33 centimeters - Definition and Overview

33 Centimeters is an amateur radio band. It ranges from 902 to 928 MHz and is unique to ITU Region 2. The 33 centimeter band is part of the UHF spectrum, and is primarily used for very local communications as opposed to bands lower in frequency.

History

The 33 centimeter band has a somewhat short history, being one of the newest amateur radio bands.

The Beginning

In 1985, the Federal Communications Commission allocated the frequency band between 902 and 928 MHz to ISM (Industrial, Scientific, and Medical) devices. In that proceeding, the band was also allocated to the Amateur Radio Service on a secondary basis meaning amateurs could use the band as long as they accepted interference from and did not cause interference to primary users.

900 MHz Cordless Phones

In the mid 1990s, many cordless phone manufacturers started producing phones that used the lower and upper ends of the 33 centimeter band for communication between the handset and base. These phones, which are regulated by Part 15 of the FCC's regulations, have made amateur use of the upper and lower end of the 33 centimeter band somewhat tricky due to the amount of these phones being used by the general population. Part 15 devices, by law, have to accept interference from any licensed radio service with which they share frequencies.

The 33 Centimeter Band Today

Today, the 33 centimeter band is rapidly becoming popular with many UHF enthusiasts. Currently, it is used by amateurs for a variety of purposes.

Amateur Television

Amateur television is arguably the most popular activity on the 33 centimeter band with some manufacturers actually producing ATV equipment for the band.

CW and Single Sideband

Amateurs who are involved in contesting use home-made or commercially available transverters to operate CW and SSB on the lower end of the band, usually just above 903 MHz. Depending on the contest, a contact made on the 33 centimeter band can get the contester more points than if the contact was made on another band.

FM Repeaters

Curiously no amateur radio equipment manufacturer has ever made an FM radio for the 33 centimeter band. Amateurs who wish to build a repeater and those who wish to use that repeater must do so using commercial equipment designed for use in the mid-800 MHz and mid-900 MHz range.

Building Repeaters

For many years, repeaters on the 33 centimeter band used a split of -12 MHz with inputs between 906 and 909 MHz and outputs between 918 and 921 MHz. Today, many new repeaters are using a split of -25 MHz with inputs between 902 and 903 MHz and outputs between 927 and 928 MHz. The reason behind this is that the selection of equipment that can be modified for a -12 MHz split is mainly limited to commercial repeaters and data radios which tend to be older, more expensive, harder to maintain, harder to find parts for, and very time consuming to modify.

With the explosion in popularity of Nextel phones with a push to talk feature, the marketplace has seen a flood of newer 800 and 900 MHz commercial mobile radios that are designed to the following specifications:

• 800 MHz Radios:
  • Transmit: 806 - 821 MHz and 851 - 870 MHz
  • Receive: 851 - 870 MHz
• 900 MHz Radios:
  • Transmit: 896 - 902 MHz and 935 - 941 MHz
  • Receive: 935 - 941 MHz

The receivers on many of these modern 800 MHz radios can be easily modified to receive higher than 870 MHz, to about 904 MHz with good sensitivity. In addition, the transmitters on many of the aforementioned 900 MHz radios can be easily modified to transmit lower than 935 MHz, to about 926 MHz with acceptable power output. With this in mind, many amateurs have opted to set up repeaters with -25 MHz splits using modified 800 MHz radios as receivers and modified 900 MHz radios as transmitters.

Using Repeaters

Amateurs who use local repeaters on the 33 centimeter band usually use commercial handheld or mobile 900 MHz radios. As shown above, these radios can transmit between 896 and 902 MHz and receive between 935 and 941 MHz. Getting many of these radios to transmit on the repeater's input frequency (between 902 and 903 MHz) and receive on the repeater's output frequency (between 927 and 928 MHz) usually requires very little modification.

Linking

Many amateurs have found the 33 centimeter band to be ideal for linking repeaters together. Some of the biggest linked repeater systems in the United States use the 33 centimeter band as their link backbone.

Propagation Characteristics

Signal propagation on the 33 centimeter band is very dependent on the transmitting and receiving antenna's line of sight. Because of this, many wide-area coverage systems like repeaters are located on top of large hills and mountains which overlook a vast area. This ensures that the transmitting antenna is higher than terrestrial obstructions such as trees and buildings. Assuming that the transmitting antenna's wavelength, height above average terrain, and effective radiated power is equal, a transmitted signal on 33 centimeters will, generally speaking, usually travel about 3/4 of the distance that the same singal would if transmitted on the 70 centimeter band.

The 33 centimeter band offers excellent building penetration characteristics since the wavelength is relatively small and can fit through windows easier than signals lower in frequency.

In many areas, the 33 centimeter band also has a very low noise floor as compared to bands lower in frequency.

Band Plan

While the national band plan as well as many State band plans call for a -12 MHz split for FM repeaters, an increasing number of States are changing their band plans to reflect the growing number of FM repeaters using -25 MHz splits. Many amateurs feel that eventually the -25 MHz split will become commonplace and all State and national band plans will adopt it. Below is an example of a more modern 33 centimeter band plan that allocates a -25 MHz repeater split while also grandfathering existing repeaters with a -12 MHz split. This band plan is roughly based on the one developed by the New England Spectrum Management Council (http://www.nesmc.org).

Sample Band Plan

Frequencies of Note

• CW and SSB calling frequency is 903.100 MHz.
• FM simplex calling frequency is 927.500 MHz.

Popular Equipment Used on the 33 Centimeter Band

Below is a list of popular commercial equipment frequently used by amateurs on the 33 centimeter band:

• Ericsson MDX
• GE Mastr II
• GE Mastr III
• Kenwood TK-481 (http://www.kenwood.net/indexKenwood.cfm?do=ProductDetails&ProdID=46&Group=1)
• Kenwood TK-980 (http://www.kenwood.net/indexKenwood.cfm?do=ProductDetails&ProdID=60&Group=1)
• Kenwood TK-981 (http://www.kenwood.net/indexKenwood.cfm?do=ProductDetails&ProdID=29&Group=1)
• Motorola MaxTrac
• Motorola Spectra
• Motorola GTX (http://www.motorola.com/cgiss/mobiles/gtx_mobile.shtml)
• Motorola MTX
• TAIT T800 Series (http://www.taitworld.com/main/index.cfm/10,0,30,html)

External Links

AR902MHz Yahoo! Group (http://groups.yahoo.com/group/AR902MHz)