[FAQ-1231] Interpreting control channel signal strength and quality chart
Page Type: REFERENCE
For each measurement of the signal strength and quality of a trunked system control channel, the Rangecast survey presents a graphical chart for easy interpretation. This report summarizes the interpretation of this chart.
Reception quality of a trunked system depends on several signal properties:
1) The signal must be consistently strong enough to be received clearly above the local RF noise background
2) For some types of trunked radio systems, the signal must not "flutter" (vary rapidly in signal strength) even if the signal is consistently strong enough to be detected
These qualities are automatically measured and evaluated by the signal survey process, with a color-coded synopsis in the "MIN" and "FLUT" columns (example below.)
"MIN" is an evaluation of the minimum observed signal strength (relative to the local RF noise background). The signal may vary in intensity, and this report - as a measurement of the minimum observed strength - tests whether the signal is consistently strong enough to be detected. The numerical value reports the minimum observed signal strength as dB above the noise floor.
Signals must adequately exceed the local background noise level. Digital signals are always noisefree, but totally disappear when close to the noise floor. In contrast, analog signals are noisefree when strong but get noisier if weaker.
In each of these fields, the background color summarizes this aspect of signal quality in an easy to understand format:
RED: the signal may not be usable
ORANGE: quality is marginal
YELLOW: quality is adequate but not ideal
GREEN: the signal passes this particular test
"AVG" is a similar report to MIN, but about the average signal strength. This column is not as important, because what matters is whether the signal is CONSISTENTLY strong enough to be clearly received, and MIN better represents that evaluation. (Whatever is found from testing the minimum signal strength, the average signal strength will clearly be just as good or better.) But since people listening to the signal directly on a radio may describe quality based on the average condition, it is useful to also display similar signal strength information about the average signal.
"FLUT" (for "flutter") reports the variation between the minimum and maximum observed signal strength. A measurement of flutter can also be considered a measurement of stability; a stable signal has low flutter, while an unstable signal has high flutter.
Signals may flutter in strength for various reasons. Some digital systems can chop or disappear entirely if flutter causes dips deep enough to approach the noise floor. On some types of trunked radio systems, extreme flutter -- even if the signal strength is very strong -- prevents clear reception of the signal, due to internal characteristics of the radio receiver. (Note that extreme flutter may also result in an extremely low minimum signal strength, in which case both fields will show red.)
The numerical value shown is the variation in dB between minimum and maximum signal strength. As before, the background color summarizes the result of this test in a red/orange/yellow/green format describing potential impacts to reception performance.
(Note: if the signal is too strong to allow for an accurate measurement of flutter, this field shows in blue with three question marks.)
"SEC" (for seconds) reports how many seconds it took the radio receiver to lock onto the control channel data signal. This is a good all-around performance test. Normally signal acquisition will be very fast, but if the radio takes more than 1 second to acquire signal, this interval (in seconds) is reported and the color will shift away from green. Note that, with marginal signals, how quickly the scanner acquires the signal will be fairly random. There are usually several rows reporting quality evaluations of the same signal from the same location at different times; when considering this column, look for the worst performance.
The color-bar section on the right is a graphical illustration of signal strength. Like on any bar graph (think of an audio volume meter), the position indicates absolute strength -- left is weak, right is strong.
As a general orientation to this bar chart,
1) Large color boxes: the level of the observed signal strength (the spread shows the variation from minimum to maximum, as the signal fluctuates in strength)
2) Gray boxes: the level of the noise floor
3) Small color dots: a printed guide in the background expressing the signal to noise ratio (receiver performance) at a certain signal strength -- essentially a meter guide that is pushed to the right by the noise floor
The large color boxes indicate the observed signal strength on this absolute scale. If the signal is stable (think of an audio meter when listening to a steady tone) there will be only one of these color boxes. If the signal fluctuates in strength (think of an audio meter when listening to voice), there will be a spread of color boxes illustrating the range (minimum to maximum observed signal strength.)
In essence, whether the signal is CONSISTENTLY strong is expressed by where the position of the left side of the color blocks (minimum signal strength), and the STABILITY of the signal is expressed by the width of the color blocks (flutter causes a spread as the strength varies, whereas a stable signal will show a narrow range.)
