial Collinear Antenna Works?” — Q&A on Ham StackExchange — Community discussion of how and why the core and shield cross-connections cause radiation in CoCo antennas.
• “Everything You Need to Know About Collinear Antenna?” — Antenna Experts Blog — General article covering collinear antennas with a section specific to coaxial-collinear types.
• High Gain Collinear Antenna for 145 MHz and 433 MHz Bands  – illustrates a practical and efficient collinear antenna design that operates around 145 MHz (VHF) and 433 MHz (UHF) frequencies — both highly active amateur radio bands.
• VHF Collinear J-Pole : The DIY Super J-Pole Antenna
• Simple High-Gain collinear antenna using coaxial cable