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Old Jul 05, 2007, 07:30 AM
Chuck 'Em and Chase 'Em
Fly2High's Avatar
United States, NY, Plainview
Joined Aug 2005
8,116 Posts
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1/4 wavelength calculation and antenna length

Guys,

I was looking to determine what length my broken antenna should be (since I did not measure it before hand and the manufacturer has given me a range of lengths to work with).

I noticed that somewhere it was said the the median 1/4 wavelength is 39.75 inches which should be for about channel 36 plus or minus.

When I did the following calculation (knowing that speed of light = wavelength x freq -> c/freq = wavelength)

I did the following :

299,792,458 metres per second /freq(Hz) = meters * (100cm/ 1m) *(1in/ 2.54cm) to get the wavelength in inches * 1/4 = 1/4 wavelength in inches

ex ch 30 = 72.390 mHz = 72,390,000Hz

299792458 / 72,390,000 *100 /2.54 * 1/4 ~ 40.76 inches

channel 36 should be about 40.69 inches

why is this wavelength LONGER than what it is stated to be for the median 1/4 wavelength?
Based on this equation, ch 55 is ~ 40.48 inches

Can anyone tell me where I went wrong???

Thanks
Frank
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Last edited by Fly2High; Jul 05, 2007 at 07:43 AM.
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Old Jul 05, 2007, 12:18 PM
Look out! The lever!
bobthenuke's Avatar
United States, AZ, Peoria
Joined Feb 2005
2,604 Posts
Frank,

Since the frequency range for the USA and Canada is Channels 11 - 60 the median number would be a split between Channels 35 and 36. If your math is correct, then the value you quoted for the median antenna length (check on it again) is probably incorrect. Keep in mind that anything even "close" to the calculated numbers will work just fine for us. I do, however, understand your desire to get it right on.

...bob
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Old Jul 05, 2007, 12:26 PM
Looking for lift...
Milford, NJ
Joined Mar 2005
489 Posts
If I just measure an existing un-altered antenna, would that give you what you need. (I'm too lazy to do the math)

Doug
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Old Jul 05, 2007, 12:44 PM
F3X
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Costa Mesa, CA USA
Joined Jun 2001
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http://www.csgnetwork.com/freqwavelengthcalc.html

Go here.
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Old Jul 05, 2007, 01:28 PM
Chuck 'Em and Chase 'Em
Fly2High's Avatar
United States, NY, Plainview
Joined Aug 2005
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the claculator returned for 72.390 (ch 30) 40.779 inches. I was off by just a little

Still this doesn't make sense that they would use 39.75 inches..

Frank
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Old Jul 05, 2007, 02:28 PM
W0X0F
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United States, CA, Los Angeles
Joined Nov 2005
1,003 Posts
Frank, inside of your rx there is a coil with a threaded ferrite plug that can be screwed in and out. This coil is used to fine tune the antenna to the exact electrical length. In theory the antenna could be any length and the coil if it were the right size could make up the difference. However in the case of your rx the antenna is cut to an approximate length and then tuned with the coil. Just solder on a piece of wire to approximate the original length of the antenna or send it back to the manufacturer and have the antenna replaced and the rx re-tuned.
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Old Jul 05, 2007, 03:11 PM
Chuck 'Em and Chase 'Em
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United States, NY, Plainview
Joined Aug 2005
8,116 Posts
You are probably right, Walt. I didn't take that into concideration. Maybe the antenna length is dependant on the ferrite plug. that may be why I read that the length is shorter than it should be.

thanks

Frank
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