Monday, March 21, 2011

Lesson 31: General Class Exam Course G9B

Here is lesson 31!  Just four more to go!  This lesson covers basic antennas.  There are a few things you will need to remember for this section.

There are two formulas you will need to know for determining the length of a 1/2 wavelength and 1/4 wavelength antennas.  For the 1/2 wave antenna:

Length (in feet) = 468/frequency in MHz

For the 1/4 wave antenna:

Length (in feet) = 234/frequency in MHz

Keep in mind that a 1/4 wave antenna is half the length of a 1/2 wave antenna and 234 is half of 468.  That should help in memorizing the formula.

There are also some feed-point impedance relationships to antenna construction that are not very intuitive and you will need to memorize.

-For ground plane antennas (think vertical antennas), downward sloping radials increase feed-point impedance.
-A dipole's feed-point impedance steadily decreases as the antenna is lowered below 1/4 wavelength above the Earth.
-A dipole's feed-point impedance steadily increases as the feed-point (where the feedline attaches to the dipole) moves from the center to the ends of the antenna.

As always, please feel free to leave any comments, suggestions, or questions in the comments box.