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Grow Light RFI

Grow Lights & RFI

Large high-power grow lights in residential areas are on the rise.  These lights are typically over 1,000 Watts and employ electronics that can act as ballast or perform some other vital function.  See photos for a typical such light and ballasts.  All photos are courtesy of Tom Thompson, W0IVJ.

Unfortunately these lights have proven to be a notorious source of RFI to the Amateur Radio service.  Based on the distances over which the noise can propagate, and tests conducted in the ARRL Lab, these devices can exceed the FCC limits by a considerable margin.  Although these tests were limited to just a few samples, one light was measured to be in excess of 30 dB over the FCC limits.

Interference from grow lights has been shown to be problematic at distances of over 1,000 feet.  This is well over three times the distance one might expect from a legal Part 15 or 18 device, i.e., one that meets the applicable FCC limits.  In addition, they are typically controlled by a timer, cycling on and off at precise times every 12 hours or so.

Historically most reports of RFI from grow lights have been from a limited number of states, particularly those known to have legalized or decriminalized marijuana.  California and Colorado, for example, are two such states.  So far, the ARRL has received more confirmed complaints from these two states than all the others combined.  It must be emphasized, however, that grow lights are being used for a wide variety of indoor horticulture.  For example, other uses include growing indoor vegetables and household, ornamental and exotic plants.  The mere presence of a grow light is not proof that illegal activity may taking place in someone’s home. 

Pertinent Federal, State, and Drug Laws

Under federal law, only the FCC can create and enforce rules regarding harmful radio interference.  Furthermore, both Part 15 and 18 of the FCC’s rules unconditionally prohibit interference to a licensed radio service caused by lighting devices.  This includes interference to Amateur radio caused by grow lights.  As an example, this advisory letter issued by the FCC not only describes some important rules, it also demonstrates the Commission’s willingness to enforce them:

While the FCC may not have jurisdiction in the enforcement of drug laws, an interference complaint to the FCC may result in an investigation.  Grow lights can also produce a distinctive noise signature that may result in an unwelcome calling card, depending on the type of plants involved.  Although marijuana may not be illegal under some state laws, it is still illegal under federal law.  Should a field investigation uncover a grow light operation involving marijuana, we simply don't know what inter-agency agreements may exist between FCC and other law enforcement bodies.  Many such growers might wish to cease operation of the offending device rather than risk a federal investigation of any kind. 

Is It A Grow Light?

Grow lights tend to exhibit several unique and identifiable characteristics.  So far, they typically produce a broad band noise, particularly across the 40m band.  It propagates over much greater distances than one might expect from devices that meet the applicable FCC limits.  Other clues include:

  • Generally grow lights are set by a timer for 12 hours on and 12 hours off but this can vary.
  • Once you have established the turn on time, listen to the noise as the lamp turns on.  It’s generally noisier with less of a flat spectrum.  This generally lasts about 5 minutes until the lamp is warm.
  • The polarization is predominately vertical even though the lamp line may be horizontal.
  • In the case of marijuana, the grow time is typically 6 to 8 weeks.  At this time, the lights may be shut off for a few days while the plants are harvested and new ones made ready. 

Initial Caveats

We typically recommend that a ham discuss any RFI problem with his neighbor as a good first step.  Obviously, even under the best of circumstances, the importance of diplomacy cannot be overemphasized during these discussions.  It’s also important never to suggest what you think  the cause might be.  If you're wrong, it often makes matters worse, regardless of what you suspect the source might be.  Simply let the neighbor know that a source of radio appears to be in their home.  It can also be helpful to demonstrate the problem with a portable radio, especially one that is or looks like an AM broadcast receiver.  Be sure not to turn up the radio so loud that it becomes offensive.

Obviously, depending on circumstances, approaching your neighbor in a case involving a grow light can be a potentially dangerous situation.  Exercise extreme caution and good judgment in this case.  For example, you may not want to directly approach a neighbor if you suspect a grow light is the source of your RFI problem.  Every case is obviously different, and no two neighbors are exactly alike, but there is simply no substitute for good judgment in any situation like this. 

What are the pertinent FCC rules?

Both Parts 15 and 18 rules contain two provisions to address interference:

  • The first is an absolute maximum emissions limit that will not protect against all interference, but which is intended to minimize the likelihood of interference.  Manufacturers of both Part 15 and Part 18 equipment must meet these requirements.
  • The second is that the equipment must be operated in a way that it does not cause harmful interference. If and when interference occurs, however, the rules then place the burden to correct the problem on the device operator.  In this case, the operator is required to take any steps necessary to stop the interference, including shutting the device off.

In the case of a grow light, the FCC rules prohibit harmful interference to a licensed radio service, such as Amateur radio, regardless if it is a Part 15 or Part 18 device.  In either case, the absolute emissions limits (which must be met by the manufacturer), should limit an interference problem a few hundred feet or less.  Based on some of the reported distances from a grow light, however, it would be a reasonable to assume that they exceed the FCC limits.  Furthermore, some reports suggest that they exceed the limits by a considerable margin.  In either case, most hams will need to start their resolution process with their neighbor – one way or another. 

Technical Solutions To A Grow Light RFI Problem

Technical solutions typically involve the addition of filters and chokes to the cables connected to the ballast.  In the case of a grow light, this typically requires that filters be added to both the AC and the Lamp Side of the ballast.  See the Grow Light Electronic Ballasts Page by Larry Benko, W0QE for details on filters being used by Larry and Tom Thompson, W0IVJ to help in these cases.  The Grow Light Ballast RFI Filter page, also by Tom Thompson, includes some additional filter info.  Links for both these pages appear below under Additional Links.

Regardless of which cure you attempt, be sure to choose a filter or other components that are properly rated for the current and voltages involved.

Although common mode chokes are another popular and effective solution in cases involving conducted emissions, the size grow light cables and associated connectors may make them impractical without additional consideration.  For example, you may need to temporally remove a connector or add an extension to the cable for the choke.

Keep in mind that split ferrites, beads and clamp-on cores are not as effective as toroids at HF.  Toroids are donut shaped, and you’ll need to pass the cable end through the center hole multiple times.  Typically the largest toroid that is readily available is 2.4 inches in diameter.  The size of the cable (and connector), therefore, may prohibit or limit the number of turns that are possible with grow lights.

Note:  In most cases, common mode chokes are easy to fabricate.  Simply wrap the cable around a toroid core several times, typically eight or more, in order make one.  However, depending on the core material, the number of turns will vary.  One should typically keep the self resonant frequency above 15 MHz.  Be sure to keep the toroid as close to the RFI source as practical.  The type of ferrite is also important.  Also, a good mix to try for HF suppression is type 31 ferrite.



Locating RF Interference at HF.  A proven and practical approach to dealing with RFI from grow lights and more, by Tom Thompson, W0IVJ.  QST November 2014, p 33.

Additional Links

AC Line Filters:

 Ferrite Materials & Common Mode Chokes:

 Grow Lights in The News



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