Communication Range of Handheld Radio
Posted: Sat Nov 19, 2016 11:17 am
Recently I read a comment in an on-line discussion forum regarding the communication range of a handheld radio. An anecdotal report said, in one user's experience, a handheld radio could "barely communicate" over a distance of a half mile. This struck me as being unusual, and it was certainly contrary to my own experience with using handheld VHF radios for many years. That remark prompted me to analyze the communication circuit for two handheld radios to communicate with each other. My findings are below.
Communication Range of Handheld Radio
A common situation in marine radio communication involves use of a handheld radio. This article attempts to analyze the range of communication for such a radio. The transmitter power of the handheld radio is assumed to be 5-Watts. The antenna of the handheld radio is physically shorter than the usual full-size antenna that would be used, and therefore we assume the antenna gain is negative in comparison to a full-size halfwave dipole antenna. Exactly how much less effective the short antenna will be is difficult to assess. Here I will assume the antenna gain is -6 dB compared to isotropic, which would about -8 dB compared to a dipole. There is no transmission line, so the transmission line loss is assumed to be 0 dB. We now analyze the transmitted signal level from a handheld transmitter as described above:
Transmitter power 5-Watt = +37 dBm, where 0 dBm is one milliWatt
Transmission line loss = 0 dB
Antenna gain = -6 dB
Radiated power = +31 dBm
For path loss we will calculate for a distance of 0.5-mile at a frequency of 157-MHz. Assuming free-space attenuation, the path loss will be
Lp = 36.6 + 20log(f) + 20log(d) (f in MHz, d in miles)
For a distance of 0.5-miles this calculates to -74.5 dB. The assumption of a free-space path loss is reasonable if the two antennas are actually in visual sight of each other and there is mostly open water as the intervening terrain.
The signal from the 5-Watt handheld radio after traveling a 0.5-mile path in freespace will be
Transmitter signal + path loss = signal at receive antenna
+31 dBM + -80.5 dB = -49.5 dBm
We assume the handheld receiver sensitivity is as good as a fixed base radio, that is, the receiver is able to produce usable signals from received signals as weak as -113 dBm, or 0.5-microVolts in a 50-Ohm antenna. Now we find the received signal strength at the receiver, including antenna gain and transmission line loss:
Signal level = -49.5 dBm
Antenna gain = -6 dB
Receiver line loss = 0 dB
Received signal = -55.5 dBm
Because the receiver sensitivity is sufficient to work with signals as weak as -113 dBm, and the available signal is much stronger, -55.5 dBm, we have a reserve gain in the path of -55.5 - -113 = 57.5 dB. This means that the signal could fade as much as 57.5 dB and the communication path would still be usable.
The analysis shows that if two 5-Watt VHF Marine Band handheld radio are only 0.5-mile apart and they are in visual sight of each other (which permits a reasonable assumption of the freespace path loss), there should be no difficulty to communicate. The transmitter power, antenna gains, receiver sensitivity, and path loss are such that there is a fade margin of 57.5 dB. This suggests excellent reliability.
If the transmitter power were decreased to 1-Watt, the transmitted signal power would decline to 30 dBm, a reduction of 7 dB. Since the path had a fade margin of 57.5 dB, the reduced transmitter power provides communication with a fade margin of 50.5 dB.
If the antennas of the radios are less effective, let us say only having a gain of -9 dB compared to isotropic, that is a net reduction of only -6 dB in the signal path, still giving a fade margin 51.5 dB.
If we assume both 1-Watt transmitter power reduction and -9 dBi antennas, then the fade margin is still 44.5 dB.
If we assume cross-polarization of the antennas, we can add about -20 dB to the path loss, but we still have a fade margin of 24.5 dB.
If we assume the path loss will be much greater than the free-space assumption, we can deduct the greater path loss from the fade margin calculated. Since there is a fade margin of 57.5-dB, a great deal of added path loss can be tolerated..
These calculations affirm my initial opinion based on experience: two VHF Marine Band handheld radios should be able to communicate over a distance of 0.5-miles with high reliability, as long as the path is a true line-of-sight path, that is, the two radios are in sight of each other. Since we assume that a handheld radio will be held at least 5-feet off the ground, we know the distance to the radio horizon will be
d = (2 x h)^0.5 where d is in miles and h is in feet
d = 3.16-miles
This affirms that the radio horizon of the two radios will overlap.
On the basis of the above calculations, two 157-MHz handheld 5-Watt radios separated by a distance of 0.5-miles and in sight of each other should have no problem communicating. Even if the transmitter power is decreased to 1-Watt, the antenna gain cut in half, and the antennas oriented for cross polarization, there should still be sufficient fade margin in the path to allow good communication.
The one factor which was not varied in the above calculations was the receiver sensitivity. Note that receiver sensitivity can be reduced if the operator of the radio sets the squelch threshold improperly. Local noise interference will also reduce the apparent receiver sensitivity. Another influence on performance will be battery status. Transmitted power will be reduced if the battery voltage is below normal.
Note that if the other station in the communication is using a fixed mount radio with a better antenna, the range improves considerably. The antenna gain of the other station improves both the receiving and transmitting paths of both stations. Since the analysis assume a loss of -6dB for the antennas, substituting an antenna with 3dB gain at the other station improves the fade margin by 9dB on the path in either direction.
