Effective Radiated Power Calculator

Calculator Inputs

Site Name

Frequency (MHz)

Transmitter Power

Power Unit

Feedline Loss (dB)

Connector Loss (dB)

Filter or Combiner Loss (dB)

VSWR

Other System Loss (dB)

Antenna Gain

Gain Reference

Additional System Gain (dB)

Reset

Formula Used

1) Convert transmitter power to dBm
From watts: P(dBm) = 10 × log10(P(W) × 1000)

2) Estimate mismatch loss from VSWR
Γ = (VSWR - 1) / (VSWR + 1)
Mismatch Loss(dB) = -10 × log10(1 - Γ²)

3) Compute net conducted power
Net Conducted Power(dBm) = Input Power(dBm) - Total Losses(dB) + Additional Gain(dB)

4) Calculate ERP and EIRP
If antenna gain is in dBd:
ERP(dBm) = Net Conducted Power(dBm) + Antenna Gain(dBd)
EIRP(dBm) = ERP(dBm) + 2.15

If antenna gain is in dBi:
EIRP(dBm) = Net Conducted Power(dBm) + Antenna Gain(dBi)
ERP(dBm) = EIRP(dBm) - 2.15

5) Convert dBm back to watts
P(W) = 10^((P(dBm) - 30) / 10)

How to Use This Calculator

Enter the transmitter output power and choose its unit.
Fill in feedline, connector, filter, and other system losses.
Enter the VSWR to estimate mismatch loss automatically.
Provide antenna gain and choose whether it is dBd or dBi.
Add any extra system gain from amplifiers or active stages.
Press Calculate ERP to show results above the form.
Review ERP, EIRP, conducted power, and the Plotly stage graph.
Use the CSV or PDF buttons to export the results.

Example Data Table

Example	Input Power	Losses Summary	Gain	Estimated ERP	Estimated EIRP
FM Site A	50 W	Feed 1.2 dB, Conn 0.3 dB, Filter 0.5 dB, VSWR 1.5, Other 0.2 dB	6 dBd	115.144 W	188.904 W
Link Site B	25 W	Feed 0.8 dB, Conn 0.2 dB, Filter 0.2 dB, VSWR 1.2, Other 0.1 dB	8.15 dBi, Extra Gain 1.5 dB	103.356 W	169.565 W
Repeater C	10 W	Feed 2.0 dB, Conn 0.5 dB, Filter 0.7 dB, VSWR 2.0, Other 0.4 dB	3 dBd	7.742 W	12.701 W

FAQs

1) What is effective radiated power?

Effective radiated power is the radiated signal level referenced to a half-wave dipole. It combines transmitter power, line losses, mismatch loss, and antenna gain. Engineers use it to compare practical transmitting systems more accurately than transmitter output alone.

2) What is the difference between ERP and EIRP?

ERP uses a half-wave dipole as the reference antenna. EIRP uses an isotropic radiator. EIRP is always 2.15 dB higher than ERP for the same physical system, because an isotropic radiator is the lower-gain reference.

3) Why do feedline and connector losses reduce ERP?

Losses remove energy before it reaches the antenna. Even a powerful transmitter can produce modest ERP if coaxial cable, connectors, filters, or combiners absorb too much signal. That is why system loss budgeting matters in radio design.

4) Why is dBi 2.15 dB higher than dBd?

dBi compares an antenna to an isotropic radiator. dBd compares it to a half-wave dipole. A dipole has 2.15 dB gain over isotropic, so converting between references requires adding or subtracting 2.15 dB.

5) Does VSWR affect radiated power?

Yes. A poor VSWR causes reflected power and mismatch loss. Some energy returns toward the source instead of reaching the antenna for radiation. This calculator estimates that effect and subtracts it from the conducted power budget.

6) Can ERP be higher than transmitter power?

Yes. If antenna gain is larger than total system losses, ERP can exceed the raw transmitter output in watts. The antenna is not creating energy; it is concentrating it more effectively in a preferred direction.

7) Which unit should I enter for transmitter power?

You can enter power in mW, W, kW, or dBm. The calculator converts the value into dBm internally, performs the loss and gain arithmetic in logarithmic form, then returns final answers in watts, dBm, and dBW.

8) Is ERP enough for full coverage prediction?

Not by itself. ERP is one major input, but coverage also depends on frequency, terrain, antenna height, radiation pattern, polarization, receiver sensitivity, clutter, and legal limits. Use ERP as part of a full link or propagation study.