Quote:
Originally Posted by staticMHZ
the shoebox one.
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You would need to lengthen the two dipoles for FM.
The center of FM is 98 MHz. So if you divide that into the speed of light you get 3 meters (actually that is 100 MHz but close enough). Then since that antenna is based on a 1/2 wave dipole with 1/4 per side, plus the end effect of the wire causes the dipole element to be physically shorter than free space by 95%.
So 1/4 of 3 meters is .75 meters or 29.5 (30 inches is easy to remember, also remember we cheated and used 100 for the center of the band.)
30 time .95 is 28 1/2 inches on each side.
Now really a plain dipole is a narrow banded antenna and now suited to cover the FM band. It's ok,if you tune to your fav station.
What you want for an element is a folded dipole. Actually you can make one if you have twin lead around or go buy one at Radio Shack for about $7.
Dipole FM Antenna - RadioShack.com
You might even have one around the house from buying an FM radio at some point.
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Now if you want to soup up the shoebox antenna instead of building his driven element buy this from Radio Shack
UHF Outline Bow-Tie Antenna - RadioShack.com
You might even want to solder a balun as close as possible to the antenna or drape the lead wire back behind the reflector and attach the balun there again with as little twin lead as possible.
It would be broad banded because it's a bowtie dipole and a vee's out at the ends like the big whisker antennas and a corner reflector is broadbanded.
Then to get fancy, don't use a flat reflector. Try a circular one, parabolic, or real simple just a one that is a Vee with about a 90 degree bend in the cardboard covered with tin foil.
Common TV Antenna Types then go down to Reflector Antennas and look at the reflector types and you will see what I mean by a 90 degree reflector. The whole page is very informative.
The parabola works better but it's so so so much easier to make one with a corner reflector behind it.
You place the radio shack folded dipole or his simple dipole in front of the corner reflector and with something non metalic you experiment on the right distance. Adjust it and move away, check your signal.
Done right a corner reflector with a single dipole element will give you about 4 to 5
db of gain, nothing to sneeze at!
So just by taking his shoebox antenna and folding the reflector in the middle to 90 degrees, with the fold horz (important!) you will add 3db of gain if you totally screw up the design!
Do it right and you will have as much gain as one of those CM 4149 antennas that Ken shows next to the corner reflector
I knew I had seen the gain of CM4149, it's 6 dbi, which is 3.85 dbd in which is the best way to judge antenna, in dbd
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