This article is a desk study of the likelihood of ground wave communications over the path VK2OMD-VK1EA, about 150km.

To be useful, S/N needs to exceed 10dB.

Ham mythology has it that ground wave is good for 100km on 80m without much qualification.

## Ambient noise

Ambient noise is very important, it is one factor of the Signal / Noise ratio which sets the limit for communication.

Using ITU-R P.368-9 we find that expected median noise figure in a residential precinct at 3.6MHz is 57dB.

Using Field strength / receive power converter we can calculate the equivalent field strength in 2kHz receiver bandwidth, it is 4.4dBμV/m. (For simplicity I have omitted the rx internal noise, it is insignificant in this scenario… but I only know that because I reviewed it.)

Note that this noise figure is based on a survey using a vertical monopole and may overestimate the noise received by a horizontal antenna.

## Ground wave propagation

Path length is 150km over “average” ground, assumed to be of type σ=0.005, ε_{r}=13.

Using ITU-R P.368-9 associated program GRWAVE.EXE, ground wave field strength was calculated for the scenario horizontal to horizontal at 10m antenna height. Implied radiated power is 3kW (65dBm) EIRP.

GRWAVE gives field strength for the above scenario at -24dBμV/m which can be extrapolated to -39dBμV/m at 50dBm (100W) EIRP.

## S/N calculation

Signal is -39dBμV/m, median ambient noise 4dBμV/m, so we can calculate S/N=-39-4=-43dB, some 53dB short of target. Expected ground wave range is about 13km, nowhere near 150km.

Vertical to vertical is about 31dB better, but still 23dB short of target. Expected ground wave range for vertical to vertical is 55km.

## Conclusion

This path is not feasible for ground wave at the freqeuncy and power mentioned, so communications will depend mainly on ionospheric paths; F2 layer propagation, meaning D layer ionisation depleted and MUF above 4MHz though E layer (or even Es at times) may provide a path, again conditional on low D layer absorption.

## References

- ITU-R. Feb 2007. Recommendation ITU-R P.368-9 (2/2007) Ground-wave propagation curves for frequencies between 10 kHz and 30 MHz.
- ITU-R. Aug 2019. Recommendation ITU-R P.372-14 (8/2019) Radio noise.