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Antenna length for a frequency modulated signal. Answered

I  am given to understand that the length of an antenna is proportional to the wavelength of the signal that is fed to it (a quarter wavelength, for example). In the case of a signal modulated by amplitude, the frequency (and therefore wavelength) of the carrier wave remains unchanged. However, in the case of a signal modulated by frequency, the signal fed to the antenna will vary in frequency (and, therefore, wavelength). My query is this : how would one go about calculating the length of an antenna for a frequency modulated signal? Its frequency is not constant, so to construct an antenna of say, a quarter of the wavelength, would be impossible.
I thank you in advance for your assistance.


5 Replies

Dr.Bill (author)2010-01-13

468 / frequency will give you length in feet and inches described as a whole number and a decimal.
This will give you the proper electrical length for a 1/4 wave antenna usable for FM.
If you want a ground plane antenna use 2808 / frequency for the vertical length and 2948 / frequency for the length of the radials. 1/4 wave antenna formulas. ARRL Lab can be contacted @ 1-860-594-0214
                                                    International 1-860-594-0355

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alexhalford (author)2009-12-31

Thank you all very much. Happy new year.

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steveastrouk (author)2009-12-31

As Re-design and Lemonie say, the modulation depth is a tiny fraction of the actual broadcast frequency - think about it, channels are only separated by 50 kilohertz, so the mod depth must be less than THAT, and the broadcast frequency minimum is 88 megaHertz.

Maths: 50 E 3/88E6 = 0.05%


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Re-design (author)2009-12-30

An antenna has a certain freq. the it is best tuned for.  But the drop off is not sharp.  Say you have an antenna for the 10 meter band.  You might tune it for the center of the band and have a 1.1 swr.  At each end of the band the swr might only rise to 1.2 and you won't loose much signal.  But use that antenna on another band and it would be unuseable unless the new band was a harmonic of the 10 meter band.

On a receiver it is even less important to have a perfect antenna.

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lemonie (author)2009-12-30

The FM frequency isn't modulated that much, it's close enough to the value given.


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