How far should your groundwave signals go?
A recent club talk touched on the distance groundwave signals go. (ie how far can you be heard without bouncing signals off the ionosphere). I’m not sure how well known it is that the ITU publishes a lot of information on radio propagation. The main focus is on broadcast and telecommunication applications but there is information in many of their publications that seems likely to be of interest to amateurs.
An ITU document, rather snappily called “RECOMMENDATION ITU-R P.368-7“, covers groundwave propagation from 10kHz to 30MHz. It provides a set of curves showing how field strength declines with distance from the transmitter over reasonably flat ground and over water at different frequencies. The tables are based on vertical radiation at a power of 1kW. A 100w amateur transmitter would deliver signal levels 10dB lower of course.
As an example, Figure 7 of the document shows propagation over medium dry ground. It appears that signals in the 160m band travel about six times further than in the 10m band, which is why the nearby medium wave band was popular with broadcasters. Very Low Frequency (VLF) signals travel even further. Those at 132kHz (about 2200m) look as though they should go something like forty times further than 10m signals.
Antennas in the VLF bands tend to be very inefficient and the UK licence power limits for the two VLF bands are also low. The power limits are based on effective radiated power (erp) though, not power delivered to the antenna as with other bands, so transmitter power amplifier levels can be increased to compensate for low antenna efficiency.