https://www.marcelpost.com/wiki/index.php?action=history&feed=atom&title=Common_mode_interferenceCommon mode interference - Revision history2026-05-17T04:57:04ZRevision history for this page on the wikiMediaWiki 1.39.17https://www.marcelpost.com/wiki/index.php?title=Common_mode_interference&diff=2562&oldid=prevAdmin at 23:58, 15 December 20162016-12-15T23:58:23Z<p></p>
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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.<br />
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For related information on the subject, see also our article on [[impedance matching]] and [[baluns]].<br />
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===Why we don't want Common Mode currents===<br />
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* high voltage at radio can cause 'RF in the shack', 'hot' microphones, RF burns<br />
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* may cause RFI to nearby devices (distorted audio, upsetting cw keyers, fluorescent lamps flicker)<br />
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* 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.<br />
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* A system that radiates rf from the feedline is also susceptible to receiving rf from other signal sources, causing additional noise in the system.<br />
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===Causes of Common-mode currents===<br />
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Common Mode Currents can be caused by:<br />
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* imbalanced antenna<br />
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* feed point imbalance to balance transition<br />
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* running the feedline through the antenna's near-field<br />
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* other nearby noise sources<br />
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* Coax shield attenuation<br />
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* Ground loops (improper grounding)<br />
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* domestic appliances fed by unshielded power cables<br />
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===What causes an antenna to not be balanced?===<br />
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* nearby trees and building structures<br />
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* difference in radiating element lengths<br />
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* differences in height above ground of the radiating elements<br />
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The imbalance can also occur in the feedline and may be caused by:<br />
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* feedline design (e.g. coax)<br />
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* distance of nearby buildings to one side of a twin-lead feedline<br />
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So in short:<br />
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{| class="wikitable"<br />
|+Common Mode Currents<br />
|-<br />
|Cause<br />
|Effect<br />
|Symptoms<br />
|-<br />
|Imbalances in the form of:<br />
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nearby trees and buildings to an antenna,<br />
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different antenna element lengths,<br />
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difference in height above ground of antenna elements,<br />
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nearby metallic objects to feedline,<br />
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inherent feedline properties,<br />
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ground loops<br />
|common mode currents<br />
|RF in the Shack,<br />
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re-radiation of rf and can cause RFI,<br />
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interfere with antenna radiation pattern,<br />
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pick up unwanted rf and create additional noise<br />
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===Minimising Common Mode currents===<br />
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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:<br />
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* chokes<br />
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* isolation (1:1 baluns)<br />
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* grounding<br />
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* redistribute the effects over the transmission line conductors</div>Admin