For UHF repeater systems operating around 435 MHz, antenna efficiency and gain are critical. Simple vertical antennas often do not provide sufficient performance, especially when wide coverage and reliable signal strength are required. To address this need, a UHF collinear antenna design adapted from the well-known Diamond BC-200 has been developed and documented by Kostadin Evstatiev (LZ1DJ).
This 3 x 5/8 Collinea antenna is intended specifically for the 420–440 MHz band and provides high gain without requiring post-installation tuning when constructed accurately.
UHF Collinear Antenna Construction
The antenna consists of a radiator, housing, mechanical support, connector, and radial system. All major dimensions and construction details are shown in the original design drawings. The structure is enclosed in a protective casing, making it suitable for outdoor installations and long-term repeater service.
Radiator Design and Collinear Structure
The radiating element is the most important part of this antenna. It is constructed from three radiating sections, each having an electrical length of 5/8 wavelength, connected in series.
Each section consists of a straight vertical radiator combined with an extension (phasing) coil. The radiator wire is made from 2.5 mm diameter copper wire, and the extension coils are wound from the same wire on a mandrel with a diameter of 8 mm.
The three radiating sections and the two extension coils can be formed from a single continuous wire, although construction is often easier if the upper sections are built separately and later joined using copper sleeves or chandelier terminals. During assembly, rubber washers are placed on each section and later fixed in position using adhesive after final alignment.
This configuration ensures that all three radiating sections operate in phase, forming a true collinear antenna with increased gain and a flattened radiation pattern.
Collinear Antenna Matching Coil and Feed
At the base of the antenna, a separate coil designated as L1 is used for impedance matching. This coil is wound on a Teflon, polyamide, or Plexiglas former and consists of 4.75 turns of silver-plated wire with a diameter of 1.5 mm.
The lower end of the coil is soldered directly to the connector body, while the upper end is connected to the vertical radiator using a terminal. A tap is taken from 3.8 turns above the grounded end and connected to the center pin of the connector. This arrangement provides proper impedance matching to standard coaxial feed lines.
Antenna Body and Weather Protection
The antenna body serves both mechanical and protective purposes. It consists of a support, a casing, and a top plug.
The support is manufactured from aluminum, while the casing is made from PVC or fiberglass tubing with an outer diameter of 20 mm, wall thickness of 2.5 mm, and a total length of 1500 mm. The casing is glued into the support using epoxy or industrial adhesive.
For long-term durability, it is recommended to protect the casing from ultraviolet radiation by covering it with a milky-white self-adhesive foil.
Radial System
The antenna uses four horizontal radials, made from stainless steel (chrome-nickel). These radials serve as the RF reference and help stabilize the radiation pattern.
They also mechanically secure the connector and are fastened using M5 cadmium-plated nuts, ensuring both electrical reliability and mechanical strength.
Performance and Gain
When built according to the specified dimensions, the antenna delivers a measured gain of approximately 6 dB across the 420–440 MHz range. Importantly, this performance is achieved without the need for additional tuning, making it ideal for repeater installations where stability and consistency are essential.
The collinear arrangement produces a low-angle radiation pattern, improving coverage over long distances and maximizing repeater effectiveness.
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