Calculator: J-Pole Antenna

The J-Pole antenna is an omnidirectional, vertically polarized antenna very popular for its durability and its slightly higher gain compared to a standard ground plane. It requires no radials, making it ideal for tight spaces or quick hoisting on a telescopic mast during an emergency.

How it works

• It is an end-fed half-wave antenna (radiator) driven by a quarter-wave matching stub.
• It can be built using twin-lead ribbon cable (“Slim Jim”), copper pipes, or aluminum rods.
• The coaxial cable connects at a specific point near the base to find the perfect 50-ohm impedance match.

Tactical J-Pole Calculator

Enter the operating frequency to get the construction measurements. (The gap and feed point measurements are starting approximations and may require slight adjustment with an SWR meter).

Center Frequency (MHz)

CALCULATE

Cutting Results

Long Element (1/2 λ) -- meters

Short Stub (1/4 λ) -- meters

Gap Spacing -- millimeters

Feed Point -- millimeters

⛺ Emergency Deployment Guide (For Non-Techs)

• Setup: The J-Pole antenna must always be mounted completely vertically.
• Recommended Height: Place it as high as possible and clear of metal roofs or large concrete structures. Tying it to the highest branch of a tree with non-conductive rope (nylon/paracord) or putting it on a wood/PVC mast is ideal.
• Isolators and Spacing: Unlike the dipole, the J-Pole is extremely sensitive to nearby metals. If you mount it on a metal mast, make sure the mounting point is below the shortest tube (the bottom of the “J”). The rest of the antenna should not touch metal.
• Feed Line (Coaxial): 50-ohm cable is recommended (RG-58, LMR-400). If you don’t have any, you can improvise with 75-ohm TV cable (RG-6) for emergencies; if the antenna is cut correctly, the radio will accept it without being damaged.