Use this tool to calculate how far a VHF radio can transmit.
Enter the antenna height and pick the units (meters or feet).
Formula
d = 4.12 * √h
where h is the antenna height in metres and d is the distance in kilometres.
Example Calculation
For a height of 12 feet, the range of a VHF radio system is 5 miles or 8 km. Note that this assumes no obstructions and no interference from other radios which will reduce the range.
Assumptions
Key assumptions are
- the transmitter has enough output power and
- the receiver sensitivity is good enough
for communication between the transmitter and receiver.
To validate these assumptions, use the antenna range calculator. This tool uses the free space path loss equations to find the max range between a Transmit-receive pair. The FSPL assumes ideal conditions of free space and will therefore represent the best possible propagation distance.
As an example, if these are the system parameters:
- Frequency of operation = 100 MHz
- Transmit Power = +20 dBm
- Transmit Antenna Gain = 0 dBi
- Receiver Sensitivity = -90 dBm
- Receive Antenna Gain = 0 dBi
- Total Cable and Other Losses = 0 dB
then the ideal range is 75 km which exceeds the range calculation above. In this scenario therefore we have adequate transmit power and receiver sensitivity.
Another example:
- Frequency of operation = 100 MHz
- Transmit Power = -10 dBm
- Transmit Antenna Gain = 0 dBi
- Receiver Sensitivity = -90 dBm
- Receive Antenna Gain = 0 dBi
- Total Cable and Other Losses = 0 dB
the ideal range in this case is 2.4 km. In this scenario, on account of inadequate transmit power and receive sensitivity, the range calculation will not be correct.
Validate the radio system parameters using this calculator prior to estimating the range using the calculator on this page.
Background
VHF (Very High Frequency) is the name for radio frequencies in the range between 30 MHz and 300 MHz. VHF band is used for FM Radio, Two-way radio, television broadcasting, Automatic Identification System (AIS) and other applications.
The VHF wavelength lies between 1 meter to 10 meters.
Antennas are typically on the order of the wavelength. Using the dipole antenna calculator, at 100 MHz, the physical length of a half wave diople is 4.7 feet or 1.4 meters.
Another example is the antenna shown below which is designed for the AIS band at 160 MHz. It is a whip antenna also known as a monopole. The monopole antenna calculator can be used to find the length of a quarter wavelength long antenna.
In this case, it is 18.4 inches. The antenna in the picture below is half wavelength long according to the product description, and hence its length is calculated to be 36.8 inches.
Also as per the product description the total length is 39 inches including the base and holder. So the calculation is consistent with the product details
✅ I really like it when the math makes sense! 🤓
References
[1] Line of Sight Propagation on Wikipedia