What is the strength of rf signal (AM or FM) when it reaches to antenna two three miles away?

Question

What is the strength of rf signal (AM or FM) when it reaches to antenna two three miles away?

Answer 1

No way to answer, other than probably in the microvolt range.

Depends on transmitter power, antenna gain, antenna height above ground, atmospheric conditions, receiver antenna type, polarization and gain, and more parameters.

Answer 2

this is what they called FSPL (free space path loss).  loss of signal strength travels line of sight in free space assuming no skyscraper buildings in the city reflecting the signal.  as a rough calculation

FSPL = 20Log(D) + 20Log(F) +32.44

FSPL = loss in dB

D = distance in km = 5km (2-3 miles)

F = frequency in MHz = 100 MHz (FM)

FSPL = 20Log(5) + 20Log(100) + 32.44

FSPL = 86

Pr = Pt / 86

if the transmitter signal is 100 watts

reciever signal is Pr = 100 / 86 = 1.16 watt

actually above Friis equation has to be modified due to buildings.

Pr = Pt Gt Gr [  w / (4 pi D)  ] ^ n

Pr = power receive

Pt = power transmit

Gt = transmitter antenna gain

Gr = receiver antenna gain

w = signal wavelength

D = distance

n = 3 to 5

thus

signal strength = (const) / (D ^ n)

for line of sight n = 2

Answer 3

in the high microvolt region, possibly low millivolt. Depends on the size of the antenna. and the power of the transmittter.

It may reach a fraction of a volt or even a volt with a big enough antenna and powerfull enough transmitter.