Calculation of radio ranges in the VHF range

With the program available here, the expected range of VHF radiotelephony systems can be calculated using the ground wave propagation. The results of this range calculator also apply approximately to the conditions in the upper SW area, for example for CB radio and amateur radio operation in the 10m band. The calculated values are mean values as they can be expected without the occurrence of special effects like Sporadic-E, Aurora or soil inversions. Of course, they can vary more or less depending on the nature of the terrain and the reflective properties of the ground. Obstacles such as buildings or trees in the line of the radio link naturally lead to shorter ranges.

If the radio visual range is smaller than the attenuation range, it essentially determines the range. Only if the attenuation range is considerably greater can ranges of up to 30% over the radio visual range be expected under normal conditions. In this area, more precise calculations are hardly possible, since, among other things, diffraction effects that fluctuate strongly due to weather conditions, but practically always occur, have a not inconsiderable influence.

The program is based on the fact that the attenuation range near the ground is essentially limited by the waves reflected from the ground. As a result of the reflection, them experience a phase jump of 180 °. The lower the transmitting and receiving antennas are, the more acute the angle between the direct waves and those reflected on the ground, so that the latter more and more appear out of phase at the receiving location at low altitude and thus counteract the direct waves. As a result, the ranges achieved on the ground are much smaller than in space. There they are essentially only limited by the free space attenuation.

Here are a few practical tips on how to use the program. The RF power delivered to the antenna socket must be entered as the transmitter power. The signal voltage applied to the antenna socket for a signal that is currently worth receiving is considered the receiver sensitivity here. If there were no HF interference level picked up by the antenna, the sensitivity specification of the receiver could be used. The range would then apply to the signal-to-noise ratio (SNR or SINAD) specified in the technical data of the device used. However, since there is always an interference level in practice, the information must be corrected in the case of sensitive receivers and good antennas in the direction of poorer values. Even without direct sources of interference, when the antenna is connected, at least solar, galactic noise and, with decreasing operating frequency, increasing atmospheric noise are present.

The distance between the center of the radiator and the ground must be entered as the antenna height (effective antenna height). In the case of large mounted to the base point radiators and a lower installation height, half the radiator length must also be added. If the antenna is located on a hill or mountain, for example, its height above the mean terrain level of the surroundings should be taken into account for realistic calculation results. In the vicinity of a slope or on the edge of a plateau, therefore, depending on the considered direction of the radio link, there may be different effective antenna heights for one and the same antenna. An indication of the altitude above sea level (normal altitude zero) would otherwise lead to incorrect results. For practical reasons, the antenna gain to be entered relates to a half-wave radiator (stretched dipole), for which a value of 0 dB must be entered. Any losses, especially cable losses, are to be deducted from profit. An entry such as "3.75-0.4" is permitted here.

The frequency dependence of the free space attenuation is just canceled out when calculating the signal voltage from the field strength. A frequency specification is therefore not necessary for these calculations.