With an HF (high frequency) radio, you can reach the entire world...if the conditions are favorable. The range of HF transmissions is affected by the ionosphere, which is like the weather: it changes constantly, it's unpredictable, and you can't do anything about it.
It depends on the codition of the sun. Is there a solar storm? Propagation peaks on eleven year cycles. The cycle is now on the downward swing. The peak was about two years ago.
The belt clipper boys with only VHF and UHF privileges are limited to very short distances, About 25 miles or so. Working through a repeater station the range can be over 100.
It is hard but not impossible to to work (talk) 100 countries in a weekend.
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The key to working distance (DX) is finding another Ham at a rare location. Lots of places are uninhabited and a Ham must go there.
The time of day and the frequency used will determine the distance covered. For example, at about 7:00AM on a frequency of 7.200 MHZ. you can work West Australians (VK6 Perth). At Noon on 14.100 you can work Europe.
(Photo courtesy American Radio Relay League)