Prevent Oscillation or Feedback in Signal Booster Installations

In addition to RF blocking tape, RF blocking patch and RF blocking cloth, there're several other methods to reduce or stop oscillation or feedback as we will discuss in this post.

What is Oscillation or Feedback?

Oscillation is generally feedback, for instance when a live speaker and microphone are very close. The screeching, deafening disconcerting sharp noise is usually the result. Cell phone signal boosters also face similar problem. For cellular boosters, oscillation has the effect of lowering antenna gain considerably or simply turn-off the system. At first, the reception is decreased within antenna. When oscillation is really bad, the cell signal booster goes off. It is indicated with a red indicator light on a typical signal amplifier unit that amplifies the signals. That steady or solid red light indicates the amplifier has shut down to prevent excessive oscillation which may damage the signal booster system and even the wireless network if the signal amplifier does not power down as required per its certification with FCC.

Oscillation takes place when the broadcasting signal emanating from inside antenna is picked up by outside antenna and forced through the cell phone booster. Oscillation in cellular signal boosters is that your cell phone can receive the highest possible signal. However, the signal would be bogus with background clatter. In case the cell phone amplifier continues facing oscillation for a longer time, such as a number of successive weeks, cell phone signal booster will stop working forthwith.

Due to this, FCC (Federal Communications Commission) requires cell phone cellular amplifiers certified by the commission to prevent interloping of cell towers and cellular networks around to prevent damage to wireless networks owned and/or operated by mobile service providers.

FCC maximum gain requirement.

An agreement FCC reached with cell phone signal booster manufacturers and service carriers was that home cell phone signal amplifiers be limited to +70dB and +50dB for car cellular signal boosters. The limit was agreed because it allows cell phone signal boosters perfectly powered without interfering with networks, reception and carrier towers. Exceptions exist for commercial and industrial signal boosters that must be installed by certified installers that must ensure no oscillation can occur that damages wireless networks.

Automatic oscillation is now a standard and meant to avoid exceedingly powerful outbound and inbound signals. The problem was manifested mostly in urban and metropolitan locations considering the reason for poor reception inside a building or home was the construction material and not distance from the carrier tower. If the outgoing or incoming signal is excessive, it could hurt the cell signal booster or the cellular tower. FCC approved automatic oscillation mechanism would easily prevent cell tower overload or the cell booster and ensure there was effective reception.

Oscillation prevention through proper installation.

During installation of cellular phone signal amplifier kit, separating interior and exterior antennas is highly critical. If outdoor antenna(s) and indoor antenna(s) are very close to one another, they will create oscillation or feedback loop where each picks the other's signals. While speaker and microphone give an ear-splitting loud screech - your cell phone amplifier won't. However, an equivalent of such public address oscillation will be taken up by any cellular device around the coverage area.

Left uncorrected, the feedback will have a huge effect on reception and transmission of cellular coverage and eventually damage the cell phone network. As mentioned, FCC ruled that cell phone signal amplifiers must have automatic detect oscillation capability. Once oscillation is ascertained, it is the FCC requirement that the cell phone reception booster should correct itself immediately.

Therefore, once signal booster has detected oscillation, the gain is powered down automatically or the cellular reception boosting ability toned down to autocorrect the problematic oscillation. The reduction in gain lowers the coverage area of indoor cell phone booster. Since reduced gain has some effect on the problematic oscillation, FCC regulation requires that the cell phone signal booster power itself off to eliminate possible damage to wireless network outside. The process might sound long but it generally takes way less than half a second.

During installation, both amplifier "shutdown" and "gain reduction" brought about by oscillation have to be avoided. To easily prevent this, internal and external antennae must be installed sufficiently far enough apart from one another.

Sufficient minimum separation distance.

One of the most important ways of preventing oscillation is finding the right place to place your external and internal antennae. If you're doing your own cell phone signal installation, the sufficient minimum separate distance can be determined by the booster's guide for that model. Installation guides help you find the minimum separation distance required to avoid oscillation issues.

Essentially, during installation of antenna of a cell booster, a vertical difference of 25 feet to 30 feet from outside antenna must be maintained. Wherever vertical separation cannot be accomplished, minimum required distance separation can be accomplished through horizontal separation.

If you choose horizontal separation you must ensure the distance is much longer than what would be required in vertical separation to accomplish minimum separation required.   You can also employ both horizontal and vertical separation to reach the perfect minimum required separation. 

You can reduce the minimum required separation also by ensuring both antennas of the cellular booster are 180 degrees apart. Having a wall or roof between the indoor and external antenna can also help shrink the minimum separation required.

Omnidirectional antennas.

Since omnidirectional antennas beam in 360 degrees, they cannot be faced opposite of the booster antenna. If you use them, you will have to find higher minimum required separation more than directional antennas would need.

Check oscillation on your booster by peering at the front lights on the base unit of the cell phone signal booster. Oscillation is indicated by red lights. During installation, if red lights are visible the distance separating the two antennas should be increased to easily correct the problem.

Minimum separation distance required tips.

If you don't take into account the minimum separation distance required, the cell phone signal booster will shut down or reduce gain. If oscillation causes the cellular amplifier to shut off, any attempt to turn it on will fail until the oscillation impediment has been cleared off.

For cell signal boosters with 50dB gain, the separation required between the antenna of the device and tower antenna is 40 feet. 75 feet is sufficient for a 60dB cellular signal booster.

Always remember the length of booster and antenna connecting cables isn't the measure of separation. Separation is basically the distance between in-door and tower antenna (aka. donor antenna, outside antenna) generally, in a straight line.

Due to the metal roof, the two antennas in a vehicle hardly face a major problem. For a vehicle, placing outside antenna at the center of the vehicle roof or at the roof's rear should be sufficient to avoid oscillation.

Note you don't need to maintain any minimum separation between any of the antennas (device and tower antenna) and the cell reception booster unit. You only need to ensure the minimum separation distance needed is maintained between tower and device antennas (aka. broadcast antenna, inside antenna) for sig. amplifier to work as required and prevent oscillation.

Minimum amplifier gain separation summary.

Always remember the stronger the cellular reception outside, the more required separation distance between indoor and outdoor antennas would be required. Recommended distances of separation include:

• 80dB amplifier - 125 feet.
• 70dB amplifier - 110 feet.
• 68dB amplifier - 80 feet.
• 65dB amplifier - 70 feet.
• 55dB amplifier - 55 feet.
• 45dB amplifier - 15 feet.
• 40dB amplifier - 6 feet.
• 30dB amplifier - 4 feet.
• 20dB amplifier - 2 to 3 feet.

Recommendations aren't cast in stone because the materials lying between outdoor and indoor antennas will determine the best distance to separate them. For instance, if the concrete separating antennas is thick enough, separation distance required won't be that long.

AGC signal boosters.

With Automatic Gain Control (AGC) feature in cell phone signal boosters you don't need to manually adjust the gain. The gain is automatically adjusted compensating for reception fluctuations with intention of preventing oscillation in case of overpowering. This is a feature in Cel-Fi or Wilson network extenders and most modern cell phone signal boosters such as indoor signal boosters EZ4G, Flare, Fusion 5x, Guardian 3 QR and vehicle signal boosters FusionTrek, Fusion2Go 3.0, Fusion2Go 3.0 RV.