The radio spectrum
A journey into frequency macro to micro
The whole terrestrial range: up to 50 GHz
used by non-public services
mostly military, also ships, airnav, amateur radio
above 10 GHz generally point-to-point connections
Radio signals can be transmitted up to about 50 GHz. An hifi stereo signal could well fit into a 40 KHz slot. So it would be theoretically possible to transmit 1.250.000 signals simultanously in a city if there was no other use of the spectrum. The lack of frequency for radio stations is a myth. The range above 3 GHz is extremely empty. Only the marked areas have been allocated for general purpose: satellite TV around 4 and 11 GHz and the new 5 GHz Wifi band. Unfortunately freqencies above 3 GHz posess disadvantages for broadcasting. Trees and buildings will not let them pass. Consequently most communication is highly directional point-to-point from dish to dish with the ability to transmit high amounts of data at once.
The broadcast-suitable spectrum below 3 GHz
Still space for 75.000 radio signals. At least the amout of publically available frequencies has grown with the expansion of cell/mobile phones (blue). UMTS and LTE can transmit internet radio. But everyone has to request his own stream - not very efficient. Large part of the broadcast spectrum (red) is still reserved for TV. Digital radio DAB/DAB+ has overtaken the former TV band III. Still, coordination is extremely careful. You could well place 100 FM, 400 DAB and 200 TV stations in a large city via terrestric if some interference in the surrounding area would be taken in account.
So what is in between? Some ranges are reserved for rescue, police, air-traffic, ships, public transport, company communication, amateur radio, GPS but it does all not take that much. Large parts go to the military. As a matter of fact, considerable amounts of the spectrum remain widely unused.
LW/MW/SW Worldwide propagation below 30 MHz
On shortwave from 3 to 30 MHz signals can travel the world- a fundamental difference. Once there were many radio stations in the broadcast ranges. Hence audio was reduced in order to place as many signals as possible. Today the bands are abandoned. Only China, missionaries and a few other stations have stayed. Digital audio DRM offers improved sound quality. But DRM receivers have never reached the market in larger numbers. Only on the radio bands there is space for 200 AM or 400 DRM stations. Unfortunately propagation is always a little unpredictable and you must switch frequencies from time to time.
Long- and Medium Wave (known as AM) have a similar problem: Local reception during daytime and a radius of about 2000 km in the night. MW has 120 channels, each one occupied by about 50 stations worldwide. Now let's take a closer look at the single channels.
MW in focus
a closer look at 3 out of 120 channels
These are 3 adjactent channels: 3 carriers surroundes by their audio in both side bands. What cannot be seen here: Usually there is more than one carrier per channel.
One MW carrier in high resolution
+/- 10 Hz where signal addition leads to constant fading
0.764990 (-10 Hz)
0.765010 MHz (+10 Hz)
This is the frequency microcosmos: Several carriers, some of them with very specific characteristics. Many are stable over years with 0.1 Hz-accuracy. It's a challenge to identify them but you cannot hear the audio of the stations.
more about offsets
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