How this calculator works
A J-pole combines an approximately half-wave radiator with a quarter-wave matching stub, so its long and short sections are nominally three quarters and one quarter of an effective wavelength. Velocity factor represents the chosen physical structure; use actual transmission-line data for a twin-lead design. The feed-tap and spacing fractions are adjustable layout estimates, not impedance matching solutions. A uniform nonmagnetic dielectric has velocity factor approximately 1/sqrt(relative permittivity), but a partly air-filled antenna needs an effective value.
Formula
lambdaeff = c VF/f; A = 3 lambdaeff/4; B = lambdaeff/4; C = tap lambdaeff; D = gap lambdaeff; VF ≈ 1/sqrt(epsilon_r).
Worked example
At 299792458 Hz with velocity factor 1, effective wavelength is 1 m, A is 0.75 m and B is 0.25 m. Tap and spacing fractions of 0.025 each give 0.025 m starting dimensions.
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Frequently asked questions
Are the feed tap and spacing exact?
No. They must be tuned with impedance or standing-wave measurements. Tube diameter, feed line, mounting and nearby metal all matter.
Can I substitute an optical-fibre velocity factor?
An optical material constant does not describe a practical RF antenna structure. Use the effective RF velocity factor of the actual conductor and dielectric geometry.
Where is the length reference?
Both section lengths are measured from the shorted bottom of the matching stub. The long section includes the upper radiating portion.