Note: the width of the color boxes is NOT an indication of signal strength (more bars does not mean a better signal!) The PLACEMENT of the bars to the right (higher signal strength) is what indicates that the signal is strong. And a stable signal (good) will show a smaller number of blocks, so the ideal signal will be a single green block on the right side of the graph.
To express how much noise is present, large gray boxes are printed (starting from the left side of the graph) up to the position expressing the absolute signal strength of the noise. In essence, any signal with an absolute strength in the gray range cannot be detected, because the signal is weaker than the background noise from that location. This is useful to see, because if your environment is noisy, you may be able to improve radio performance by relocating to a quieter location, or finding ways to reduce the amount of background RF near the radio equipment.
For signal strength, what matters most is not the absolute strength, but the strength above the local RF noise background. To illustrate this aspect, in the bar graph, a color background (red/orange/yellow/green) is printed in small dots. The position of these colors is pushed to the right in noisy environments, by the amount of noise present -- think of the color dots as a background quality guide, floating above the noise level, showing how strong an absolute signal is needed to obtain a certain quality of reception in that particular environment. Then, the large color squares showing the actual observed signal strength is printed in the color of this background guide, so you can easily see the quality of the received signal.
In environments with substantial RF noise (such as near a broadcast facility, or other electrical equipment that emits signals in the radio frequency spectrum), a higher absolute signal strength is necessary to obtain the same quality of reception. This is because the radio receiver must attempt to hear the signal above the local RF noise -- compare to trying to listen to someone speaking in a noisy room, where the person cannot be heard if speaking at a normal level that would be perfectly clear in a quiet room. Therefore, if the environment is noisy, the actual signal strength must be higher for the signal to be received clearly.
In this way, the bar graph visually encapsulates a lot of information about the stability and strength of the observed signal, the background RF noise levels, and how this background RF noise may impact the quality of reception.
In the example shown above:
* The last line shows a PERFECT SIGNAL -- a single green box. That there is a single box means the signal is stable (no flutter), and that this box is green indicates that the signal strength is far enough above the background noise that the radio should be able to clearly detect the signal.
* The first line shows a WEAK BUT STABLE SIGNAL -- a single orange box. The signal is stable (one box), but the signal strength is low enough that the radio may not work well for detecting this signal.
* The third line shows a STRONG SIGNAL WITH SOME FLUTTER -- three green boxes. Note that the background noise level (expressed with the gray boxes on the left) is higher, which has pushed the small dots to the right (compare to the last line; due to the background noise, a signal must be stronger in absolute terms to have the same signal to noise ratio). But the flutter is still small enough that the quality metric (FLUT column) is green, indicating that this will not impact reception.
* The fourth line shows a STRONG SIGNAL with moderate flutter (FLUT column yellow), but A LOT OF BACKGROUND NOISE - so much, that the radio's ability to detect this strong signal may be impacted (the series of large boxes showing signal strength are colored red to orange.)
* The sixth line shows a SIGNAL WITH EXTREME FLUTTER (FLUT column red). The observed signal strength varies in quality between orange and green; although the signal is sometimes strong enough to be clear (at maximum), the fact that the signal is not consistently strong will impact performance, and the overall evaluation of signal strength should be based on the minimum (orange.)
Note that the signal at maximum shows as green, even though (in absolute terms) this is a weaker signal strength than the minimum on the fourth line (which was colored red.) This is because the fourth line was in a noisy environment, while the sixth line was in a quiet environment, and a signal of adequate strength in a quiet location can be drowned out by noise in a bad location.
* The second line shows a single green box on the far right side of the graph, but that does not necessarily mean a stable signal; the last position simply means "stronger than the measurement range of the device" with no further detail about the actual signal strength. Also note that the FLUT (flutter) column shows as "???" indicating that the flutter could not be measured, because THE SIGNAL WAS TOO STRONG TO ALLOW FOR ACCURATE MEASUREMENTS.
Rangecast FAQ - ID 1231 - last updated 03 February 2019 Images shown - [block]