Differences in RF Scan Results and Ensuring Good Signals
You Have Identical Systems But Different Results – Why?
The first step in using a wireless system is to perform a frequency scan, to find the open space in your area. We often receive inquiries from customers comparing the scan differences between receivers. While we attempt to keep the performance equal among all of our units, there are many variables that make scan “perfection” difficult to attain. Before they leave our factory, all receivers and transmitters are aligned and quality tested for proper operation within a 2-3% tolerance for sensitivity, level, frequency response, power output and more.
Scans may appear different due to the fact that the results are a “peak-hold” function, meaning each time the scan occurs, the previously noted RF level may increase but will never decrease. The RF measured at that moment will be the peak of the radio signal detected. The level of RF at a given frequency will rarely remain constant. If one receiver has been through the scan function 10 times and another only once, the first unit will likely show more signal level than the second.
The main integrated circuit that controls the RF metering is a commonly used electronics component. The RSSI (Received Signal Strength Indication) output is decently linear at the middle of its range but somewhat non-linear near the lower and upper limits of use. Given the relatively low resolution in the typical wireless microphone receiver display and the limitations of the RF metering, some differences will be noted. Most often the differences noted in the metering, either showing too much signal or too little on one unit as compared to another, are minor. It can be possible for one device to show a slightly high RF signal, while the other shows slightly low signal, making the differences seem greater than they actually are in reality.
Over time some components and factory alignments may “drift” slightly from the optimum settings. Although this is rare and in most cases minor, this could change the perceived metering when comparing one unit to another. If a component fails and the receiver sensitivity is compromised, the RF meter display should show a significant difference between a poor performing receiver and one that is within specification.
Even with the tight tolerances, there are still variations in component specifications that will impact the displayed RF metering, or RSSI, and we’ll explain the reasons why and how to work with them.
The Science of RSSI
RSSI, or received signal strength indicator, is a relative measurement of the radio signal strength at the receiver antenna. The signal strength increases as the receiver gets closer to the transmitter. Each time the distance between the transmitter and receiver is decreased by half, the amount of signal strength quadruples. This effect is known as the Inverse Square Law. Since RSSI approximates the RF energy at the receiver antenna, the level on the meter can be artificially high due to additional signal from background noise and interfering sources, and it is those variables -external to the receiver - that will affect the quality of your reception and thus sound.
The amount of actual range of a wireless microphone system will be determined by many factors. Transmitter power output, receiver sensitivity, antenna type and placement, signal type, signal frequency and physical obstructions will all impact the usable range. In most analog FM-based (Frequency Modulation) wireless mic systems, the transmitter audio gain can also adversely affect the range, due to Signal to Noise Ratio (SNR).
The overall sensitivity of the receiver is more important than what the RF meter displays. To test the actual sensitivity of two like receivers, place a transmitter a specified distance from the receivers, ideally being fed from the same antennas, under test and walk away with the receivers while monitoring the signal. This exercise is called a “walk test.” In dynamic use, the meters will remain within a reasonable tolerance (1 to 2 pixels apart) and the squelch (audio muting) will occur within 10 feet of each other. If not, there may be a fault with one unit or the other (sometimes both, but rarely).
If the antennas are removed from the receivers and the scan results show signal all along the band, the meter may need adjustment. Any receiver that showed a signal with the antennas removed would be expected to show more signal at all other times when in use. The lowest signal (1 pixel) shown on the RF meter should be 1uV (a millionth of one volt) so it doesn’t take much error either way to be deemed “off,” especially when comparing results to another unit.
Another possible issue to consider is the transmitter antenna location and handling. This is especially true with the handheld version of transmitter, but generally true of all transmitters. This is where we get into the discussion of Attenuation.
Improving Range with RF Attenuation
As you know, what most refer to as “wireless mics” are actually wireless hand-held transmitters. A transmitter is any device that sends out a wireless signal – electromagnetic waves via a transmitting antenna – to another device that interprets those waves, called a receiver.
Attenuation is a reduction in strength of a radio frequency (RF) signal during transmission and is measured in decibels (dB). Material surrounding radio signals is referred to as the “medium” through which the signal travels. Attenuation measurement in decibels is ten times the input signal power divided by the signal power at an output of a specified medium. For radio frequencies, attenuation refers to the way in which the strength of waves lessens while going through open air, walls, the human body, and other features present in the environment.
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How Should I Be Holding The Transmitter and Why?
Antennas like ones used in the handheld transmitters don’t contain boosters (like you see in cell phones or radio towers) that can amplify a signal. They passively radiate, assuming that they have a clear field to do so. Holding the transmitter at the bottom, as shown above, will greatly reduce the RF level that is radiating out. Your hand, being a dense medium, prevents the waves from radiating adequately to reach the receiver.
The graph below shows the RF level of an HH transmitter at a given distance from the antenna, but connected to an RF spectrum analyzer, which measures output in dBμV, or decibels relative to one microvolt. It’s a simple way to measure differences, which can be small, in signal output. The antenna was covered and you can see that the level measures 70.0 dBμV:
Next, the transmitter is held in a way where the antenna was not obscured. A position like this, with your hand closer to the capsule, is the correct way to hold it.
The RF level now measures 100 dBμV, which shows that the signal being put out by the transmitter is not impeded. Big difference.
So, holding your hand-held transmitter incorrectly can result in a loss of 30 dB – or more, which will definitely affect the operating range of the system. This can be avoided by simply holding the transmitter in a way that does not obscure the antenna. Don’t forget to tell your pastor, praise band members and others contributing to services and events about the correct way to hold and speak/sing into a handheld transmitter!
In Summary
2. Differences are likely to be due to small variations in circuits….
3. …But sometimes can be due to space between your transmitter and receiver
4. Sound quality takes into consideration your space and, if using wireless mics, how the mic is held by the user
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Karl Winkler is a 30-year veteran of the professional audio industry and is currently Vice President of Sales and Marketing for Lectrosonics, Inc. Karl has provided system design consultations and hundreds of frequency coordinations for theaters, TV and feature film productions, major touring acts and houses of worship.