Northwest Fisheries Science Center

Electromagnetic Interference

Electromagnetic Interference

Electromagnetic interference (EMI) is often more apparent with PIT–tag monitoring systems installed in streams than with those installed at dams.  At the dams, antennas for both juvenile and adult systems are shielded, while antennas in stream monitoring systems are both large and unshielded.  Since EMI is often directional, moving an unshielded antenna just a few meters or rotating it can significantly impact EMI levels. 

Ambient EMI

The strength of ambient EMI is a critical factor for determining the read range of the system and is therefore an important consideration during final placement of antennas. 

We have installed systems at sites where ambient EMI levels were consistently high enough to prevent detection of some tagged fish.  Therefore, we recommend a test deployment using the PIT–tag interrogation system to evaluate ambient EMI at different potential locations before final selection of the stream monitoring site.

Test deployments should use the same antenna that will be installed in the monitoring system or an antenna of similar size and shape.  The antenna should be placed in the same location and be oriented in the same direction as planned for the permanent installation.  Test deployments should continue for 1–7 d because sources of ambient EMI can be transitory or cyclic.

Internal EMI

Since PIT–tag detection relies on electromagnetic fields, any EMI that is at or near the operating frequency of the monitoring system can significantly reduce or prevent tag reading.  All parts of a PIT–tag monitoring system can generate significant levels of EMI. 

For example, EMI can be generated by the transceiver or transmitted through AC power lines.  EMI can also be caused by coupling or radiated from components such as the power–system battery or the battery charger of a laptop computer. 

Often, different arrangements or components must be tested to minimize the impact of the EMI before settling on a final design. 

Sometimes EMI can be quieted by grounding the instrument box or transceiver or by replacing a component with one from a different manufacturer. 

High power output by the transceiver can also produce internal EMI, even in the most electronically quiet transceivers.  Shorter cables generally produce greater read distances and may mitigate this problem, since they deliver sufficient current to the antenna without increasing transceiver power output.