Reply to post:

"I was led to understand that AM "bends" which is why radio HAMS can make contacts and get their QSL cards from around the globe, the down-side is it's bandwidth hungry."

A big subject - but here are a few pointers from memory and Googled links. Apologies in advance for errors and omissions.

The range of Long Wave transmissions and daytime Medium Wave transmissions are limited by the extent of their ground waves beyond the line of sight.

Radio signals from about 1MHz to 30MHZ are also reflected by the ionosphere. There are considered to be several ionospheric layers - the higher the transmission frequency the higher the layer that naturally reflects it. That's why Short Wave transmissions have a potentially longer range than Medium Wave transmissions.

All the ionosphere reflections vary by day, season, and sunspot cycle. Medium Wave changes between daylight and darkness. The higher Short Wave frequencies vary by sunspot cycles.

Above 30MHZ can be considered line of sight. However exceptional atmospheric conditions can produce longer ranges by low level ionospheric reflections.

Those higher frequencies can also bend round the contours of the ground to reach further than the predicted line of sight.

Hills and buildings might cause multiple reflections that affect the received signal's quality - even a digital one. Hence the UK demonstration proof of concept of RADAR was by the flutter effect caused by an aircraft interacting with a transmission.

LW/MW/SW transmissions are narrow bandwidth - purely because the overall available space for many channels in their part of the spectrum is quite small. Therefore there is a limit to the highest audio frequency they can carry. Once you get to FM and the TV bands then the bandwidth that can be made available for each station is much larger.

AM (amplitude modulation) signals suffer distortion from noise spikes. FM (frequency modulation) is relatively immune to noise spikes until the signal has become quite weak. The wider an analogue FM station's bandwidth - the better the audio quality.

The old VHF 405 line TV pictures and sound in the UK used amplitude modulation. Both suffered when a passing car ignition system generated noise spikes. The picture signal "peak white" showed the noise as white spots.

Changing TV standards to FM for the sound - and making the AM picture signal "peak black" - alleviated those problems. Black spots are less noticeable in a picture. Car ignition systems also became better suppressed.