| Scenario | PTX | Gamp | Gant | Lcable | Lmisc | EIRP (approx.) | ERP (approx.) |
|---|---|---|---|---|---|---|---|
| Wi‑Fi / ISM link | 0.1 W | 0 dB | 5 dBi | 1 dB | 0.5 dB | ≈ 0.224 W (23.5 dBm) | ≈ 0.136 W (21.35 dBm) |
| VHF handheld + whip | 5 W | 0 dB | 2 dBi | 0.3 dB | 0 dB | ≈ 7.4 W (38.7 dBm) | ≈ 4.6 W (36.6 dBm) |
| UHF base + Yagi | 25 W | 3 dB | 11 dBi | 2 dB | 1 dB | ≈ 315 W (55.0 dBm) | ≈ 192 W (52.83 dBm) |
| Microwave point‑to‑point | 1 W | 10 dB | 24 dBi | 1.5 dB | 1 dB | ≈ 1.41 kW (61.5 dBm) | ≈ 0.861 kW (59.35 dBm) |
| Broadcast-style chain | 500 W | 0 dB | 8 dBd | 1.2 dB | 0.8 dB | ≈ 3.27 kW (65.14 dBm) | ≈ 1.99 kW (62.99 dBm) |
- P(dBm) = 10·log10(P(mW))
- EIRP(dBm) = P_TX(dBm) + G_amp(dB) − L_cable(dB) − L_misc(dB) + G_ant(dBi)
- ERP(dBm) = EIRP(dBm) − 2.15 (because 0 dBd = 2.15 dBi)
- Optional free-space estimates (using EIRP in watts at distance r meters): S = EIRP / (4πr²) and E = √(30·EIRP)/r
- Enter your transmitter output power and select its unit.
- If you have an external amplifier or system gain, enter it in dB (otherwise keep 0).
- Enter antenna gain and choose dBi, dBd, or linear ratio from the datasheet.
- Add feedline/cable loss plus any extra losses (filters, combiners, connectors).
- Optionally add distance to estimate free-space power density and E-field at that range.
- Click Calculate. Download CSV/PDF to document the scenario.
Why ERP and EIRP Matter in System Design
Effective radiated power summarises how strong your transmitter appears in the far field after antenna gain and system losses. ERP is referenced to a half‑wave dipole, while EIRP is referenced to an isotropic source. Many regulatory limits and coverage models start with one of these two numbers. In link budgets, this transmit-side term sets the baseline for received power overall.
Working in dBm Avoids Common Power Mistakes
Adding watts directly is error‑prone because gains and losses are multiplicative. Converting power to dBm turns multiplication into addition, so every amplifier gain increases the total and every cable loss reduces it. This calculator shows the intermediate antenna‑input power, which is a checkpoint for troubleshooting. If you enter dBm or dBW, the same arithmetic applies.
Interpreting Antenna Gain: dBi, dBd, and Linear
Antenna datasheets may list gain in dBi or dBd, and some simulation tools output linear ratios. The calculator normalises everything to dBi internally, then derives ERP by subtracting 2.15 dB. This prevents mixing reference systems when comparing omni antennas, dipoles, and directional arrays. Verify whether gain values are peak or pattern‑dependent.
Loss Budgeting for Feedlines and RF Hardware
Feedline loss changes strongly with frequency, length, connectors, and bending radius. Treat every inline component as a loss term: duplexers, filters, lightning protection, and splitters can each add measurable attenuation. Entering realistic losses often reduces predicted radiated power more than expected, improving safety margins. Shorter runs and low‑loss cable can beat a higher‑power amplifier.
Distance-Based Estimates for Compliance Screening
When you enter a distance, the tool estimates free‑space power density and electric field using EIRP. These values are useful for first‑pass checks, test setup planning, and comparing sites. Real environments introduce reflections and absorption, so field measurements remain the final authority. Use consistent units when comparing against limits.
Reporting, Traceability, and Scenario Comparison
Engineering decisions benefit from repeatable documentation. Use the download buttons to store assumptions, reproduce calculations, and share consistent numbers with reviewers. Comparing multiple runs helps you choose antenna gain versus amplifier gain, or identify whether reducing feedline loss gives the best improvement per cost. Keeping a dated record supports audits and change control.
1) What is the difference between ERP and EIRP?
EIRP references an ideal isotropic radiator, while ERP references a half‑wave dipole. They differ by 2.15 dB: ERP(dBm)=EIRP(dBm)−2.15. Use whichever unit your standard, regulator, or planning tool specifies.
2) When should I enter antenna gain in dBd?
Use dBd when your datasheet explicitly states gain relative to a dipole. The calculator converts dBd to dBi internally by adding 2.15 dB, then reports both ERP and EIRP for clarity and compliance checks.
3) How do I enter cable loss if I only know efficiency?
If you know efficiency η as a percent, convert to a loss factor: loss_linear = 1/η. For example, 80% efficiency becomes 1/0.8=1.25. Select linear loss and enter 1.25, or convert to dB as 10·log10(loss_linear).
4) Are the distance-based field estimates accurate everywhere?
They are free‑space, far‑field estimates from EIRP, useful for screening and comparisons. Indoors, near metal, or in urban areas, reflections and absorption can change results significantly. Use calibrated measurements for final compliance decisions.
5) Why can ERP or EIRP appear as a negative dBm value?
dBm is logarithmic. Any power below 1 mW is negative in dBm, even though the physical power is positive. For example, 0.1 mW equals −10 dBm. Low-power IoT systems often produce negative dBm outputs.
6) Does this calculator replace a full link budget?
It covers the transmit-side radiated power terms and optional free‑space field estimates. A full link budget also includes propagation loss, receiver antenna gain, receiver losses, noise figure, bandwidth, and required SNR. Pair these outputs with your path model.