The Twin-lead J-Pole antenna is a classic and highly efficient vertical antenna widely used by amateur radio operators on the VHF and UHF bands. The attached diagram shows a traditional J-Pole constructed using 300-ohm twin-lead, a method that combines electrical efficiency with mechanical simplicity. This antenna design has been popular for decades because it delivers reliable performance without the need for radials or complex matching networks.
What Is a Twin-Lead J-Pole Antenna?
A J-Pole antenna is essentially a half-wavelength vertical radiator fed through a quarter-wavelength matching stub. The “J” shape is formed by the long radiating element and the shorter parallel section at the bottom. In this design, both sections are created using 300-ohm twin-lead, with one conductor partially removed to form the radiating element.
The antenna is end-fed, vertically polarized, and can be connected directly to 50-ohm coaxial cable, making it ideal for FM, repeater, and general VHF/UHF operation.
Understanding the Sections Shown in the Diagram
The attached image clearly identifies the two electrical sections of the antenna.
The half-wave section is the main radiating element. Its approximate length is calculated using:
Half-wave length ≈ 5540 ÷ frequency in MHz (cm)
This section determines the operating frequency of the antenna and provides efficient radiation with a low angle of takeoff.
The quarter-wave section forms the impedance-matching stub. Its length is approximately:
Quarter-wave length ≈ 2400 ÷ frequency in MHz (cm)
Because twin-lead has a velocity factor lower than free space, this section is shorter than a true free-space quarter wavelength. This matching section transforms the high impedance of the half-wave radiator down to a value suitable for 50-ohm coax.
Twin-Lead J-Pole Antenna –Feed Point
The antenna is fed near the bottom of the matching section using 50-ohm coaxial cable such as RG-58 or RG-8. The feed point is adjustable along the stub, allowing fine tuning for the lowest SWR. This adjustable feed is one of the reasons the J-Pole is easy to match without additional components.
The diagram also notes that the coaxial feed should be kept as short as practical, especially at VHF and UHF, to minimize feed-line losses.
Construction Using 300-Ohm Twin-Lead
In this design, one conductor of the twin-lead is cut away at the top, as shown in the diagram. This creates the half-wave radiator while the remaining parallel section acts as the matching stub. The twin-lead serves three purposes at once: it provides the radiator, the matching network, and mechanical support.
The top of the antenna can be lightly tacked or secured, as it is a high-impedance point and carries minimal current.
Dimensions for VHF (2-Meter Band – 144 MHz)
Using 144 MHz as the design frequency:
Half-Wave Radiator Quarter-Wave Matching Section
Practical Notes –The total antenna height will be slightly longer due to spacing and feed connection. It is recommended to cut the twin-lead slightly longer and trim during SWR adjustment.
Dimensions for UHF (70-cm Band – 435 MHz)
Using 435 MHz as the design frequency:
Half-Wave Radiator Quarter-Wave Matching Section=
Practical Notes –At UHF, construction accuracy becomes more critical. Keep leads short, solder joints clean, and the coax feed as short as possible to reduce losses.
Grounding and Outdoor Installation
For outdoor installations, the lower end of the antenna may be grounded for lightning protection, as indicated in the diagram. Grounding the coax shield and the base does not significantly affect antenna performance but improves safety and reduces static buildup.
Performance Characteristics
The Twin-lead J-Pole antenna offers excellent real-world performance for its simplicity. Because it is a half-wave radiator, current maximum occurs above ground, which typically results in lower noise and better efficiency compared to quarter-wave verticals with radials.
Its radiation pattern is omnidirectional in the horizontal plane, making it ideal for repeater access and general local communication.
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