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J-Pole Antenna Calculator

Design your optimal J-pole antenna for a chosen frequency using our smart J-pole antenna calculator.

Clear
Half-wave radiator length982.0787 mm98.21 cm, 3.222 ft — plus a quarter-wave matching stub of 0.491 m — the stub transforms the high end-fed impedance down to 50 Ω
Free-space wavelength2.0675 mc ÷ f
Velocity factor applied0.95a real element is a few percent shorter than the free-space figure, because of end effect and conductor thickness — thicker elements need more shortening
Nominal feedpoint impedancematched by the stubin free space and well clear of ground — height above ground shifts it considerably
The classic 143 ÷ f rule0.9862 mthe traditional half-wave dipole formula, which bakes in a velocity factor of about 95%
Quarter wave491.0394 mmthe length of a ground-plane radiator, or a matching stub
At double the frequency491.0394 mmhalf the length — antenna size is inversely proportional to frequency, which is why low-band antennas are so large
Bandwidth guidancea few percent for a thin dipolethicker or caged elements broaden it considerably; a thin wire dipole is a narrowband device

The formula

half-wave dipole ≈ 143 ÷ f(MHz) metres

Why a half-wave dipole is short of a half wave

A resonant dipole is about 95% of a free-space half wavelength. The shortfall is the end effect: capacitance between the element tips and their surroundings makes the antenna electrically longer than it is physically, so it must be cut shorter to resonate. Thicker elements have more end capacitance and need more shortening, which is why the velocity factor is a range rather than a constant.

The traditional 143/f(MHz) formula for a half-wave dipole in metres has that 95% factor already baked in — which is why it does not match c/2f.

Feedpoint impedance depends on the antenna type: about 73 Ω for a free-space half-wave dipole, roughly 36 Ω for a quarter-wave over a good ground plane, and very high for an end-fed half wave, which is exactly why a J-pole needs its quarter-wave matching stub. All these figures shift substantially with height above ground.

Antenna size scales inversely with frequency, and there is no way around it: a resonant 80-metre-band dipole really is about 40 m long. That single fact shapes every practical compromise in low-band antennas.