Thursday, July 15, 2010

Lesson 31: Technician Class Exam Prep T9A

Here is lesson 31 and the T9A questions from the exam question pool.  The T9A questions deal with antennas.  You may have the best transceiver on the market, but if you have a bad antenna the quality of the transceiver won't mean a thing.  You will want to familiarize yourself with some basic antenna principles before you try to get on the air.  You at least need to know the basic principles for building a dipole antenna.  The exam question review will help, but they really don't go into the detail you need.  A good book is Basic Antennas: Understanding Practical Antennas and Design published by the ARRL.  It breaks down the basic theory and gives some good project ideas.

There are two formulas you will need to know for the exam.  The first is to find the length of a quarter wave vertical antenna in inches.  The formula for this is:

Length(in) = 12(234/freq(MHz))

The other is to find the length of a half wave dipole in inches.  The formula for that one is:

Length(in) = 12(468/freq(MHz)

For once, these formulas come in handy for everyday Ham use, unlike some of the others.

Until next time...

73,
Andy
KE4GKP

5 comments:

1. Thanks Andy when will you up date your general class because i want to jump in to it because i want it before my13 birthday
KPB

2. Andy, I am loving your course. So Helpful.
Can you tell me how this 1/4 or 1/2 length formula came to be the standard? it is a little "off". Using the 1/4 wave for 146 Mhz: The actual Wavelength of 146 Mhz based on the extact vaule of "c"/146 *10^6 = 2.0533 M
1/4 Wave = 0.5133 M = 20.208 inches.

12 * (234 / 146) = 19.223 Inches.

Am I missing something?

3. I'm on the road, but I will try to answer the best with an iPhone in Vance County, NC. for the most part, the formula is a swag. It provides a fairly accurate length across all frequencies. The moral of the story is that a 1/4 wavelength vertical for 146 MHz is about 20" and will provide a relatively low SWR. Unfortunately, I was an archaeology major and daydreamed through math. Hope that helps. 73. -Andy

4. Thanks, Andy. I figured it was for a "close enough" for a band. Glad you could confirm.
Finished your Tech Lessons today and did a practice test on eHam - passed. Think I'll go through your videos again before my real test on Jan 14. Can't thank you enough for all the effort you have put into helping us folks that are getting started in the field.