Why is the range of an FM signal shorter than AM signal? I thought it was due to the high frequency of FM signal getting absorbed by physical barriers. Is it so? Does power of a signal has any role to do with range?
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asked Dec 21, 2014 at 7:26
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3\$\begingroup\$ FM and AM are just modulation techniques and have nothing to do with frequency. Please be more specific. \$\endgroup\$– MajenkoCommented Dec 21, 2014 at 10:15
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2 Answers
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If you are comparing AM and FM standard broadcast bands then there are a couple of reasons. AM broadcast is typically about 1MHz and FM about 100MHz and this makes a big difference.
In free space (no obstacles like the earth), the power loss from a transmiting antenna to a receiving antenna (assuming perfect isotropic antennas) is: -
Loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d)
Where f is in MHz and d is in kilometres. This equation tells you how many dB of power loss you can expect at a given distance with a given carrier frequency.
If f = 1MHz the 1st term becomes 0dB. However, if f = 100MHz then the 1st term becomes 40dB i.e. 100MHz transmissions incur 40dB more loss at the receive antenna compared to 1MHz transmissions.
It's not that less power is transmitted but that the receiving antenna has a smaller aperture to capture power heading its way. The aperture is a measure of surface area i.e. an antenna operating at lower frequencies has a bigger effective surface area in which to capture power and convert to an electrical signal.
The 2nd reason is due to the ionosphere - signals at or about 1MHz can readily bounce off the ionosphere back to earth whereas at 100MHz, there is very little signal reflection back to earth.
answered Dec 21, 2014 at 10:45
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\$\begingroup\$ In addition to the greater free path loss already mentioned, the limited diffraction of high frequency EM waves also hinders the FM reception. On the other hand, diffraction is more pronounced in the case of AM frequencies. Thus AM radio can bend over obstacles like mountains more easily. So, although an AM receiver may seem to be in the 'shadow' of mountains, reception is still possible. \$\endgroup\$ Commented Jan 26, 2020 at 16:18
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The range of FM transmission depends on the transmission power, channel conditions and the antenna gain. AM medium wave broadcasting uses 300 Hz to 3 MHz band while FM radio is associated with VHF band. Path loss is generally high for FM radio broadcast. At night time,the shortwave AM radio signals can travel farther due to ionospheric reflection and this is not possible with FM radio. With only a transmit power of 0.2 W, considering station frequency of 106.4 MHz and station bandwidth 30 kHz with AWGN model, a broadcast distance of 500 m can be possible provided the antenna gains are huge, of the order of 14.5 dBi. The range can further be extended and relaxations be made on antenna gains if transmit power is made high.
