J-Pole Antenna Calculator
Calculate practical dimensions, feed point location, and antenna characteristics for a J-Pole antenna.
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Inputs
Live
Math
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Related
Enter parameters and click Calculate to view results
Formula & Theory
Radiator ≈ 143/f, Matching Stub ≈ 71.5/fThis formula is used to calculate antenna parameters for j-pole antenna calculator.
Overview
The J-Pole Antenna Calculator helps amateur radio operators, RF engineers, antenna designers, emergency communication teams, and students calculate the practical dimensions of a J-Pole antenna. By entering the operating frequency, the calculator determines wavelength, radiator length, quarter-wave matching stub length, total antenna height, feed-point location, recommended element spacing, feed impedance, and estimated antenna gain. It is widely used for VHF, UHF, amateur radio, public safety communication, repeater systems, emergency communication networks, and fixed wireless installations.
Input Guide
Enter Frequency exactly in the units shown by this j-pole antenna. Check the operating band, unit prefix, and decimal position before calculating; these are the inputs used by the formula.
- Frequency — use MHz.
Output Guide
The results describe the calculated j-pole antenna values for the inputs you entered. Check each value against the available space, selected components, feed system, and operating conditions before making a final design decision.
How This Calculator Works
The J-Pole Antenna uses Radiator ≈ 143/f, Matching Stub ≈ 71.5/f. Supply Frequency (MHz) in the displayed units, then use the calculated values as the first engineering target for this specific antenna types design or analysis.
Design Notes
The J-Pole antenna is a popular end-fed vertical antenna consisting of a half-wave radiator and a quarter-wave matching stub. Its design eliminates the need for a conventional ground plane while providing an omnidirectional radiation pattern and low-angle radiation that is ideal for terrestrial communication. Because the antenna is end-fed, impedance matching is achieved by selecting the correct feed-point position along the matching stub. Practical antenna performance depends on conductor diameter, element spacing, mounting height, nearby conductive objects, feed-line routing, common-mode current, weather conditions, and construction accuracy. Although this calculator provides reliable starting dimensions, final tuning is required for optimum VSWR and radiation efficiency.
Build and Tuning Notes
Use the calculated radiator length and matching stub dimensions as the initial construction values. Leave the radiator slightly longer than calculated and gradually trim it while monitoring the resonant frequency. Adjust the feed-point position along the matching stub to obtain the desired impedance match, typically around 50 Ω. Install the antenna in a clear location away from nearby metallic structures and use a common-mode choke or current balun on the coaxial feed line to minimise feed-line radiation. After installation, verify resonant frequency, return loss (S11), VSWR, and impedance using an antenna analyser or vector network analyser (VNA). Fine-tune both the radiator length and feed-point location until optimum antenna performance is achieved.
Frequently Asked Questions
What is a J-Pole antenna?
A J-Pole antenna is an end-fed vertical antenna that combines a half-wave radiator with a quarter-wave matching stub. It provides omnidirectional coverage without requiring a conventional ground plane and is widely used for VHF and UHF communication.
Where are J-Pole antennas commonly used?
J-Pole antennas are commonly used for amateur radio, VHF and UHF repeaters, emergency communication systems, public safety networks, base stations, APRS, and general-purpose terrestrial radio communication.
Why is the feed-point location important?
The feed-point position determines the antenna impedance. Moving the feed point up or down along the matching stub changes the impedance, allowing the antenna to be matched to a 50 Ω coaxial feed line.
Does a J-Pole antenna require a ground plane?
No. The quarter-wave matching stub performs the impedance transformation, allowing the antenna to operate efficiently without the radial ground plane required by a conventional quarter-wave monopole.
What are the advantages of a J-Pole antenna?
J-Pole antennas are simple to build, offer low-angle omnidirectional radiation, provide good efficiency, require no ground radials, and are well suited for amateur radio, repeater, and fixed communication applications.
Why can measured antenna performance differ from calculated values?
Actual performance may vary because of conductor diameter, feed-point adjustment, mounting height, nearby conductive structures, cable routing, common-mode current, environmental conditions, manufacturing tolerances, and measurement uncertainty. Practical tuning and field measurements should always be performed before final installation.
Alex Warren
B.Sc. in Electrical & Electronic Engineering (EEE)
Alex specialises in antenna design and wave propagation. His expertise helps ensure these calculators present practical RF concepts, useful design estimates, and clear engineering guidance for students, HAM operators, and wireless professionals.