J-Pole Antenna Calculator
Design J-pole antennas for any frequency band with precise dimensions
Calculate J-Pole Antenna Dimensions
Common bands: 2m (144-148 MHz), 70cm (420-450 MHz), 10m (28-29.7 MHz)
Typical range: 0.6 - 0.96
VF = 1/√εᵣ
J-Pole Antenna Dimensions
Example: 2-Meter J-Pole Antenna
Design Parameters
Frequency: 145 MHz (2-meter band)
Material: Copper wire (VF = 0.96)
Wavelength: λ = c/f = 299,792,458 / 145,000,000 = 2.07 m
Calculated Dimensions
A (Long section) = 0.75 × 2.07 m × 0.96 = 148.9 cm
B (Short section) = (2.07 m / 4) × 0.96 = 49.6 cm
C (Feed point) = (2.07 m / 50) × 0.96 = 5.0 cm
D (Spacing) = (0.045 × 2.07 m) / 2 = 4.7 cm
J-Pole Structure
Long Section
3/4 wavelength (0.75λ)
Main radiating element
Short Section
1/4 wavelength (λ/4)
Matching section
Feed Point
50Ω impedance point
Coax connection
Spacing
Element separation
Non-critical dimension
Common Velocity Factors
Design Tips
Mount at least 6 feet from metal objects
No ground plane required
Use choke balun near feed point
Omnidirectional radiation pattern
Vertical polarization
Understanding J-Pole Antennas
What is a J-Pole Antenna?
A J-pole antenna is a vertical antenna that gets its name from its J-like shape. Invented in 1909 by Hans Beggerow, it's essentially a half-wave antenna fed at one end through a quarter-wave matching section.
How Does It Work?
- •The long section (3/4λ) acts as the main radiating element
- •The short section (λ/4) provides impedance matching
- •Feed point provides 50Ω impedance for coax connection
- •No ground plane required unlike other verticals
Velocity Factor
The velocity factor (VF) describes how fast electrical signals travel through a material compared to the speed of light in vacuum. It affects the physical dimensions of the antenna.
VF = 1/√εᵣ
where εᵣ is the relative dielectric constant
Applications
- •Amateur radio communications
- •Maritime mobile installations
- •Emergency communications
- •Portable and fixed installations