POTA-CHOKE

The POTA-Choke is a compact and rugged enclosure designed for a high-performance HF common mode choke, specifically optimized for Parks on the Air (POTA), portable amateur radio operations, and field deployments.

The enclosure securely houses an FT240-31 ferrite core with bifilar windings and provides mounting for two SO-239 connectors. Its compact design allows it to be attached directly to an antenna mast using a bungee cord or placed close to the radio to suppress unwanted common mode currents.

Whether you're operating a portable vertical, dipole, EFHW, or other HF antenna, this enclosure offers a practical and durable solution for building a professional-grade common mode choke.

Motivation

Many amateur radio antennas—including Verticals, Dipoles, End-Fed Half-Waves (EFHW), Yagis, G5RV, ZS6BKW, Windom, Doublets, Cobwebs, and Inverted-V antennas—can generate unwanted common mode currents on the outside of the coaxial cable shield.

These currents can lead to:

• Increased RF noise

• Distorted radiation patterns

• RF feedback into the radio station

• Reduced antenna performance

A properly designed common mode choke presents a high impedance to these unwanted currents while allowing the desired differential RF signal to pass with minimal loss, significantly improving antenna performance and reducing interference.

Operation

The choke uses a bisectional bifilar winding on an FT240-31 ferrite core, providing excellent suppression of common mode currents across the HF spectrum.

Typical performance:

• 25–45+ dB suppression from 160 m to 10 m

• Nearly 20 dB suppression on the 6 m band

• Suitable for power levels up to 1 kW when constructed using AWG18 (0.75 mm²) PTFE-insulated wire (600 V).

This design is based on proven amateur radio choke designs and offers low insertion loss while maintaining excellent broadband performance.

Features

• Compact enclosure optimized for POTA and portable operation

• High-performance bifilar winding design

• Uses an FT240-31 ferrite core for excellent HF common mode suppression (3–30 MHz)

• Supports two SO-239 female connectors

• Low insertion loss

• Internal winding support for secure ferrite mounting

• Rugged and compact enclosure

• Drain/vent hole to reduce internal condensation and corrosion

• Integrated bungee cord slots for quick mounting to antenna poles or masts

• Suitable for ladder line, open-wire, or coax-to-balanced transitions where excellent symmetry is required

Recommended Antennas

This choke works exceptionally well with:

• Vertical antennas

• Dipoles

• End-Fed Half-Wave (EFHW)

• Yagis

• G5RV

• ZS6BKW

• Windom antennas

• Doublets

• Cobweb antennas

• Inverted-V antennas

• Other HF coax-fed antennas

Intended Use

Designed primarily for POTA, SOTA, field days, emergency communications, and portable HF stations, but equally suitable for fixed station installations where a compact, weather-resistant common mode choke is desired.

Materials

• PLA filament

• 4 × M4 × 20 mm Phillips pan head screws

• 4 × M3 washers

• 8 × M3 × 19 mm Phillips pan head screws

• 8 × M3 nylon lock nuts (Nyloc)

• 4 × M3.5 fork-type cable lugs (spade terminals)

• 2 × SO-239 UHF female 4-hole flange panel connectors

• 1 × FT240-31 ferrite toroid core

• 4 m of 1 mm enameled copper wire (polyester enamel that burns off at approximately 150 °C for easy soldering)

Tools

• 3D printer

• Deburring tool or hobby knife

• Phillips screwdriver

• Needle-nose pliers

• Soldering iron and solder

• Wire stripper

• Cable lug crimping tool