Ground wave path budget

Last Updated on October 30, 2026 by John Berry

Here’s a path budget tool for assessing ground wave links in the 1.8 to 30MHz bands. It actually works down to 10kHz, so can be used for the 135kHz and 475kHz bands, but that’s not my focus for now.

Background

The GRWAVE Fortran application was for many years the standard method for calculating path loss assuming ground wave propagation.

In recent years, the ITU has deprecated GRWAVE. The current Recommendation, ITU-R P.368-10 (08/2022), names NTIA’s LFMF-SmoothEarth program as its official software.

On running both programs side by side across 1.8 to 30 MHz, both polarisations, and sea, average and poor ground, out to 500 km. LFMF agrees with GRWAVE to within about 1.5 dB everywhere, and usually much less.

The path loss calculator I’ve developed here accesses an instance of LFMF which runs on a virtual server. Since it uses third-party code and my server, there are some conditions around its use by visitors and these are noted below.

Using the tool

Users can enter their technical parameters into the ground wave path budget tool and get an assessment of link performance. This is ultimately assessed by way of a fade margin above receiver threshold. If the fade margin is positive, the link will work at that distance.

Conversely, the path budget tool can be used to assess the maximum distance possible given the extant technical parameters for a zero decibel fade margin. A zero decibel fade margin is where the link just works. This maximum distance is the last field in the calculator.

Technical issues

LFMF only supports antenna heights up to 50 m, so the calculator caps heights at 50 m.

In line with other calculators on this site, I focus on the calculation of a fade margin. Fade margin = Tx power – Tx feeder loss + Tx antenna gain – path loss + Rx antenna gain – Rx feeder loss + processing gain – Rx sensitivity. If the fade margin is positive, the path or link using that path will work. If it’s negative, it won’t. I’ve also included a goal seek routine to report the propagation distance for 0dB margin.

Rec. ITU-R P.368’s reference antenna is a short vertical monopole on perfectly conducting ground, which has a gain of 4.77 dBi. A real ground-mounted vertical will be somewhat lower gain because of ground losses. If the result is not to understate the signal by up to about 5 dB at each end, care is needed in modelling appropriate antenna gains

LFMF needs statement of surface refractivity. I’ve fixed this at 315 N-units. 315 N-units is GRWAVE’s default and the value most of NTIA’s own test cases use. Surface refractivity variation results in path loss variance of less than 1dB from 1.8MHz to 30MHz.

At short distances LFMF returns a negative path loss. I have therefore enforced a calculation limit of one wavelength.

Here’s the calculator.

Privacy and use

In providing this calculator, I assume that you are a bona fide licensed radio amateur who has a hobbyist interest in exploring electrical science. This calculator is provided to you free of charge without liability or guarantee.

When you use the ground wave path calculator, your IP address and the values you enter are sent for processing on an Amazon Web Services server in London. That server runs the LFMF-SmoothEarth program. These data are kept in system service logs for up to 30 days, to diagnose faults, and prevent abuse, and they are deleted automatically when the logs expire. Individual entries can’t be deleted sooner. My lawful basis for keeping these data is my legitimate interest in providing the calculator to you and keeping it working.

If you are unhappy about the use of your data, do not use the calculator.

Notes on parameters

Ground type

The algorithm in the calculator relies on the characteristics of the ground between transmitter and receiver. You must define the conductivity and the relative permittivity of the ground. There are two possible ways of setting these.

∎. Set one of the standard pairs of values from six in the drop-down list.
∎. Enter your own values in Siemens per metre for conductivity and a number for relative permittivity. You can find more about both in the adjacent page.

Antennas

Ground wave propagation depends a lot on antenna characteristics and the ground around the terminals. Take care in entering appropriate values for gain and height.

Environmental noise

At LF/MF/HF, noise from the environment dictates he ability to communicate. It does so by effectively degrading the receiver threshold. Noise reduces as 1/f with the greatest noise in the 160m (1.8MHz) and 80m (3.7MHz) bands, and least noise in the 10m band. An appropriate value of threshold degradation must be entered. The default is zero, but this is massively optimistic.

Receiver threshold

You should enter an appropriate receiver threshold from the receiver manufacturers data. Enter a value for 10dB S/N representing SSB speech or 0dB S/N for data. Any processing gain (when using FT8 for example) is then applied in the field for Data Processing Gain. Typically FT8 has a processing gain of 21dB over the 0dB S/N in the 2.5kHz receiver bandwidth.

Data processing gain

Modern data modes can decode signals significantly below the 2.5kHz ‘normal’ SSB noise. Here’s a diagram that explains the relative bandwidths, specifically of JT65/Q65. Together with coding and a priori decoding, this gives the data system enhancement or data processing gain.

The JT65/Q65 threshold is represented as a single value for discussion. Modes differ in their respective gains. The gain indicates the level at which data will be recovered with an ‘arbitrarily low’ [good] bit error rate. The indicated signal report (on WSJT-X or other application) is calculated from the ratio of the received signal level and the 2.5kHz bandwidth threshold. That’s typically in the range -10 to -28, and sometimes as low as -32dB.

Importantly, in the calculator, if the margin is positive, communication will be possible.