CompTIA Network+ (N10-009)Network ImplementationHard
An access point has a transmitter power output of 20 dBm. The signal passes through a cable with 2 dB of loss before reaching an antenna with 6 dBi of gain. What is the resulting Effective Isotropic Radiated Power (EIRP)?
- A26 dBm
- B24 dBm
- C28 dBm
- D12 dBm
Show answer & explanationAnswer & explanation
Correct answer: B. 24 dBm
EIRP is calculated by adding antenna gain and subtracting cable/connector losses from the transmitter's output power: 20 dBm (TX power) + 6 dBi (antenna gain) − 2 dB (cable loss) = 24 dBm EIRP.
Why the other options are wrong
- A. This results from an arithmetic error, such as only accounting for half the loss.
- C. This ignores the 2 dB cable loss entirely (20 + 6 + 2 miscalculation), giving an inflated value.
- D. This would result from subtracting the gain instead of adding it (20 − 6 − 2), which is incorrect math.
EIRP Calculation
Effective Isotropic Radiated Power is the total power radiated in the direction of maximum antenna gain, calculated as transmitter power plus antenna gain minus cable/connector losses (in dB/dBm).
- Formula: EIRP = TX Power (dBm) + Antenna Gain (dBi) − Cable Loss (dB)
- dB values are additive/subtractive, not multiplicative
- Regulatory bodies often cap maximum allowed EIRP
Memory trick: Power in, gain adds, loss subtracts — EIRP is the final signal strength.