900 Mhz Interference: What Are the Causes and How to Avoid It?

FACTS CHECKED BY  Jose George​

Some things create disturbances in electrical circuits quite often. Though several things create this disturbance, RFI and EMI are more common. EMI is electromagnetic Interference, while RFI is Radio Frequency Interference. Everything that uses power can create RFI disturbance. Thankfully, the interference in the wireless microphone system occurs only due to three frequency bands. These are 

  • Broadcast band (In the United States, it is between 54 and 698 MHz)
  • 2.4 GHz
  • 900 MHz or 33-centimeter band

Some wireless audio devices and microphones use some other bands. Nevertheless, we are not going to discuss them here.

Three different sorts of wireless interference

There are some common interference issues. 

Natural Radiators

Some natural factors emit electromagnetic energy. These energies are lightning, intense solar magnetic storms, dust, or snowstorms. However, these radiators do not exceed low frequencies except a few.

Unintentional radiators:

They are artificial technology that also emits such energy. At times, these issues are purely unplanned. For example, electrical contacts of a breaker. Sometimes, devices emitting EMI are so small that one cannot filter them. Although the FCC and product makers know about this, they can’t help it.

Intentional radiators

These are the artificial technologies that use radio energy for sending communication signals.

Common Sources of Interference 

Natural RadiatorsUnintentional RadiatorsIntentional Radiators: Broadcast Band(54-698mhz)Intentional Radiators 900mhz and 2.4Bands
Lightning and electrostatic discharges Power lines and transformersPrimary Users: FM/AM radio boosters and translators, Aural TV transmitters, Fixed and mobile broadband services, Digital and analog TV transmittersWi-Fi devices, Bluetooth devices, Zigbee devices, and wireless USB devices
Solar magnetic storms of high intensityMicrowaves, X-rays, MRIs, Breakers, Arc welders, consumer electronicsSecondary Users: Amateur radios, licensed wireless microphones, broadcast Tv camera control signalsCordless phones, baby monitors, medical telemetry devices, hobbyist devices
Dust and snowstormsBunched power cables, fluorescent fixtures, high voltage lightningTertiary Users: fixed and mobile TVBDs, medical telemetry devices, unlicensed wireless microphones.RFID readers, intercoms, remote controls

Note: The Government places Broadcast band 54-698MHz in a hierarchy.

  • 900 Mhz Interference–Primary users: 

The Government gives some spectrum bands either on lease or as a property to some users. Therefore, these primary users have high interference protection. Examples are digital and analog TV transmitters, cellular services, 2-way radios, etc.

  • Secondary users: 

These include licensed users of the spectrum. However, the primary users have more rights than the secondary users. E.g., amateur stations, licensed wireless microphones, etc.

  • Tertiary users: 

These include unlicensed users. Thus, they are not protected from interference. E.g., unlicensed wireless microphones, fixed and mobile TVBDs. The Federal Communication Commission or its-certified facilities regulate these users indirectly.

The TV broadcast band has a unique nature. You can get a variety of licensed and unlicensed services in this band. Some licensed bands have some or a single owner with exclusive property rights of the frequencies. On the contrary, unlicensed bands like 2.4 GHz and 900 MHz have several occupants that use spectrum under a particular set of rules. TV broadcast bandstands in contrast to all such bands.

900 Mhz Interference--communication signals

Image: communication signals

How to Avoid Wireless Interference when using a Wireless System 

You can prevent any undesired signal and harmful interference. Read these basics here to get protection from interference.

900 Mhz Interference–Signal blockage

The transmitter and receiver antennas must have a good line of sight. So, try to avoid any metal objects, walls, and people. Keeping both transmitter and receiver antennas in the same room is better. Also, keep them elevated to prevent any obstructions. The salty water in the human body absorbs signals from the wireless mic. In addition, folding hands around the handheld transmitter antenna reduce output by 50 percent. Again, the strength of the transmitted signal also decreases when the antenna of the bodypack transmitter folds.