For more background see some prior articles on these topics:
Radio Horizon
http://continuouswave.com/radio/radioHorizon.html
Marine VHF Radio Communications
http://continuouswave.com/whaler/reference/VHF.html
Conversion of Receiver Sensitivity
From micro-volts to dBm
http://continuouswave.com/radio/dBm.html
Communication Range of Handheld Radio
A common situation in marine radio communication involves use of a handheld radio. This article attempts to analyze the range of communication for such a radio. The transmitter power of the handheld radio is assumed to be 5-Watts. The antenna of the handheld radio is physically shorter than the usual full-size antenna that would be used, and therefore we assume the antenna gain is negative in comparison to a full-size halfwave dipole antenna. Exactly how much less effective the short antenna will be is difficult to assess. Here I will assume the antenna gain is -6 dB compared to isotropic, which would about -8 dB compared to a dipole. There is no transmission line, so the transmission line loss is assumed to be 0 dB. We now analyze the transmitted signal level from a handheld transmitter as described above:
Transmitter power 5-Watt = +37 dBm, where 0 dBm is one milliWatt
Transmission line loss = 0 dB
Antenna gain = -6 dB
Radiated power = +31 dBm
For path loss we will calculate for a distance of 0.5-mile at a frequency of 157-MHz. Assuming free-space attenuation, the path loss will be
Lp = 36.6 + 20log(f) + 20log(d) (f in MHz, d in miles)
For a distance of 0.5-miles this calculates to -74.5 dB. The assumption of a free-space path loss is reasonable if the two antennas are actually in visual sight of each other and there is mostly open water as the intervening terrain.
The signal from the 5-Watt handheld radio after traveling a 0.5-mile path in freespace will be
Transmitter signal + path loss = signal at receive antenna
+31 dBM + -80.5 dB = -49.5 dBm
We assume the handheld receiver sensitivity is as good as a fixed base radio, that is, the receiver is able to produce usable signals from received signals as weak as -113 dBm, or 0.5-microVolts in a 50-Ohm antenna. Now we find the received signal strength at the receiver, including antenna gain and transmission line loss:
Signal level = -49.5 dBm
Antenna gain = -6 dB
Receiver line loss = 0 dB
Received signal = -55.5 dBm
Because the receiver sensitivity is sufficient to work with signals as weak as -113 dBm, and the available signal is much stronger, -55.5 dBm, we have a reserve gain in the path of -55.5 - -113 = 57.5 dB. This means that the signal could fade as much as 57.5 dB and the communication path would still be usable.
The analysis shows that if two 5-Watt VHF Marine Band handheld radio are only 0.5-mile apart and they are in visual sight of each other (which permits a reasonable assumption of the freespace path loss), there should be no difficulty to communicate. The transmitter power, antenna gains, receiver sensitivity, and path loss are such that there is a fade margin of 57.5 dB. This suggests excellent reliability.
If the transmitter power were decreased to 1-Watt, the transmitted signal power would decline to 30 dBm, a reduction of 7 dB. Since the path had a fade margin of 57.5 dB, the reduced transmitter power provides communication with a fade margin of 50.5 dB.
If the antennas of the radios are less effective, let us say only having a gain of -9 dB compared to isotropic, that is a net reduction of only -6 dB in the signal path, still giving a fade margin 51.5 dB.
If we assume both 1-Watt transmitter power reduction and -9 dBi antennas, then the fade margin is still 44.5 dB.
If we assume cross-polarization of the antennas, we can add about -20 dB to the path loss, but we still have a fade margin of 24.5 dB.
If we assume the path loss will be much greater than the free-space assumption, we can deduct the greater path loss from the fade margin calculated. Since there is a fade margin of 57.5-dB, a great deal of added path loss can be tolerated..
These calculations affirm my initial opinion based on experience: two VHF Marine Band handheld radios should be able to communicate over a distance of 0.5-miles with high reliability, as long as the path is a true line-of-sight path, that is, the two radios are in sight of each other. Since we assume that a handheld radio will be held at least 5-feet off the ground, we know the distance to the radio horizon will be
d = (2 x h)^0.5 where d is in miles and h is in feet
d = 3.16-miles
This affirms that the radio horizon of the two radios will overlap.
On the basis of the above calculations, two 157-MHz handheld 5-Watt radios separated by a distance of 0.5-miles and in sight of each other should have no problem communicating. Even if the transmitter power is decreased to 1-Watt, the antenna gain cut in half, and the antennas oriented for cross polarization, there should still be sufficient fade margin in the path to allow good communication.
The one factor which was not varied in the above calculations was the receiver sensitivity. Note that receiver sensitivity can be reduced if the operator of the radio sets the squelch threshold improperly. Local noise interference will also reduce the apparent receiver sensitivity. Another influence on performance will be battery status. Transmitted power will be reduced if the battery voltage is below normal.
Note that if the other station in the communication is using a fixed mount radio with a better antenna, the range improves considerably. The antenna gain of the other station improves both the receiving and transmitting paths of both stations. Since the analysis assume a loss of -6dB for the antennas, substituting an antenna with 3dB gain at the other station improves the fade margin by 9dB on the path in either direction.
For more background see some prior articles on these topics:
Radio Horizon
http://continuouswave.com/radio/radioHorizon.html
Marine VHF Radio Communications
http://continuouswave.com/whaler/reference/VHF.html
Conversion of Receiver Sensitivity
From micro-volts to dBm
http://continuouswave.com/radio/dBm.html