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Choosing an Antenna

Choosing an Antenna. Rule #1: All antennas are not created equal. Types of Antennas. Generally speaking antennas fall into two categories: Omnidirectional Directed Requirement: an Omnidirectional antenna. Omnidirectional Antennas. There are numerous choices in omnidirectional antennas

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Choosing an Antenna

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  1. Choosing an Antenna Rule #1: All antennas are not created equal

  2. Types of Antennas • Generally speaking antennas fall into two categories: • Omnidirectional • Directed • Requirement: an Omnidirectional antenna

  3. Omnidirectional Antennas • There are numerous choices in omnidirectional antennas • Dipoles • Monopoles • Hoops • Slim Jims • J poles • Etc…

  4. The Glorious Monopole • The monopole and dipole are the easiest to design and are fairly small in size • Best of all, they cost less than $5 at Radio Shack

  5. Preliminary Thoughts Rule #2: An antenna has to be designed for the frequency of broadcast • What frequency will the user choose? • I figured 50% of FM stations are between 90-100MHz, 35% >100MHz, and 15% <90MHz

  6. Preliminary Thoughts • The home user needs a free frequency on the FM band • It seemed reasonable that the user would be forced to pick a station on the fringes of the FM band for good reception

  7. Finding the Right Size • An 88MHz monopole antenna one wavelength in size is just over 11 ft. • A more reasonable size is the quarter wavelength monopole of just 2ft. 9in. • For a 108MHZ monopole quarter wavelength antenna the size is 2ft. 3in.

  8. Finding the Right Size • One eighth wavelength antennas were also possible • The sizes are 16.5in. And 13.5 in. respectively

  9. Necessary Specs • Broadcast Warehouse said the transmitter needed a 50ohm non-reactive antenna Rule #3: Designing an antenna is a process of compromises

  10. The Impedance of Monopoles

  11. The Importance of Impedance

  12. Finally an Antenna • I have the right size and the best impedance matching. What else is left? • The transmitter has two outputs for the antenna and the antenna has one input. • Where does the extra output go?!

  13. Fun with Physics An infinitely large, perfectly conducting plane…

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