Incorrect Antenna type and its placement

Don’t ignore the importance of receiver antennas in wireless operations. If there are issues in antenna placement, selection, and cabling, insufficient signal strength will occur. As a result, there are frequent dropouts at the receiver end. Nowadays, modern diversity receivers are in use instead of single antennas. Still, the placement of proper antennas is important.

  • Always place the receiver antennas at least one-quarter of a wavelength apart. One wavelength is about 20 inches at 600 MHz, and thus, one quarter is 5 inches. One wavelength distance is much better. 
  • Place the receiver antennas at a wide “V” angle. This angle ensures better pickup when the transmitter is moving.
  • Keep the antennas close to transmitters with the line of sight. At times, antennas are frequency band-specific. So, check its frequency band before connecting.
  • At times, the receiver is far from the performance area. In such cases, use beam antennas at some remote position to get a clear line of sight to the transmitters.
  • If your antennas need to be far from the stage, it is good to use beam antennas. These antennas pick signals from that direction for superior performance.
  • If a co-axial cable connects your antenna and receiver, there is a self-signal loss. To avoid this, use an in-line antenna amplifier to nullify this within the line. Also, make sure that the total loss is not more than 5db.
900 Mhz Interference--Radio Telescope antenna

Image: Radio Telescope antenna

900 Mhz Interference– Poorly coordinated frequency set

At times, poor coordination in wireless frequencies causes microphone disturbances. So, to improve that, make sure that wireless frequencies meet two conditions.

Frequencies must not match local TV channels. Most television transmitters work up to one million watts. But microphone systems work only up to 50 mW. So, to avoid the adjacent channel interference, avoid local TV channel frequencies.

 So, what does “local” mean here?

Local depends on the TV transmitter’s coverage area and the wireless system’s location.   This distance can be up to 50-60 miles. However, if you have an indoor set-up, it’s good. The building structures weaken the TV signals too much.

900 Mhz Interference--cordless radio wireless microphones and transmitter system

Image: cordless radio wireless microphones and transmitter system

Pick compatible frequencies. To ensure this, choose the “channel” or “group” frequencies set for wireless systems. As you use channels of the same group, you can be sure of harmony in small set-ups.

However, you must use a frequency coordination computer program for big wireless set-ups. Big set-ups mean wireless microphones and in-ear monitors.

Remember that you cannot pick one frequency set and use it for long. Due to some unexpected disturbances, you may need to adjust your wireless frequencies quite often.

900 Mhz Interference– Poor Battery Management

For wireless mics, transmitter battery life is critical. Most battery makers use lithium or alkaline single-use batteries due to their high output voltage. High output is essential to avoid signal dropouts. However, their life is too small. On the other hand, rechargeable batteries give 20% less voltage than signal-use batteries even after a full charge. Despite this fact, battery makers use cheap batteries to cut costs.

Comparing the battery’s output voltage with the transmitter’s voltage is good. With this, you can ensure that the battery will last for a complete performance. Lithium-ion batteries are suitable for 9-volt applications. At the same time, you must pick Ni-Mh or Ni-Cad batteries for a couple of hours.

It is good to use rechargeable batteries if you have a staff to manage things. However, maintain proper upkeep of these batteries to make most of them.

900 Mhz Interference–Improper Gain set-up

Proper input gain is also essential in a wireless device, and too high or too low gain causes distortion. In wireless devices, there is a gain control switch on transmitters. With this, you can gain low input to prevent overload and high to give a proper signal level.

It is pretty easy to set this wireless transmitter gain. Set this gain to the loudest input signal to light up the peak indicator lights. In wireless systems, this indicator is available on the receiver. So, check the peak indicator while the performer is playing or singing. If it flashes regularly, you have to reduce the gain. On the contrary, raise the transmitter gain if the indicator does not flash at all. Finally, make sure that the peak indicator on the receiver flashes at times only.

Conclusion

Wireless systems make performers free of cords and cables. However, they have become a big headache for the sound crew at the same time. Mainly, setting a well-behaved wireless system for touring experts and newbies is pretty challenging. We can help you with your wireless systems as we deal in high-quality cable assemblies. Feel free to reach us.h us.

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