It all started with a good idea that had a problem.
In the days before FM arrived in Australia, when commercial radio stations fought it out for ratings, the engineering of the transmitter site was a big deal. Because everyone was limited to the same transmitter power (originally 2 kilowatts and later 5), there was an engineering battle to find the best site, so as to get the most coverage.
In AWA, we owned one of the two commercial stations in North Queensland, 4TO, in Townsville. Our competitor was 4AY, whose nominal home was the much smaller town of Ayr, but who regarded themselves as a Townsville station and moved the transmitter as close as they could.
4TO had a problem, they had to vacate their old transmitter site on the eastern edge of Townsville. The local manager spent much time searching for a better location, and one day he spotted an unmarked road that headed towards the coast. It turned out to be the access road to the new Australian Institute of Marine Science on Cape Cleveland. And it was in just the right place - half way between Townsville and Ayr, and right on the coast, on a peninsula where we would get the benefit of a sea water path to many of the towns they wanted to reach.
So far so good, but then the problems started. AIMS had been given buffer zone protection from real estate development, and that protection was a new National Park. So we weren't allowed in the National Park. There was some land on the peninsula outside the national park, but it was very close to a long distance radio receiving station. Which had been put there to keep it away from all sources of radio interference.
That's where I got involved. You have to think of how on earth you can persuade them to accept the unthinkable, a transmitter almost next door. We decided that the only solution was to design everything so that we could use this argument:
We know we're very close, but we've designed our transmitter to reduce the signal on your land so much that it will be less than you get now from our old transmitter.
In the end, that won the day, but we had to figure out how to design it, how to calculate what it would do, and how to measure it once we'd built it.
AM antennas are directional, and are normally designed so that maximum power goes towards your listeners, and minimum power towards other stations on your channel and towards places where nobody lives. My boss found a location where minimum power towards the receiving station would also point to where nobody lived, but we had to do more.
This is where "near-field" comes in. When you're very close, those radio fields can't add up the same way as they do further out, because with the power being transmitted from more than one mast, you are a significantly different distance from each mast.
And that's really all that I had to do. In principle, because one mast was 10% further away from the receiving station than the other, and because the signal drops in proportion to the distance, all that I had to do was make the current in the further away mast 10% bigger, and the signal from the two masts would (in theory) cancel out completely.
Of course the real world is more complicated - there are more exact ways to calculate the near-field, there was a whole area to be protected not just a single point, and the two masts were located in slightly different directions from the receiving station. But it still turned out that the simple theory was good enough, and once we'd installed it, what I measured at the receiving station was pretty much exactly what I'd predicted.
4AY wasn't at all happy that we achieved local coverage in their old home town of Ayr.
Times move on, and politics happens. In later years, 4AY moved in across the road from us, having got permission to be inside the National Park, with similar arrangements for the receiving station. Further on, AWA decided to sell all its radio stations, and 4TO was sold to 4AY, who had no need for almost identical adjacent transmitter sites, so they closed ours and now transmit both stations from their site.
And the receiving station has since closed down.
So that's why, today, you can still drive along the Bruce Highway past the AIMS turnoff and get really bad (and now needlessly bad) radio reception in your car.