Near vertical incidence skywave

Since getting our amateur radio licences a few years ago, a small group of us living in and around Bristol have become interested in communicating with each other via radio, as well as learning and implementing about what's needed to make that possible. Most of us would say we're curious about doing new things with radio, or otherwise have a hacker mindset. Over time, several goals have emerged:

At first we tried VHF/UHF radios, but natural obstructions between us severely limited line-of-sight propagation. In order to communicate that way, we'd need to use one of the local 70cm repeaters. Even though that worked very well, we wanted to avoid the use of third-party infrastructure. Instead, we use the shortwave bands, taking advantage of the near vertical incidence skywave (NVIS) propagation path. Operating with NVIS involves transmitting the radio signal upward, where it refracts as it's passed through the ionosphere, which reflects it back down to earth for reception in an area roughly 500km across. Combined with ground wave propagation, this often means the group can reliably communicate using digital and voice modes with only five watts, no matter what natural obstructions exist between us.

Of importance is the ever-changing value of foF2, the critical frequency of the F2 layer and the highest frequency which the ionosphere will reflect vertically. Ionosondes are special radars whose purpose is to examine the ionosphere. The ionograms that they produce report the value of foF2 as well as the maximum usable frequency (MUF) for communication between stations of various distances apart. Usually, but not always, this means that we tend to use the 40m band in the UK daytime and the 80m band at night.

So far we've experimented with several parts of our setup with the aim of using the NVIS propagation path effectively:

We have impromptu hack-'n'-chats several times per month on JS8Call at 7.078MHz USB on 40m or 3.578MHz USB on 80m.