Among the many antenna designs available to amateur radio operators, the coaxial collinear antennastands out for its rare combination of simplicity, low cost, and excellent performance. Built almost entirely from ordinary coaxial cable, this antenna is capable of producing real gain over a standard quarter-wave vertical while maintaining an omnidirectional radiation pattern that is ideal for local and regional VHF and UHF communication.
The antenna shown in the below attached diagram is a classic example of a vertically stacked coaxial collinear design. Variations of this antenna have been built and refined by radio amateurs around the world for decades, and many commercial base-station antennas use the same underlying principle, hidden inside fiberglass radomes.
What makes this design especially attractive is that it requires no traps, coils, or complex matching networks. When built carefully, it works “right the first time” and delivers consistent results.
Collinear antenna – Basic Operating Principle
A collinear antenna works by stacking multiple radiating elements in a straight line so that their radiation adds constructively in the horizontal direction. In simple terms, the energy that would normally spread upward and downward is compressed toward the horizon, which is exactly what is needed for terrestrial communication on VHF and UHF.
In the coaxial collinear antenna, the coax cable itself becomes both the radiating element and the phasing system. By cutting the coax into precise electrical lengths and selectively joining the inner conductor of one section to the outer shield of the next, each half-wave section is kept in phase with the others. This in-phase current distribution is what produces gain.
Each additional half-wave section increases the effective aperture of the antenna, resulting in a stronger signal at low elevation angles.
Feed Point and Ground Plane Arrangement
At the bottom of the antenna is a standard female RF connector, typically an SO-239 or N-type connector depending on frequency and power requirements. The coaxial feedline from the transceiver connects directly here, making the antenna electrically simple and mechanically robust.
A set of four quarter-wave radials forms the ground plane. These radials are usually cut slightly longer than the theoretical quarter-wave length, often by about five percent, to account for end effects and practical mounting conditions. The radials are soldered or mechanically connected to the connector ground and angled slightly downward, which helps stabilize impedance and improve the radiation pattern.
This ground plane section plays a crucial role. Without it, the antenna would be difficult to match and would suffer from feedline radiation and unpredictable behavior.
Quarter-Wave Matching Section
Immediately above the feed point is a quarter-wave section of coaxial cable. This part of the antenna acts as a matching and transition section between the feedline and the stacked half-wave radiators above it.
Because this section is electrically a quarter wavelength long, it helps transform the impedance seen at the feed point to something very close to 50 ohms. This eliminates the need for an external matching network and keeps SWR low across the intended operating band.
The accuracy of this section is important, especially at UHF frequencies, where small errors in length can have a noticeable effect on performance.
Collinear antenna –Half Wave Radiating Sections
The main body of the collinear antenna consists of multiple half-wave coaxial sections stacked vertically. Each section is cut to an electrical half wavelength, taking into account the velocity factor of the coaxial cable being used.
At the junction between sections, the outer shield of the upper section is carefully stripped back and soldered to the inner conductor of the section below. This soldered junction ensures that RF currents are correctly phased from one section to the next.
These joints are the heart of the antenna. When they are done correctly, current maxima occur at the correct points along the antenna, and all sections radiate in phase. When done poorly, losses increase and the antenna may perform worse than a simple quarter-wave vertical.
Open-Ended Top Section
The top of the antenna is left electrically open. This open end is intentional and is part of the overall electrical design. It allows the current distribution along the stacked sections to develop properly and prevents unwanted reflections that would otherwise distort the radiation pattern.
While electrically open, the top must be mechanically sealed to prevent moisture from entering the coax. Water ingress is one of the most common causes of failure in home-built coaxial antennas.
Collinear antenna –Calculating the Correct Lengths
To calculate antenna dimensions accurately, you must account for the velocity factor (VF) of the coaxial cable.
Typical values:
- RG-58: ~0.66
- RG-213: ~0.66
- Foam dielectric coax: ~0.78–0.85
Half-wave length (meters) = 150 / f₀ × VF
Quarter-wave length (meters) = 75 / f₀ × VF
Where f₀ is the frequency in MHz.
Always measure and cut carefully—errors of even a few millimeters matter at UHF.
A properly built coaxial collinear antenna offers:
- Gain: 4–8 dBi, depending on number of sections
- Radiation pattern: Low-angle, omnidirectional
- SWR: Typically below 1.5:1 if built correctly
- Bandwidth: Moderate, ideal for FM and digital modes
Radiation Pattern and Gain
When properly constructed, the coaxial collinear antenna produces a low-angle, omnidirectional radiation pattern that is well suited to FM, digital modes, and repeater work. Compared to a simple quarter-wave ground plane, the improvement is immediately noticeable, especially on marginal paths.
The actual gain depends on the number of half-wave sections used. Two or three sections provide a modest improvement, while four or more sections can produce a significant increase in effective radiated power toward the horizon. However, there is a practical limit, as losses in the coax and mechanical complexity eventually outweigh the benefits.
Assembly and Recommended Materials
To protect the antenna and ensure durability, it is recommended to install the structure inside aplastic (PVC) orfiberglass tube.An economical alternative is to usefiberglass telescopic fishing rods , easily found at fairs and popular stores.Never use carbon fiber rods , as carbon is conductive and will impair signal transmission.The joints between the segments must besealed against moisture , preferably withthermoplastic adhesive applied with a heat gun.This protection prevents oxidation and maintains the electrical integrity of theCollinear Antenna , even in outdoor environments subject to climatic variations.
Despite its simplicity, awell-designed homemade collinear antenna can achieve thesame performance as commercial models from renowned brands.The secret lies in the precision of the cuts, the speed factor of the cable, and the quality of the connections.
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- https://www.qsl.net/iz0dxd/antennacollineare.pdf