Subelement J: Antennas— Topic 66: Transmission Lines
Question 3-66J6
Element 3 (GROL)If a transmission line has a power loss of 6 dB per 100 feet, what is the power at the feed point to the antenna at the end of a 200 foot transmission line fed by a 100 watt transmitter?
Explanation
Decibels (dB) express a ratio of power. A 6 dB power loss signifies a reduction in power to one-fourth (1/4) of its original value.
The transmission line is 200 feet long, and the loss is 6 dB per 100 feet. Therefore, the total power loss over the 200-foot line is 6 dB + 6 dB = 12 dB.
To calculate the final power:
1. **First 6 dB loss (over the first 100 feet):** 100 watts * (1/4) = 25 watts.
2. **Second 6 dB loss (over the next 100 feet, applied to the remaining power):** 25 watts * (1/4) = 6.25 watts.
Thus, the power at the antenna feed point is approximately 6 watts.
* **A) 70 watts** and **B) 50 watts** are incorrect as they do not correspond to the calculated dB loss. A 50 watt output would represent only a 3 dB loss, not 12 dB.
* **C) 25 watts** would be the power remaining after *one* 6 dB loss (100 feet), not the total 12 dB loss over 200 feet.
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