Three spacing limits
The maximum spacing is the tightest of three limits: the off-air RSSI floor, the amplifier's nominal gain and its maximum gain. The tool names the limit that binds, so you can see which input to change to move the answer.
Amplifier Spacing Calculator
Work out how far apart leaky feeder line amplifiers can sit, how many a run needs and what pad each one takes, from cable loss, coupling loss and an off-air RSSI floor.
Free on a Standard account, with no time limit. No credit card.
Walkthrough
See it working
Step 02
Set the link parameters
Enter the operating frequency, run length, pilot level, minimum off-air RSSI and target amplifier input level. Then set the distance from the cable, body loss, connector loss at each amplifier and the coupling planning level.
Step 05
Read the results
The results show the maximum spacing and the limit that binds, the amplifier count, and a trace of every segment with pad settings, signal levels, reserve gain use, cascade noise figure and DC current draw.
Step 06
Export the test sheet
Export a PDF commissioning test sheet with the predicted values for every segment and blank columns for field measurements, or copy a text summary to the clipboard.
Amplifier class and radiating cable selection with the resulting spacing and signal trace.
Overview
What the Amplifier Spacing Calculator does
Leaky feeder (radiating cable) systems in mines and tunnels need line amplifiers at intervals to make up for cable loss. Space them too far apart and radios near the end of each segment fall below the signal level they need; space them too close and the job carries more hardware and power than it needs. Because the spacing sets the amplifier count, it also drives the cost of a quote. A manufacturer's reference spacing is usually based on cable loss and amplifier gain alone, so it does not show whether a radio a few metres from the cable still receives enough signal.
Read the full overview
The Amplifier Spacing Calculator works out the maximum spacing from three limits and uses the tightest. The off-air signal at the radio must stay above your RSSI floor, the loss in a segment should stay within the amplifier's nominal gain, and it must never exceed the amplifier's maximum gain. Cable loss and coupling loss come from manufacturer datasheet data in the noIM₃ component catalogue, and the off-air budget allows for distance from the cable, body loss and connector loss at each amplifier.
Each amplifier is modelled as a pilot-tone AGC stage with an input pad, so the tool gives the pad setting for every segment along with the signal levels, reserve gain used, cascade noise figure and DC current draw. Branches fed from splitters run as their own amplifier chains, and segment boundaries can be moved to match real access points. The results export as a PDF commissioning test sheet with the predicted values and blank columns for field measurements.
Capabilities 7
Off-air signal budget
The off-air signal at the end of each segment allows for cable loss, connector loss, manufacturer coupling loss at the 50 or 95 per cent planning level, distance from the cable and body loss. Coupling loss is corrected for the operating frequency.
AGC amplifiers with inline pads
Each amplifier is a pilot-tone AGC stage with an input pad sized so the gain block sees your target input level. The tool gives the pad setting for every segment and flags segments that use reserve gain or would saturate the amplifier.
Datasheet cable data
Radiating cables come from the noIM₃ component catalogue, which holds manufacturer datasheet curves for cables from suppliers such as RFS and CommScope. The tool will not size a run when the datasheet does not give a coupling figure the calculation needs.
Splitter branches and custom layouts
Equal splitters feed branches that run as their own amplifier chains, with positions shown along a single distance from the head end. Segment boundaries on the main line and each branch can be edited to place amplifiers at real access points and equipment locations.
Noise figure, DC current and stated assumptions
The line noise figure cascades every segment's cable and pad loss with each amplifier at the gain it applies, so it grows with the number of amplifiers, and DC current draw is totalled per chain. The amplifier class figures, body loss and connector loss are editable defaults marked as assumptions, to replace with your own equipment's figures. A splitter's loss over the ideal split is yours to type, or the most lossy figure the makers publish for an equal splitter of that size, band and impedance, named on the page.
Commissioning test sheet
A PDF test sheet lists every segment on the main line and each branch with the predicted pad setting and signal levels, plus blank columns for measured values and a pass or fail result. A text summary of the results can also be copied to the clipboard.
