Wednesday, March 30, 2011

Lesson 33: General Class Exam Course G9D

Here is lesson 33 and the G9D section.  This lesson goes over some aspects of specialized antennas.  There are two antenna types you will need a basic understanding of: Log Periodic Antennas and Beverage Antennas.

A log periodic antenna looks a lot like a type of Yagi antenna, but the log periodic antenna is capable of handling a much larger bandwidth than a Yagi.  The elements of a log periodic antenna are basically a series of dipoles connected to each other along a boom.  The longest element is cut to 1/2 wavelength of the lowest desired operating frequency.  From there the length and spacing of the elements is determined by a logarithmic formula.  Thus the "log" in log periodic antenna.

A Beverage antenna is basically a very long wire.  Beverage antennas are great for low HF directional receiving but are very bad for transmitting.  Beverage antennas are characterized by their long length, low height above the Earth, and their ability to be very directional receiving antennas.  They can be extremely long, miles in some cases.

NVIS, or Near Vertical Incidence Skywave, is a term you will need to be familiar with as well.  An NVIS antenna is a horizontally polarized antenna that is placed between 1/10 and 1/4 wavelength above the Earth.  The result this has is that the majority of the signal strength is radiated more upward than off to the sides which allows for shorter skip distances during the day.

The one and only number you need to memorize for this section is the gain of two 3 element vertically stacked Yagis 1/2 wavelength apart.  By stacking Yagis in this fashion, the two Yagis will produce a 3 dB gain over just a single 3 element Yagi.

No sweat!

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



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