In an RF link, how does cable loss affect EIRP?

• A. Higher cable loss reduces the power delivered to the antenna, lowering EIRP.
• B. Cable loss has no effect on EIRP.
• C. EIRP is independent of any losses.
• D. Cable loss increases EIRP.

Answer: (A)

Cable loss in the feed line reduces the power that actually reaches the antenna, so the amount radiated in a given direction drops. EIRP is determined by the power delivered to the antenna multiplied by the antenna’s gain. In practice, if the transmitter outputs Pt and the feed line introduces a loss L (in dB), the power at the antenna is Pt minus L, and the EIRP becomes Pt minus L plus the antenna gain. Therefore, higher cable loss lowers EIRP. For example, with 20 dBm leaving the transmitter and 3 dB of feedline loss, the antenna sees 17 dBm, and with 5 dBi of antenna gain, the EIRP is 22 dBm. If there were no loss, the EIRP would be higher by the amount of the loss.