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:

• We depend on no infrastructure other than our own equipment. Infrastructure in this sense may include a repeater or a centralised network configuration or, for the purists, the mains electricity supply. Practically, this means that by just by putting up an antenna and hooking it up to a radio and computer running suitable software, the resulting node can discover and join a decentralised communication network.
• We try to transmit at low power not only so that people can join in whatever the level of their licence, but also because we're interested in running our stations from battery power. Low power operation means we're interested in weak signal modes and efficient antenna systems. One way to take advantage of these weak signal modes is to connect a computer to the a radio to operate in digital modes.
• As much as possible, we want to use equipment we've made ourselves. It keeps costs low, but more importantly, by solving problems during building we learn more. On the software side, we aim to use no proprietary software in our experiments, because amateur radio technologies should be open and extendable.For example, as well as using data modes and protocols devised by other people, we'd like to extend them for our own purposes, and create new data modes of our own.

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:

• Antennas we've built so far include dipoles and inverted vees. Being horizontal and at low height, these tend to be successful for NVIS. We've had success with end-fed half-wave antennas too. In 2024, we plan to upgrade our dipole antennas with reflectors, as well as to build full-wave horizontal loop antennas for NVIS.
• Related to antennas, learning about ionospheric propagation between our home stations or other points on the surface of the Earth, as well as general solar conditions, is important to understand effective NVIS operation.
• As for radios, we're currently constructing the (tr)uSDX, a cool 5W transceiver that operates on five shortwave bands. And for power, several of us are interested in how best to power our radios for portable operation.
• Modes we use are usually JS8Call to chat, FT8 or WSPR to test propagation, or occasionally plain old voice. In the future we'd like to try packet radio (AX.25) or create our own modes for data transfer.

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