How this calculator works
EIRP describes the isotropic transmitter power that would give the same strongest-direction radiation as the actual antenna. Add gain in dBi to transmitter power in dBm and subtract feed losses in dB. Cable attenuation can be entered directly, per metre, or estimated using the manufacturer LMR-400 relation at 25 C and matched impedance. Use a current datasheet for other cable models. Connector count adds a separate loss.
Formula
EIRP_dBm = P_tx,dBm + G_dBi - L_feed,dB. P_dBm = 10 log10(1000 P_W). LMR-400 loss in dB/100 ft = 0.122290 sqrt(f_MHz) + 0.000260 f_MHz.
Worked example
A 1 W transmitter is 30 dBm. With 10 dBi gain and 3 dB total loss, EIRP is 37 dBm or 5.011872 W. With no loss it is 10 W.
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Frequently asked questions
Does a high EIRP mean more total antenna power?
Antenna gain concentrates radiation into some directions; it does not create transmitter power.
Are dBi and dBd the same?
No. This calculation requires gain relative to an isotropic radiator, in dBi.
What does the loss solve-for return?
It returns the total loss implied by the two powers and gain, without allocating it between cable and connectors.