Inputs and outputs
What goes in, what comes out
Inputs 13
- Amplifier class (generic AGC line amplifiers, editable)
- Radiating cable from the component catalogue
- Operating frequency
- Total run length
- Head-end pilot level
- Minimum off-air RSSI at the radio
- Target amplifier input level
- Distance from the cable, body loss and connector loss at each amplifier
- Coupling loss planning level (50 or 95 per cent)
- Optional splitter branches with tap position and branch length
- Optional custom segment boundaries
- Amplifier figures, body loss and connector loss, each marked as an assumption
- Splitter loss over the ideal split, typed or taken from makers' published splitter datasheets
Outputs 9
- Maximum amplifier spacing and the limit that binds
- Amplifier count for the main line and every branch
- Inline pad setting and amplifier gain for each segment
- Segment by segment trace of conducted and off-air signal levels
- Reserve gain use and saturation warnings
- Cable loss, coupling loss and off-air budget at the operating frequency
- Line noise figure, cable included, and DC current draw per chain
- Results summary ready to copy
- PDF commissioning test sheet
Standards & methodology
- Radiating cable longitudinal loss modelled as K₁√f + K₂f, fitted to manufacturer datasheet curves
- Manufacturer coupling loss at the 50 and 95 per cent planning levels, with the distance falloff from the datasheet reference distance radiating cable makers publish: 20 log₁₀ for the 95 per cent figure, 10 log₁₀ for the median (Eupen application note)
- ITU-R P.372 clause 1.2.1: noise factor of a line loss and the cascade
Use cases
When to use this tool
- 01Sizing the amplifier chain for a new leaky feeder system in an underground mine
- 02Working out amplifier spacing for a road or rail tunnel
- 03Counting amplifiers across a main line and splitter branches for a quote
- 04Comparing radiating cables or amplifier classes before committing to a design
- 05Placing amplifiers at real access points and equipment locations
- 06Preparing a commissioning test sheet for field technicians
- 07Checking an existing network for off-air margin and reserve gain use
How does the calculator decide the amplifier spacing?
It works out three limits and uses the shortest. The off-air limit is the segment length at which the signal a radio receives near the end of the segment falls to your RSSI floor. The gain limit is where the segment loss uses up the amplifier's nominal gain (the pilot level minus the target input level), and the maximum gain limit is where the amplifier can no longer restore the pilot level. Connector loss at each amplifier is taken off before the remaining budget is converted to a cable length.
Why can the spacing come out shorter than the manufacturer reference?
A reference spacing is usually set by cable loss and amplifier gain alone. Once you allow for the 95 per cent planning coupling level, a radio a few metres from the cable and loss from the user's body, the signal near the end of a long segment can fall below the RSSI floor first. The calculator shows all three limits so you can see which one sets the answer.
What is the 95 per cent planning level?
Radiating cable datasheets give coupling loss at two levels. The 50 per cent figure is the median along the cable, and the 95 per cent figure is the loss not exceeded at 95 per cent of points along the cable, so it is the more conservative figure for design. You can choose either level.
What is the inline pad?
Each amplifier module has an attenuator ahead of its gain block. The pad is set so the gain block sees your target input level, and the gain block then restores the signal to the pilot level. The calculator gives the pad value for each segment, which is the figure set at installation and is printed on the test sheet.
Where does the cable data come from?
Radiating cables load from the noIM₃ component catalogue, which holds manufacturer datasheet curves for longitudinal loss and coupling loss. The tool reads longitudinal loss from the datasheet table between its published points (a K₁√f + K₂f fit of the table only outside them) and coupling loss at the selected planning level. If a datasheet does not give a figure the calculation needs, the tool says so instead of assuming a value.
Does the calculator model intermodulation, DC voltage drop or coverage?
No. It covers amplifier spacing, signal levels, pad settings, cascade noise figure and DC current draw. Intermodulation, DC power feed and full network simulation are handled in the Leaky Feeder Designer, and 3D coverage prediction in the Leaky Feeder 3D Coverage tool.
Does any of my data leave the browser?
The spacing calculations run in your browser. The radiating cable library is loaded from the noIM₃ component catalogue when the tool opens, but your run details are not sent to the server.
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Further reading
- FDMA vs TDMA Underground: Why Amplifier Group Delay Favours FDMA on Leaky Feeder and DAS Cascades FDMA has margin where TDMA has a budget. Slot timing, delay tolerance and amplifier group delay on leaky feeder and DAS cascades, with the standards' numbers.
- Designing Leaky Feeder Coverage for Underground Mines A working guide to leaky feeder design underground: the two loss numbers behind every layout, amplifier spacing, uplink and downlink budgets, and mine geometry.
Free on any account
Free on a Standard account, with no time limit. No credit card.