Deep Dive: G9B12
The correct answer is A: 8 feet. The approximate length for a 1/4 wave monopole antenna cut for 28.5 MHz is 8 feet. Length = 234/f (MHz) = 234/28.5 = 8.21 feet ≈ 8 feet. For amateur radio operators, this is a basic monopole length calculation. Understanding this helps when building vertical antennas.
Why Other Answers Are Wrong
Option B (11 feet): Incorrect. 11 feet is too long - that would be for about 21 MHz, not 28.5 MHz. Calculation error. Option C (16 feet): Incorrect. 16 feet is too long - that would be for about 14.6 MHz, not 28.5 MHz. Calculation error. Option D (21 feet): Incorrect. 21 feet is way too long - that would be for about 11 MHz, not 28.5 MHz. Calculation error.
Exam Tip
1/4 wave monopole at 28.5 MHz = 234/28.5 = 8 feet. Think 'M'onopole 'L'ength = '2'34/'F'requency (half of dipole formula). Length = 234/f (MHz) = 234/28.5 = 8 feet. Not 11 ft, not 16 ft, not 21 ft - just 8 feet.
Memory Aid
1/4 wave monopole at 28.5 MHz = 8 feet. Think 'M'onopole 'L'ength = '2'34/'F'requency. Length = 234/f (MHz) = 8 feet. Standard monopole length formula (half of dipole formula).
Real-World Example
A 1/4 wave monopole for 28.5 MHz: Length = 234/28.5 = 8.21 feet ≈ 8 feet. This is the approximate length for a quarter-wave monopole at this frequency (10 meters). The formula 234/f (MHz) is half of the dipole formula (468/f) because monopoles are half the length. This is a standard monopole length calculation.
Source & Coverage
Question Pool: 2023-2027 Question Pool
Subelement: G9B
Reference: 2023-2027 Question Pool · G9 - Antennas and Feed Lines
Key Concepts
Verified Content
Question from the official FCC General Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the G9B topic.