From my mid-childhood on, I've been fascinated by the possibilities of shortwave. With no infrastructure like satellites or telephone lines in between, thousands of miles can be bridged, using just God's creation in between. The best I've heard up to now was a double echo - the signal of Deutsche Welle going twice around the globe...
To give you a taste, I've collected some information:
Such distances can only be covered, because a layer in the upper atmosphere (about 60-400 kilometers high) is ionised (hence the name Ionosphere) by X-rays coming from the sun. This means, that electrons are rushing around freely without being bound to specific atoms, like in a metal, and so the air can conduct electricity and so it can reflect radio waves - whatever goes up from the surface is being reflected back to quite distant places. Considering the diameter of the earth and the maximum height of the ionosphere (the F2 layer at 400 km = 250 miles), the maximum distance that can be covered by such a hop is about 4000 kilometers (2500 miles). But the surface of the earth, especially salted water of the oceans, also reflects radio waves quite well (that's why radar works) and so by bouncing up and down, the globe is eventually covered.
Unfortunately, things aren't quite that simple. The capability of the ionosphere to reflect radio waves is limited by two factors:
Which frequencies are "low" or "high" depends on several factors:
Of course that depends on the hardware. The best starting point for web links I found that covers the topic is the Shortwave Listening Resource Center of Thomas Hood (amateur radio callsign NW7US). The feature I like most is the Database Lookup, where you can enter a frequency and get a list of stations broadcasting on this frequancy, highligthing the current time of the day. Thomas gets his data from the High Frequency Co-ordination Conference.
If you don't live in a concrete shelter (unfortunately many modern cities consist of those) you can expect such a simple radio to pick up many regional (up to 500 or 1000 km away) and the major international broadcasters from up to 4000 kilometers away, which here in central Europe means BBC London, Deutsche Welle, Radio France, Radio Moscow, Voice of America, Kol Israel and others who have relays in or near Europe. If you are lucky, you can catch strong intercontinental signals such as HCJB (Ecuador, South America), Radio China International or Radio Australia (yes, they sometimes come really strong in here, despite [or perhaps because?] that's halfway round the globe). If the tuning is not digital, you will need fine fingers not to quickly jump over interesting stations.
Cheap (less than 50 or 100 US$) digital world receivers look nice, but can't do much more. If you have the fingers for it, better spend the money on a good analog device. Apparently, old fashioned tube radios have remarkable qualities on shortwave - don't throw away such a thing!
like the better SONY models (I use a ICF 2001-D, in March 1991 it was about 600 US$ in an entertainment electronics superstore in downtown Munich - I have no relationship with SONY other than being a satisfied user of their products), you can expect to hear, using the built in telescope antenne or a few meters of wire out of the window, most international broadcasters from all over the world, like Radio Nacional do Brasil, Radio New Zealand (only during good conditions), many stations from the arabic world, Korea (both north and south), Vietnam, India, South Africa, etc. etc. If you are lucky, you can catch major regional broadcasters from neigbouring continents, such as the domestic services of Ghana, Nigeria, Botswana, Tadzhikistan (did I spell it right?), sometimes even Columbia, etc.
To get into this category of receivers, you have to spend at least 200 US$ (on the German market, might be cheaper in the US or Far East). Price equals value pretty much, so if you have compared the prices of different dealers, you can assume to get what you pay for.
This means 2-way communication, so you need a licence for it, but it's still amazing to know what's possible, even if you can only listen. If you are o.k. with morse code (in most countries you have to be, in order to get the licence), then chances are much better, because it easier to distinguish a beep from no beep, than to understand a spoken voice [in a foreign language with odd accent].
With 5 watts and a simple ground plane antenna (3.5 metres rod up plus a
few wires down) on top of the house, I got well around inside Europe and a
few times I reached the United States - people didn't believe I had only that
With 10 watts and a 120 meter long wire antenna, I saw others talking (voice) around southern Germany, with the same transceiver and a 3 element (8x8 meters fork-like) beam antenna we teletyped from Germany to Venezuela.
I heard someone in a car on his special cliff on a caribbean island using 50 watts and a car antenna (typically a 3 meter rod) talking with europeans.
1000 Watts and 3 element (8x8 meters fork-like) beam antenna is the typical equipment of a high end amateur, which enables daily voice contact to most places of the world - often enough at the price of neighbours getting fourious, because they hear that fellow squeaking out of their TV set or so...
Besides hobby enthusiasts, who listen in just for the fun of hearing far away stations, I know of (or can imagine) the following groups of people using shortwave technology:
Disclaimer: all these links lead to other people's websites, which I can not take resposibility for. The links are provided for your convenience and shall not be construed to mean any type of endorsement.
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