System Capacity Design
Average and peak RF power through a shared passive device for a mixed fleet of transmitters. Separate incoherent power-sum and coherent envelope (PEP) peaks, deterministic and statistical occupancy peak models, thermal dissipation, and pass or fail against the device average and peak ratings.
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Walkthrough
Device tab with presets, topology selector, input and output ports, per-port insertion loss, and separate average and peak power ratings.
Overview
When a large radio or microwave system funnels many transmitters through one shared passive device — a switch matrix, a combiner, a multiplexer, a circulator — the question that actually sizes that hardware is not how many transmitters there are, but how much power passes through it. Average power sets the thermal load and the device continuous rating. Peak power sets the device peak rating. Get either wrong and the shared device runs hot, derates, or fails in service. A spreadsheet that simply multiplies power by the number of radios overstates the average badly and says nothing useful about the peak.
The noIM₃ Composite RF Power Loading Calculator answers the question from a systems-integrator point of view. You describe the shared device and build a per-service table of transmitter power, count, and duty cycle, and the tool returns the duty-weighted average composite power, the two physically distinct peak powers, the thermal dissipation, and a pass or fail against the device ratings. It is built for the engineer specifying the distribution hardware, not for circuit-level design.
The two peaks matter because passive-device datasheets rate them separately. The incoherent power-sum adds the time-averaged powers of the carriers that are keyed at once and is the figure for the average power rating. The coherent peak envelope power is the worst case when the carrier voltages align in phase and reaches the square of the sum of the square roots of the carrier powers, which for N equal carriers is N times the incoherent figure and is the figure for the peak rating. The peak can be driven as a deterministic worst case or as a statistical occupancy peak at a chosen confidence, using the same offered-load definition as the Erlang B and C calculators so a traffic-sized fleet flows straight into the power budget.
The average composite power is the sum over every service of count times duty cycle times per-transmitter power. Because it is duty-weighted it captures the real difference between a low-duty voice fleet and a continuous data link of the same power, and it is the figure that drives both the device average power rating and the heat dissipated in the rack.
The incoherent power-sum adds the powers of the simultaneously keyed carriers and is the figure for the average power rating. The coherent peak envelope power is the worst case when carrier voltages align in phase, N times higher for N equal carriers, and is the figure for the peak power rating. Both are shown together with their peak-to-average ratios so you size against the right one.
Deterministic mode keys a fixed maximum number of the highest-power transmitters at once for a conservative worst case. Statistical mode treats each transmitter as an independent on/off source and reports the peak at a P90, P95, or P99 confidence from the occupancy distribution, the realistic peak for a large fleet where not every radio keys at the same instant.
The statistical model uses the offered load equal to the sum of count times duty as the expected number of transmitters simultaneously keyed, the same definition the Erlang B and C calculators use. A fleet sized for traffic in those tools can be carried straight into this power budget without redefining the activity assumptions.
A combiner funnels every carrier onto one output, so that port carries the full composite. A switch matrix distributes the carriers across many outputs, so the worst-case stress is only the busiest output’s fan-in, not the total — the calculator assumes the worst-case grouping at the busiest output and checks the rating there, and a live schematic draws the carriers, the device, and the highlighted busiest port. A multiplexer is treated as a frequency-selective common port. Filtered and resistive combining are modelled separately, with the resistive case dumping ten times log ten of the carrier count in dB into the isolation loads.
The average composite power is split through the device insertion loss into the heat dissipated inside the device and the power delivered past it. The dissipated figure is the watts of heat the shared device adds to the rack, ready to feed into a communications-rack heat-load schedule.
The average load is checked against the device average power rating and the peak load against the device peak power rating. Each reports the headroom in dB and as a percentage of the rating with a clear pass or over indication, and a warning is raised when either rating is exceeded. A third check covers incident traffic: the sustained simultaneous-keying load is also checked against the average rating, catching the fleet whose duty-weighted average passes but whose sustained all-keyed load would not.
The tool models power loading and thermal dissipation only. It does not model passive intermodulation, intermodulation products, frequency-dependent loss, route differences through a switch matrix, or per-carrier modulation PAPR — the coherent envelope assumes constant-envelope carriers, so enter per-carrier PEP for digital modulations. The boundary is stated in the tool rather than implied, with a pointer to the PIM and Intermod calculators for those questions.
Inputs and outputs
Inputs 13
Outputs 12
Use cases
Passive-device datasheets rate average and peak power separately. The incoherent power-sum adds the time-averaged powers of the carriers and is the figure for the average (CW) rating. The coherent peak envelope power is the worst case when the carrier voltages align in phase and reaches the square of the sum of the square roots of the powers, N times higher for N equal carriers, and is the figure for the peak rating. The tool shows both so you can size against the correct one.
Use the deterministic peak for a conservative worst case, where a fixed maximum number of transmitters key at once. Use the statistical peak for a large fleet where not every transmitter keys at the same instant; it treats each transmitter as an independent on/off source and reports the peak at a confidence level (P90, P95, P99) from the occupancy distribution. For a small number of high-consequence carriers the deterministic worst case is usually the safer choice.
Duty cycle is the fraction of time a transmitter is keyed. The average composite power is duty-weighted, so a voice fleet at fifteen per cent duty contributes far less to the average and to the heat load than a continuous data link of the same power. The peak power, by contrast, depends on how many transmitters are keyed at once rather than on the long-run duty cycle.
No. It models power loading and thermal dissipation only. It does not model passive intermodulation (PIM), intermodulation products, frequency-dependent loss, differences between the routes through a switch matrix, or per-carrier modulation PAPR (enter per-carrier PEP for digital modulations). For intermodulation questions use the PIM and Intermod calculators; the boundary is stated explicitly in the tool.
No. It is built for the systems integrator specifying the shared distribution hardware — the switch matrix, combiner, multiplexer, or circulator — not for PCB or component-level design. The inputs and outputs are framed around device ratings, thermal load, and equipment selection.
Related
Free on a Standard account, with no time limit. No credit card.
