I was an electronics technician in the US Navy.
First, frequency is independent of amplitude. Frequency has
to do with the distance between sequential crests of a sine
wave. The greater the distance, the lower the frequency.
Amplitude has to do with the heighth of the waveform.
See this Google search for images and sites on sine waves:
Amplitude has to do with the signal strength, which has an
effect on distance in that, the stronger the signal, the
further it can travel.
Given two signals of equal strength and different frequencies,
lower frequencies travel further than higher ones. AM radio
signals, in the range of 520 to 1,710 KHz, can often be picked
up at distances of 100-300 miles, while FM frequencies of
88 MHz to 108 MHz are limited to what's known as line-of-
sight transmission, topping out at around 50-60 miles, at
best. Khz = thousands of Hertz, or cycles per second, and
Mhz = millions of Hertz (Hz), so the high end of the AM
frequency band, 1,710 Khz = 1.71 Mhz, vs the 88-108 Mhz
range of the FM band.
Note that AM and FM differ in the type of modulation they
use, but this is irrelevant to the distance which the signal
can travel based on frequency. The frequency and strength of
the transmitted signal, which is called the 'carrier wave',
is what determines the distance of propagation. AM and FM
differ in that the music or voice content is added to the
carrier wave by modulating (varying) the amplitude or the
frequency of the transmission wave with the audio signal.
AM uses amplitude modulation, and FM uses frequency. The
quality of the audio resulting from frequency modulation
is much better than that resulting from amplitude modulation,
so that AM audio is, by definition, not as clear as FM audio,
but this difference is due to the form of modulation, and
not the frequency of the carrier.
Given two AM signals of equal strength and different
frequencies, the lower frequency will travel further and
have greater clarity at equal distances from the respective
transmitters. The same would hold two for two FM signals
of different frequencies and equal strength.
Also note that higher frequencies tend to form a more
'coherent' signal, like a flashlight vs a lantern, which
make them tend to travel only in a straight line, or
line-of-sight. Lower frequencies travel in what's called
a 'ground wave' during the day, and a 'skywave' at night,
allowing them to travel around the curvature of the earth,
while higher frequencies cannot.
A discussion on the Wikipedia page about AM radio notes:
"Medium wave and short wave radio signals act differently
during daytime and nighttime. During the day, AM signals
travel by groundwave, diffracting around the curve of the
earth over a distance up to a few hundred miles (or
kilometers) from the signal transmitter. However, after
sunset, changes in the ionosphere cause AM signals to
travel by skywave, enabling AM radio stations to be heard
much farther from their point of origin than is normal
during the day."
This page from the Great Yarmouth Radio Club notes:
"The ground wave follows the curvature of the Earth and its
range does not depend upon the height of the antenna. However,
the range does depend upon the transmitter power and also upon
the operating frequency. Low frequencies travel further than
high frequencies. Thus under ideal low noise conditions (noon,
during winter), it is possible to communicate over distances of
about 500 nautical miles at 2 MHz by using a 100 W transmitter.
At 8 MHz, under the same conditions and using the same transmitter
power, the maximum range is reduced to about 150 nautical miles.
Note that ground wave propagation is much less efficient over
land than it is over sea because of the much lower conductivity
of the ground and other factors. Consequently, ranges over land
are greatly reduced.
Ground wave communications vary daily and with the seasons.
Greatest communication ranges are achieved during the daytime
in winter because background noise levels are lowest during
Additional information may be found from further exploration
of the links provided above, as well as those resulting from
the Google searches outlined below.
Searches done, via Google:
"frequencies travel further"
am frequency band