Common mode currents can cause numerous problems including directional pattern distortion, unpredictable SWR readings and undesirable radio frequency emissions.  Common mode type currents can be generated by several types of asymmetry in antenna-feed line systems as well as other factors involving your transmitter, feed line and antenna.

    The purpose of a transmission line is to deliver the maximum power generated by the transmitter to the antenna.  A variety of factors influence the efficiency which the antenna system operates.  Factors such as cable velocity, core resistance, the dielectric used to separate the conductor from the shield, core diameter, etc., all come into play when RF flows at the time of transmission.

     Unwanted currents can leak and/or reflect back from the load (antenna) which results in a current flow on the coax shield in a reverse direction, back towards the source (transmitter ).  These currents can be effectively dealt with by installing a device referred as a RF choke tuned to the operating frequency.  This RF choke operates effectively between 25Mhz and 29 Mhz.

     Although each installation varies, it has been found that this RF choke will increase transmitter efficiency by reducing, and in most cases eliminating, common-mode currents from riding on the outside of the coax shield, and secondly to some extent reflecting the power back at the intended radiator (antenna).  Ultimately this will keep the SWR at an acceptable level thereby delivering the maximum power to the antenna.

     Some tests have also shown that certain performance characteristics increase as more RF power is applied, again results may vary with each installation.  For best results, the choke should be mounted to the mast (or support structure) directly under the antenna.  Placement of this device at the base of the antenna will dissipate the current at the source of reflection as well as reflect the currents back at the load.  In rare circumstances some installations may require an additional RF choke installed at the output of the transmitter which will further reduce any residual currents.

    Ground wire approaching 1/4 wavelength can (and in most cases will) produce highly undesirable results by acting as a very effective radiator.  A quite common example is when your radio room is located on the second floor (or above) and the ground wire is ran down to ground.  Not only will the ground wire act as a antenna, the transceiver's cabinet and other components (e.g. microphone, antenna tuners, etc.) may become RF hot.  The same RF can also leach into telephone lines, cable television systems, personal computers and virtually anything which shares a common connection such as ground.