This time we discuss the remaining part of the UHF spectrum. It contains all current mobile telephone bands, bands for cordless phones and several amateur bands. This frequency range is also used for radar and satellite communication. The famous 2450MHz frequency used by microwave ovens also lies in this part of the spectrum.
Apart from radio amateurs and some really old cordless phones, there is probably nothing in this part of the spectrum that you could hear on a (conventional) wide band receiver. Many wide band receivers (like the AOR AR8600 and Icom IC-R9500) have a tuning range to 3GHz and beyond, but there is very little you can enjoy in most of that range.
In the USA the upper part of the UHF TV band (800-880MHz) was withdrawn from broadcasting in the 1980s. Part of this spectrum was used for landmobile communication (such as trunked networks), but a very large part was allocated to cellular telephone (mobile telephone). In the 1980s this was still an analog system that you could pick up with a suitable scanner. But in the USA they banned listening to these frequencies. In the early 1990s they even required scanner manufactures to remove those frequency ranges from their scanners. At first this requirement was very ineffective. It was comparatively easy to restore the restricted frequency bands (often by cutting a jumper), but it was also possible to pick up most of these channels on an image frequency. Many scanners had poor image rejection and you could hear the cellular phone conversations by tuning 42.8MHz higher (if the scanner had an IF of 21.4MHz). Requirements on scanners were gradually made tougher until analog mobile phone services had almost disappeared.
The restricted bands are 824-849MHz and 869-894MHz. As all mobile networks in the USA are now digital, those frequency range restrictions serve no useful purpose anymore. However, they make it harder to use wide band receivers as test equipment. They also make receivers harder to repair. This legislation is crap. It should have been revoked ten years ago. Somebody just has to challenge it in court.
In Europe the band between 860 and 960MHz was used mainly for cordless and mobile phones. In the late 1980s The Netherlands started its third generation mobile phone network. It used the bands 890-915MHz and 935-960MHz. This system was still analog, but it offered many more channels than its predecessor in the 460MHz band. In the early 1990s handheld mobile phones arrived and mobile phones became available to the masses. From 1995 this telephone network was gradually replaced with the digital GSM network using the same bands. See the GSM frequency bands.. These bands have been extended 10MHz down, so they are now 880-915MHz and 925-960MHz. GSM is a digital system, so an ordinary scanner cannot receive it (at least not demodulate into intelligible speech). Even though the encryption is rather weak,the required equipment to eavesdrop on GSM phones is still way out reach of hobbyists.
Europe used the analog CT1 system for cordless telephones. It used frequency bands 914-915MHz (handsets) and 959-960MHz (base stations). Later the bands 885-887MHz and 930-932MHz were added. As these bands overlap with the GSM bands, these phones may no longer be sold and in Germany it is even forbidden to use them. CT2 was the first generation digital cordless telephone system using the band between 864 and 868MHz. The USA also has cordless phones in the 900MHz band.
Between 1700 and 1900MHz there is another center of activity for mobile and cordless telephones. GSM-1800 uses the bands 1710-1785MHz and 1805-1880MHz. This is used in Europe and many other countries. The USA uses GSM-1900 (1850-1910MHz, 1930-1990MHz). Modern cordless phones use the DECT system (which is digital). This uses the frequency band 1800-1900MHz in Europe, 1920-1930MHz in the USA.
UMTS is mainly used for mobile Internet. The main frequency band in Europe is UMTS-2100 (1920-1980MHz, 2110-2170MHz). An additional band is planned around 2.6GHz.
Inmarsat satellite telephones use frequencies in the 1600MHz band. In the past they used FM modulation and could be picked up with a suitable wideband receiver and a suitable antenna. Today the system is completely digital.
Radio waves with frequencies above 1GHz (less than 30cm wavelength) are often called microwaves. Microwave bands are often designated with a letter, rather than UHF, SHF or EHF. The following microwave bands cover today's part of the spectrum. Next week we will see many more.
At 900MHz a quarter wave antenna is just over 8cm long and can easily be hidden inside a mobile phone. At the upper end of the UHF spectrum it would be 2.5cm long. There is no need to electrically shorten an antenna to make it suitable for a handheld device. High-gain Yagi antennas have a much more manageable size than on VHF. Parabolic dishes are not yet very common in this part of the spectrum, but they are in the SHF bands. The satellite systems that operate in the UHF band are usually narrow band (GPS, digital radio, satellite telephone) and do not require parabolic dishes.
