Common Mode Choke

Workshop Build-athon

Saturday, April 6, 2024

9:00 am – 11:00 am

Ham Radio Outlet (training room) 5730 W Good Hope Road

A Common Mode Choke (CMC) should be part of any antenna system using an End Fed antenna near the entry point to the ham shack or portable transceiver. The purpose of a Common Mode Choke is to remove stray currents traveling along the coaxial feedline shield towards the transceiver. Without one, noise and outside RFI may be present masking low-level signals. A CMC also reduces or eliminates RFI generation coming from the feedline into your home. And a CMC may also slightly improve SWR on non-resonant antennas or odd length feedlines.

Placement of a Common Mode Choke is critical to its effectiveness as well as the effectiveness of the antenna. Do not place a CMC immediately following the impedance matching transformer of an End Fed Half Wave or End Fed Random Wire as both depend on a portion of the feedline acting as a counterpoise. A CMC should always be placed near the radio end of the feedline.

A note about random length antennas. While “random” may be interpreted as “any length,” in fact it is not. The length should never be the resonant length associated with any operable bands you intend to operate. Certain lengths have been proven to be more effective than others. Consult this guide for the optimal length needed for your specific antenna considerations.

Join us on the first Saturday in April to build this useful and inexpensive addition to your ham shack or portable antenna grab bag. Noted below is the parts list necessary to build the CMC. The enclosure can be purchased at any hardware of big box home improvement retail store.

Parts List

Please bring small tools and soldering equipment to this Build-athon. Screw drivers, pliers, snips, soldering iron are all that is necessary. (Note: we do have soldering irons available). As always in a workshop environment, wear safety glasses and watch out for pointed objects and hot soldering irons.

Reservations are required. Guests are allowed to attend and participate if accompanied by a member.