The High gain collinear antennaremains a favorite among radio amateurs and RF engineers who want high gain and omnidirectional coverage without resorting to complex beam designs. The drawing below 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.
This antenna is easy to construct, delivers reliable performance, and can be adapted for portable or fixed-station use. The clever use of 0.64-wavelength (λ) elements and 0.25-wavelength phasing sections ensures that all segments radiate in phase, maximizing signal strength in the horizontal plane.
Collinear antenna – Design Concept
A collinear antenna consists of multiple radiating elements stacked vertically. Each element adds gain by reinforcing the horizontal radiation pattern. In this design, two 0.64λ elements are connected by a 0.25λ phasing section made of coaxial cable or matching tubing.
This phasing section ensures that current in the upper element flows in the same direction as the current in the lower element, creating a constructive interference pattern that significantly enhances the effective gain. The antenna works as a balanced radiator, eliminating the need for a separate ground plane — a major advantage over quarter-wave monopoles.
Why 0.64 λ Elements Instead of 0.5 λ?
Although a half-wavelength element resonates well, using a 0.64-wavelength section improves both gain and impedance match. At 0.64 λ, the current distribution along the radiator produces a slightly compressed vertical lobe, giving the antenna a flatter radiation pattern and therefore greater range. This configuration typically yields 2 to 3 dB more gain compared to a standard half-wave design. Moreover, it provides a convenient 50 Ω feed impedance, which is ideal for direct connection to modern transceivers or coaxial feeders.
Dimensional Details
The dimensions shown in the schematic are tailored for two amateur bands:
For 145 MHz:
-
- Radiator length (A): 125 cm
- Phasing section (B): 50 cm
- Separation gap (S): 10 cm
- Diameter of tubing (D): 12 mm
For 433 MHz:
-
- Radiator length (A): 42 cm
- Phasing section (B): 16.5 cm
- Separation gap (S): 6 cm
- Diameter (D): 6 mm
These proportions maintain the electrical relationships of 0.64λ for radiating sections and 0.25λ for the phasing stub. The feed point (Z = 50 Ω) is located near the junction between the lower radiator and the phasing section.
Collinear antenna –How It Works
When RF energy enters the feed point, it drives the lower radiator section, which behaves as a 0.64 λ resonant element. The signal then travels through the 0.25 λ phasing line, introducing a 180-degree phase shift, effectively reversing the current direction.
Because the upper element’s current is naturally 180° out of phase at its junction, this reversal realigns both currents in phase, allowing both radiators to reinforce each other in the horizontal direction. The result is a strong, broad horizontal radiation pattern with minimal upward energy — ideal for long-distance terrestrial communication.
The finished antenna exhibits:
- High forward gain (around 5 – 6 dB over a half-wave dipole)
- Omnidirectional horizontal coverage
- Good impedance matching at 50 Ω
- Mechanical simplicity and portability
Some beginners consider replacing the 0.64 λ sections with 0.25 λ elements to make the antenna shorter. However, ¼-wave elements depend on a ground plane and cannot function independently in a stacked structure. Using them would cause out-of-phase currents, poor impedance match, and significant radiation loss. That’s why every efficient collinear design — including this one — relies on self-resonant half- or 0.64-wavelength elements.
Feed Point and Impedance Adjustment
In this collinear antenna design, the feed point is located exactly at the junction between the lower 0.64λ radiator and the 0.25λ phasing section. At this location, the antenna presents a natural impedance close to 50 ohms, making it suitable for direct coaxial feed without a complex matching network.
However, in practice, slight adjustments may be required to achieve the lowest SWR. The connection between the coax feed line and the lower radiator can be shifted slightly up or down — usually within a few centimeters — to fine-tune the impedance. Moving the feed point upward slightly increases impedance, while shifting it downward reduces it.
This adjustment compensates for variations in tubing diameter, insulation thickness, or environmental factors such as nearby metal objects. Once the optimum feed position is found, it should be soldered or clamped firmly for long-term stability.
Many builders also use a small 1:1 choke balun (made from 4–6 turns of coax wound on a ferrite core) at the feed line entry. This prevents common-mode currents from traveling down the outer shield of the coaxial cable, ensuring the antenna radiates purely from its intended elements.
When properly tuned, the SWR can easily reach below 1.3:1 across the target frequency band, offering excellent efficiency and compatibility with standard VHF/UHF transceivers.
Reference
Design based on practical implementation from original schematic source.
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