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Special: Creating Your Own Home FM Antenna
by Marc S.A. Glasgow, aka The CyberPoet®
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How to Build An FM Antenna - Floating Graphic

* Much Better Reception*

* Cheap *

* Simple Do It Yourself Project *


Why
Perhaps you want to get a specific station that you're having problems with receiving. Perhaps you just want to get better reception in general. Either way, this page will teach you how to build a home antenna of the caliber of the pro's for under $15. 
Requires
An FM receiver with a 300 Ohm or 75 Ohm antenna connections on the unit. If you have a 75 Ohm antenna connector, you will need to buy a 300 Ohm to 75 Ohm adaptor (Radio Shack). 

Supplies
100 feet of 300 Ohm twinlead wire. Available from Radio Shack's special order department (1-800-the-shack) and from their website (www.radioshack.com), part number 910-2071 (per foot quantity, 8 cents per foot, so order quantity 100, total cost $8 plus s/h). 
Picture of 300 Ohm Twinlead Wire sold at Radio Shack.

Optional Supplies (recommended, but not needed):
soldering iron & solder
electrical tape
Tools
Wire cutters and wire strippers 
OR 
exacto knife
OR
sharp kitchen knife
The Theory
FM waves, like any other wave, travels at the speed of light, and has a specific length to the signal waves (called wavelength). For some reason that is beyond my understanding, antennas built to multiples of 5/8th of the wavelength get the strongest signals  - even stronger than full wavelength antennas; using a multiple of the 5/8th length increases the strength a bit with each multiple, up to the limit of resistance on the wire (in other words, an antenna 6 times the 5/8ths length might work, but anything beyond 12 times probably can't send the signal from the ends down the wire to the radio). 
You can lead the antenna wire out through a window and tack it to the underside of overhang of the roof, or to the wall, which will also help the signal by getting the wall out of the way. The higher you can put the antenna, the better the signal will be. Also, the antenna doesn't have to be perfectly straight; it can go around a corner without a problem, just make sure each side of the antenna, measured from the corner, is a multiple of the 5/8th wave length. 
The Math
To build an antenna that centers on a specific station (such as a weak station you want to receive better), use the frequency of that station in the following formula. Otherwise, use the central frequency of the FM dial (98 Mhz) for the formula to get complete coverage. In Tampa, Mix 96 transmits at 95.7 Mhz (95.7 on your FM dial), 98 Rock uses 97.9 Mhz, 103.5 Thunder uses 103.5 Mhz, et cetera. 

Formula:
300 / (frequency in Mhz) * (5/8) * (1/2) = length in meters of the ideal 5/8ths antenna using twinlead 300 Ohm wire. 

To simplify the formula: 
93.75 / Freq in Mhz = length in meters of the ideal 5/8th's antenna. 

So, to get the length of the ideal 5/8ths antenna for the full FM radio spectrum (centered on 98Mhz):
93.75 / 98 = .95663265306 meters, or 95.66 centimeters
(which converts to 37.66 inches). 

To build a stronger antenna, multiply that result by a whole number. 
Example:  0.9566 meters * 5 = 4.783 meters
So an antenna made of (5) 5/8ths wave sections for 98 Mhz would be 4.783 meters long (which is 15'  8 5/16 ")


Time To Cut
Having figured out the ideal length, now it's time to cut the wire. Measure out the length you calculated (or using 4.783 meter figure we came up with for a pretty good general FM antenna), and cut the wire. 
Strip the wires on each end, and twist them together well. If you want to be really gung-ho, solder them together. 
300 Ohm Wire Graphic showing leads


Lead Down
Now that you have the basic antenna, we need to add in a way of hooking it up to the radio. To do that, figure out where you're going to hang the antenna, and where it would be most convenient to bring another wire down off it to the radio. 
Having figured that out, now we have to cut the antenna wire again, this time cutting only one side of it and bringing out the wires (see picture):
300 Ohm Wire Graphic showing how to cut one inch out of center

Now take a new length of the wire (make it as long as you need it to be to go from where the antenna is mounted down to the back of the stereo), strip the ends as shown in the Time To Cut section, and attach one wire from each side to the antenna:
 
Graphic of how to connect vertical cable to horizontal cable to make antenna

 
 
NOTES:
The lead down wire does act as an antenna, but since it travels vertically, it receives significantly less signal than the antenna loop, which should always be placed horizontally (i.e. - parallel with the ground), preferably as high up as possible. If you really want to insure that the antenna is perfect, make the lead down wire a 5/8ths of a wavelength too...
Best of luck!
 
 
Copyrighted Material.
© 2000,  Marc S.A. Glasgow, aka The CyberPoet®
All rights reserved under U.S. and international law.

Marc Glasgow is a Macintosh Consultant serving the Tampa Bay area since 1990.
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