RF Interference and LEDs

RF Interference and LEDs

Posted by Marinebeam on 2nd Sep 2018

Recently the US Coast Guard issued a warning to the maritime community regarding the potential interference of VHF-FM radio and AIS reception from LED lighting. As you might imagine, this has resulted in a flood of inquiries to Marinebeam regarding this issue.

In the business we refer to this radio interference as EMI, or Electro-Magnetic Interference. It is also sometime referred to as RF (Radio Frequency) interference. Another term heard often is EMC, or Electro-Magnetic Compatibility, which is a device’s ability to meet the minimum specification for performance, emissions, and immunity for its intended application.

Since 2007, Marinebeam has been a leader in producing low-EMI LED products, and takes special care in design and testing to avoid these issues. But, it is somewhat of a black-art, so read on to understand more.

So what is EMI, and why do LEDs have the potential to cause it? The LED emitters are passive diodes, and they themselves don’t cause the interference, but rather it is the associated control electronics that can cause the RF emissions. You see, LEDs are very sensitive to changes in voltage, including the rising and falling voltages found on boats. Too much voltage (like when charging batteries), and the LED will be permanently damaged. Too little voltage (as when the battery is discharged), and the LED will not provide enough light for its intended use. So, there must be some sort of regulator to provide the LEDs with a constant drive current, regardless of the incoming voltage level. Without this type of control the LED is just not suitable for use on boats.

These electronic drivers are essentially switch-mode power supplies that control voltage and current by switching a transistor on and off very rapidly. This is referred to as Pulse Width Modulation, and is the basis for almost all modern power supplies, including those used with laptops, smart phones, and televisions. It is this switching sequence that creates the potential for RF emissions. Each time the transistor changes states, there is a small spike of transient voltage that can manifest itself as EMI. EMI can either be radiated (through the air as a radio wave), or conducted (via wires running throughout the boat).

The good news is that due to the increased use of LEDs in such things as automobiles and mobile phones, where EMI is a major concern, the electronics industry has developed components, topologies and attenuation methods that significantly reduce EMI from switch-mode drivers down to benign levels. The bad news is that is a complicated subject, and the typical internet supplier based in Asia has no understanding of this issue as it relates to boats, AIS, or VHF radios, and therefore makes no effort to deploy these techniques in their cheap commodity LED products. Folks who know Marinebeam, and have used our products, know that we take this issue seriously. One look at all of the extra components on the back of our LED bulbs reassures the customer that Marinebeam uses all of the available methods in a concerted effort to reduce EMI.

How can we measure EMI and determine whether its levels are low enough to prevent interference with the boat’s radios, AIS and other systems? There are several published standards that define the safe and practical thresholds for EMI on boats. With more and more switch-mode devices appearing on the bridge of commercial vessels, and in the proximity of communication antennas, these standards are very specific and very well developed. The most stringent of these marine standards is IEC60945, the marine navigation and radio-communication equipment and systems specification. This standard sets specific limits on RF emissions, with special emphasis and limits in the VHF and AIS frequencies, as well as the L-Band frequencies used by GPS. Because this is the most applicable standard for the use of LEDs in navigation light applications for recreational boats, this is the specification that Marinebeam uses for its formal testing. This third-party lab test is actually a series of tests performed at different frequencies, and in different planes (X, Y, and Z). Third-party lab tests are expensive, and while not all of our bulbs are externally tested, these specifications and EMI thresholds provide EMC protocols for in-house design and testing.

So, is that it? Just produce a low EMI LED light that meets the IEC60945 specs and the problem is solved? No, unfortunately not. Could we have a device with very low and normally imperceptible EMI that could still present a problem in a specific application or location? The short answer is yes. As an example, your laptop charger --a switch-mode power supply device—is CE and FCC certified to meet all sorts of very strict EMI specs, and normally runs quite happily on the boat without causing any radio interference, but now imagine strapping that laptop charger to your VHF antenna. All bets are off in this case, and there will likely be plenty of interference. It makes sense really, as we all know that antennas and radio receivers are devices specifically designed to collect very faint and very low-power radio signals from very far away. So, even the very faintest signal from miles and miles away can be collected and amplified. Just like all forms of electro-magnetic radiation, radio waves follow the inverse-square law. That is to say that their signal strength decreases with the square of the distance between the source and the antenna. Or said another way, each time you halve the distance to the antenna, you quadruple the strength of the signal that the antenna is collecting. So, the point is that when it comes to reducing the potential for interference, distance between your antenna and your LED is your best friend.

“Wait” you say. “My anchor light is right next to my antenna. Will that be a problem with a Marinebeam LED?” After 11-years of supplying low-EMI masthead LEDs, we can say that it likely won’t be a problem. It depends on the distance, the antenna, the coax type, wire length, wire proximity, any un-shielded connectors, etc. The odd thing about cases of RF interference from low EMI LEDs is that it tends to be yacht-specific, meaning that when we re-test the offending LED, it passes the EMC protocol with flying colors. We generally find that the solution boils down to either the proximity to an antenna or signal booster, or general wiring housekeeping.

Proximity is pretty clear. While folks understandably hate to spend any time at the mast-top, arranging for just a few more inches of distance between the antenna and the LED will typically solve things. Wiring housekeeping is another thing all together. RF signals have the ability to couple to nearby wires and cause issues with conducted interference. We see too many yachts where the hidden space behind the electrical panel is a catch-all for all sorts of devices, wiring, loose antennas, etc. It is not uncommon to find signal amplifiers, radio splitter boxes, and FM antennas, along with dozens of power and signal wires amassed in a big spaghetti pile behind the panel. Proper mounting and separation schemes for power and signal wires can help solve all sorts of EMI and cross-talk issues.

As a final thought, if you should ever have any issues with EMI that you believe is emanating from a Marinebeam product, we will be happy to assist you to solve it.