Propagation in this part of the spectrum is mainly line of sight. Tropospheric propagation (beyond line of sight) is not as significant as it is in the UHF TV band. Signals in this range are not significantly attenuated by weather conditions, but they are blocked by buildings that obstruct the line of sight.
There are several specific frequencies of electromagnetic energy that are emitted or absorbed by particular gas molecules. These frequencies are called spectral lines. There are many of these in the visible light range. One of the lowest spectral line frequencies is 1420MHz and it belongs to the Hydrogen molecule. It goes without saying that this frequency is extensively monitored by radio astronomers.
The band between 960 and 1215MHz is used for aeronautical navigation, which includes radar. Most commercial and military aircraft contain transponders. When a radar transmitter sends a signal to them they send a response containing their own identification. The radar will receive a echo from any aircraft, so it can show a dot on the screen to show that there is something in the air. In addition it can receive the information from the transponder, so the radar screen can also show which aircraft it is (plus its altitude reading). Those transponders (secondary radar) operate on 1090MHz uplink and 1030MHz downlink.
GPS transmits several L-band frequencies, but the main signal picked up by most ordinary GPS receivers is on 1575MHz. If you listen to this frequency with an ordinary receiver, you hear absolutely nothing. This is a spread spectrum signal with an extremely low data rate. Each GPS satellite contains an extremely accurate atomic clock and these clocks have to be synchronized to within a few nanoseconds. The GPS signal contains the satellite's clock time and an accurate representation of its orbit. A GPS receiver receives several satellites at once (typically at least four) and derives the current position from the time difference between the signals.
This part of the spectrum contains several amateur bands. The 33cm band is only allocated in the USA and only since the 1980s. This band is ISM band in Region 2 (America), but not in Region 1 (Europe). The non-satellite parts of the 13cm band are allocated differently in the USA and in Europe (and for a change we have more spectrum in this band than the Americans).
While the 70cm band is just wide enough for a single TV channel, the 23cm band offers much more room for television. It is even possible to use FM modulated TV in this band. Analog satellite TV receivers can directly receive FM modulated TV on 23cm when connected to a suitable antenna and preamp. In Europe, the 13cm band is also widely used for television.
Some commercially made amateur equipment is available for the 23cm band (it is abundant for the 70cm, 2m, 6m and HF bands), but the other bands require home construction or modifying non-amateur equipment.
The upper UHF range contains several radio broadcast bands. The main band is the band 1452-1492MHz, mostly referred to as the L-band. This is one of the two bands used for T-DAB, Terrestrial DAB (the other one is TV band III). The intended use is local stations. The only problem is: DAB is not very well suited for local stations. DAB stations occupy a bandwidth of 1.75MHz, which is enough for 6 to 10 radio programs. A single local station with a single transmitter location and a single program cannot use DAB efficiently. This may work for large cities that have lots of local stations anyway, but it will not work in a small village with just one local radio station. The L-band can also be used for S-DAB (satellite DAB, but as far as I know, this is used nowhere in the world). The 23 European DAB channels in this band are LA through LW. Canada uses different DAB channels in this band. In the USA this band is not available for radio broadcasting.
There is a satellite radio system in the L-band. It is called WorldSpace. These satellites are mainly directed to developing countries, but they can be picked up in Europe. The vast majority of channels require a subscription. The signal can be picked up in The Netherlands without a real satellite dish, but you do need an unobstructed southward view.
In America they have XM Satellite Radio in the 2332.5-2345MHz band. This is also a subscription-based service. In urban areas it uses terrestrial (earth-bound) repeaters to provide coverage to locations without satellite coverage. The system can be used in vehicles, so it does not require the antenna to be pointed at the satellite.
The most widespread ISM (Industrial, Scientific and Medical) application of radio waves by far is the ordinary household microwave oven. There is nothing special about the 2450MHz frequency however, it is not the resonance frequency of water molecules. Liquid water absorbs microwave energy of just about any frequency. It is just that this band is an ISM band worldwide and that magnetron-based transmitters can be mass-produced very cheaply for this band.
As is often the case with ISM bands, low power secondary users are often permitted to use this band without a license. Radio amateurs may also use part of this band, but only for satellite communication, which is less likely to cause interference to local low power users. Low-power users include Wifi (wireless LAN) and Bluetooth. But some other unlicensed uses are also permitted, such as cordless phones and miniature radioctontrolled aircraft.
See you next time when we discuss the SHF band.
I added a band diagram.