High above Earth’s surface, roughly 60–1000 km up, is an intriguing part of Earth’s upper atmosphere called the ionosphere. High-energy light (mostly ultraviolet and X-rays) causes electrons to be stripped away from gas molecules and neutral atoms, forming ions (and free electrons). The incoming light is most intense at the upper edge of the atmosphere (before it is absorbed), but the density of atoms and molecules is higher at lower altitudes (with atmospheric density highest at the Earth’s surface), leading to a peak in ionization in an intermediate region. Much of the interesting action in the ionosphere is in the regions between 60–300 km up, where the electron density is highest (though still very low compared to sea level).
Under many conditions, radio waves between 160 m and 10 m (with frequencies of 1.8–30 MHz) can be refracted by the ionosphere, enabling wireless communication around the globe. This long-distance propagation is, at least to me, a wondrous phenomenon.
Effectively, there is a low-frequency limit below which the radio waves are strongly absorbed by the atmosphere. At a sufficiently high frequency, the radio waves will not refract back down to Earth, and will simply pass into space. However, in the Goldilocks zone between those two frequencies (the lowest usable frequency and maximum usable frequency), propagation can occur.
With the amount and intensity of sunlight reaching the ionosphere changing throughout the course of the day, year, and solar cycle, the maximum and lowest usable frequencies will change as well. Additionally, since not all of the globe is illuminated at the same time, these limiting frequencies will vary spatially. Consequently, the maximum usable frequency in one location may be below the lowest usable frequency somewhere else, and no radio contact can be made between those points at that time.
Scattered around the world are many, many radio stations operated by licensed amateurs (sometimes also called hams; etymology unclear). One aspect of the hobby which many amateurs enjoy is making contact with amateurs in other countries around the world. Just like birders have a life list of the species of birds they have seen, amateur radio operators often keep a list of other countries and territories they have contacted, splitting this list further by frequency and operating mode (Morse code, voice, or digital). Currently, there are 340 recognized entities worldwide. Of those 340, many are small reefs, islands, or archipelagos, and may not have any permanent population—such as Heard Island, making them very rare. The last time Heard Island was heard on amateur radio was in 1997. It’s presently the longest-inactive of the 340 entities and ranks around #5 on most-wanted lists. Many amateurs have been looking forward to this expedition for a long time, and have been very generous in supporting it financially.
On Heard Island, our team will put up several amateur radio antennas at Atlas Cove, and set up approximately 6 radios. We will then make contacts with as many stations as we can on the various amateur frequencies, in a combination of voice, Morse code, and digital modes, using the callsign VKØEK. Contacts are extremely brief which helps keep the throughput high, giving more stations a new entity for their list and us a more statistically significant sampling of the ionospheric conditions.
Here’s how a voice contact might proceed:
[VKØEK]: Victor kilo zero echo kilo, listening up
[Din of thousands of stations calling with their callsigns]
[VKØEK]: Kilo zero bravo bravo charlie, five nine4
[KØBBC]: Five nine, thanks
[VKØEK]: Thank you
It’s not a long, drawn-out conversation, but is enough to be logged on both ends as having happened. Under ideal circumstances, within a minute or two, that contact will be shown on a near-real-time map of contacts from Heard Island. With luck and the cooperation of stations around the world, we should be able to log >100,000 contacts over the three-week period and gather some very interesting data about which frequencies work to which places at which times.
Of course, one other advantage of the amateur radio operation is that it is yet another means of communication in the case of an emergency. While we hope that no emergency communications are needed of any type, and we have a number of satellite communications options, amateur radio provides one more level of redundancy, and has been shown to be reliable in places where little or no infrastructure exists (e.g. following major earthquakes, hurricanes, etc.).
The ionosphere does amazing things, and our amateur radio operation will both yield data on the ionosphere as well as make many thousands of amateur radio operators happy that they were able to contact a new entity.
*** Notes and References ***
 For a point of reference, airplanes generally fly at a height of 10–13 km, the highest jet aircraft flight record is 37.6 km, and the International Space Station is at a height of roughly 340 km; even high-altitude weather balloons and rarely exceed 40 km.
 In the US, getting an entry-level amateur radio license requires passing a 35-question multiple-choice test on terminology, regulations, basic electronic theory, and operating practices, and is roughly equivalent to a written driver’s exam. Knowledge of Morse code is not required. For more on US licensing, see this page.
 Islands and outlying territories beyond certain distances from the main entity are considered separate, so Hawaii, Alaska, Puerto Rico, and the US Virgin Islands all count as separate entities even though they are US states, territories, or possessions. The gritty details on criteria for listing as separate entities is found in section 2 here.
 “Five nine” is a signal report, meaning “I hear you loud and clear”.