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 Dipoles Monopoles Hoops Slim Jims J poles Etc…

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

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

Preliminary Thoughts

The home user needs a free frequency on the FM bandIt seemed reasonable that the user would be forced to pick a station on the fringes of the FM band for good reception

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.

Finding the Right Size

One eighth wavelength antennas were also possibleThe sizes are 16.5in. And 13.5 in. respectively

Necessary Specs

Broadcast Warehouse said the transmitter needed a 50ohm non-reactive antenna

Rule #3: Designing an antenna is a process of compromises

The Impedance of Monopoles

The Importance of Impedance

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?!

Fun with Physics

An infinitely large, perfectly conducting plane…