Last Updated on July 29, 2026 by John Berry
The path loss for a tropospheric path is made up of several components: free space loss, loss due to diffraction over the Earth and obstacles, and height gain. Together with the transmitter output power and the receiver threshold, they give the tropospheric propagation path budget. The result is in terms of margin: margin above receiver threshold.
If the margin is positive, the path will work with the technology proposed yielding the percentage availability wanted. If it is negative it won’t: it will yield a lesser availability.
Diffraction and height gain
Here’s a calculator that assesses radio paths for given technologies. There are three primary variables: frequency, path length, and height gain. I give further guidance on the variables below the calculator.
This tool for evaluating system technologies and equipment parameters.
| Parameter | Value | Comment |
|---|---|---|
| Frequency [f] | MHz | |
| Transmission losses | ||
| TX power [1] | Power output in Watts. Converted to dBW on Calculate. | |
| TX Power [2] | 20 | dBW |
| TX feeder losses [3] | Enter negative, as loss in dB from transmitter to antenna | |
| Antenna gain at TX [4] | dBi | |
| Path length [d] | In kilometres (km) | |
| Free space path loss [5] | -125 | In dBi using the Free Space Path loss formula. |
| Percentage time for path viability [6] | Percentage chance of a QSO. 50% for normal operation and 5% for tropospheric DX. | |
| Excess loss over free space [7] | -124 | Calculated from Recommendation ITU-R P.526-15. Expressed as negative decibels. |
| Percentage for locations variability | To accomodate locations variability at distant station pick 50% (normal, 0dB enhancement), 5% (DX, 13dB enhancement). | |
| Path loss improvement for given percentage of locations [C] | 0 | Low percentage locations enhances path loss by decibel value indicated. |
| Effective height in metres at TX [8a] | Effective antenna height at the TX in metres above the surrounding terrain | |
| Height gain at TX for given percentage time [8] | -17 | Gain in dB available at the TX end. Calculated for stated percentage of time. |
| Effective height in metres at RX [9a] | Effective antenna height at the RX in metres above the surrounding terrain | |
| Height gain at RX for given percentage of time [9] | -17 | Gain in dB available at the RX end. Calculated for stated percentage of time. |
| Antenna gain at RX [10] | dBi | |
| Feeder loss at RX [11] | Enter negative. From array to RX. Perhaps balanced by the presence of the low noise amplifier. | |
| Transmission loss as total gains minus total losses [12] as [3] + [4] + [5] + [7] + [8] + [9] + [10] + [11] + [C] | -200 | Sum of losses and gains. To be used in margin calculation. |
| System Value | ||
| Receiver threshold [13] | In dBW. The 2.5kHz sensitivity of a typical ham rig for 0dB signal to noise. | |
| Coding/bandwidth gain [14] | Bandwidth factor. Typically 10log (2500/necessary bandwidth), plus data coding gain. SSB=-10, FT8=+21. See pages on Data Modes. | |
| Antenna system threshold degradation (-ve) or improvement (+ve) [15] | Considering system noise with the low-noise amplifier in place. Considering poor site noise. See System Noise and Threshold Degradation. | |
| Effective threshold [16] as [13] – [14] – [15] | -186 | In dBW. Including threshold degradation or improvement. |
| Link Budget Results | ||
| System Value [17] as [2] – [16] | 206 | Decibels. As the maximum loss (positive) between TX output and RX input for viable operation. |
| Margin [18] as [17] – [12] | 6 | System Value minus Total Loss. As dB above the effective threshold. Positive values mean path will work. |
This calculator gives a means of modelling scenarios so that radio amateurs might plan their systems on the various bands. Some of the settings need some research to get a realistic result.
Percentage time
I suggest using 50% of time initially. At a default margin of -56dB there is no way that 300km path will work. Start by reducing the path length to 100km. Suddenly, things become possible. Then play with the various parameters to see how each changes the performance. For DX, I suggest 5% of time. In the DX case, the excess loss due to diffraction will plummet. And there will be some height gains available, but even for high sites/antennas these will be modest because of a flattened Earth.
Research the receiver threshold for your rig. Note that to enable the threshold to be used for various speech and data systems, you should enter a value corresponding with a 0dB Signal-to-Noise ratio in a 2.5kHz bandwidth for SSB. As a guide, I’ve given the typical Icom IC-9700 value of -166dBW.
Locations variability
The prediction model using Rec. ITU-R P.526-10, initially as nomograms and then implemented here as formulae, is based on a two-dimensional geometry modified by Earth bulge. It’s effectively a bald-earth assessment from a point to an area and that’s accurate enough for radio amateurs interested in determining distance over hundreds of kilometres. But there’s a local condition at the distant station to be considered.
In making a prediction to an area, the path loss will be a single value representing that area. In reality, the value of signal level at points across the area will vary. Simply, the loss into a valley will be dramatically different from that to a hilltop. This gives rise to the notion of locations variability.
The flat-Earth models have their origins in worst-case interference modelling to enable analysis of low percentage of time, high percentage locations signals. Typically, the propagation curves developed represented signal levels exceeded for 50% of locations within an area of 500 metres by 500 metres. Locations variability typically follows a long-tail distribution that can be approximated to a straight line in the centre region. This is shown in Figure 5 of Rec. ITU-R P.370-7 and subsequently in Rec. ITU-R P.1546-3 where 50% corresponds to 0dB in path loss and 5% indicates -13dB.
To accomodate locations variability, I have included the notion of 0dB for 50% and -13dB for 5% of locations within the above calculator. The path loss is therefore reduced for low percentage locations making distance greater. Values of locations variability do also vary as log f, but I’ve added a flat value for now.
Other inputs
For data modes, you will need to enter the relevant coding and bandwidth gain. I give some guidance on these figures on my Data Modes page. FT8 is typically 21dB. Q65B is 27dB. And of course, SSB voice is -10dB corresponding with a necessary 10dB signal to noise ratio. This is a much discussed and disputed variable. I give more values on my page about data modes and the Shannon limit.
Then you need to consider any threshold degradation. I discuss this on the System Noise page. If you use a low noise amplifier at the antenna, if may be that you’ll get a threshold improvement. And if you are unfortunate enough to have a bad environmental noise figure, you will need to find a suitable degradation from my Radio Noise page.
Interpreting results
Once you have decent values in each input field, press Calculate. If the margin is positive, all is well. If it’s negative, you’ll need to re-visit each variable and question it’s value and what might be done to improve things. Like spending a cash budget, you can spend the tropospheric propagation path budget to optimise performance.
Note that the approach here to path calculation is one of point-to-area. It’s from a point to stations at a distance away. The accuracy of the prediction is likely to be better than 10dB. To get accurate predictions of one point to another, you will need to invoke numerical methods using a digital terrain and buildings model.
The approach here does not include anomalous propagation like ducting, tropospheric scattering, and aircraft scatter. Often these mechanisms provide DX propagation over longer distances than simple modification of k. This model does however illustrate well the rapid degradation of signal with increased diffraction loss due to increasing Earth bulge.