Common mode interference

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In transmitters and antenna systems, common mode interference needs to be properly addressed as they can be the source of all kinds of interference to their own or nearby electronics.

For related information on the subject, see also our article on impedance matching and baluns.

Why we don't want Common Mode currents

  • high voltage at radio can cause 'RF in the shack', 'hot' microphones, RF burns
  • may cause RFI to nearby devices (distorted audio, upsetting cw keyers, fluorescent lamps flicker)
  • RF over the feedline will also re-radiate and can interfere with nearby antenna elements (e.g. in a Yagi) and disturb the antenna's radiation pattern.
  • A system that radiates rf from the feedline is also susceptible to receiving rf from other signal sources, causing additional noise in the system.

Causes of Common-mode currents

Common Mode Currents can be caused by:

  • imbalanced antenna
  • feed point imbalance to balance transition
  • running the feedline through the antenna's near-field
  • other nearby noise sources
  • Coax shield attenuation
  • Ground loops (improper grounding)
  • domestic appliances fed by unshielded power cables

What causes an antenna to not be balanced?

  • nearby trees and building structures
  • difference in radiating element lengths
  • differences in height above ground of the radiating elements

The imbalance can also occur in the feedline and may be caused by:

  • feedline design (e.g. coax)
  • distance of nearby buildings to one side of a twin-lead feedline

So in short:

Common Mode Currents
Cause Effect Symptoms
Imbalances in the form of:

nearby trees and buildings to an antenna,

different antenna element lengths,

difference in height above ground of antenna elements,

nearby metallic objects to feedline,

inherent feedline properties,

ground loops

common mode currents RF in the Shack,

re-radiation of rf and can cause RFI,

interfere with antenna radiation pattern,

pick up unwanted rf and create additional noise

Minimising Common Mode currents

If we cannot alter the amount of generated common mode currents we have a few methods that in varying degrees of success can minimise the effects of common mode currents:

  • chokes
  • isolation (1:1 baluns)
  • grounding
  • redistribute the effects over the transmission line conductors