Last Updated on July 24, 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. |
| Amplitude scintillation fading margin [C] | Not included in calculation. [Allowance for small variations in the refractivity of the path.] | |
| 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] | -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 you read the pages on diffraction and height gain carefully to learn how to the calculator computes these variables. These set values for various percentage time availability. If the aim is to assess a system for normal operation, I suggest using 50% of time. For DX, I suggest 5% of time. In the DX case, the excess loss due to diffraction will plummet. And there will be 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.
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.