33-centimeter band: Wikis

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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.

Contents

History

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

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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, must accept interference from any licensed radio service with which they share frequencies.

900 MHz Wireless Networking

In the early 1990s, wireless computer networking was just becoming affordable. Several competing technologies emerged that made use of the 902-928 MHz ISM band, such as:

  • NCR/AT&T/DEC/Lucent WaveLAN. 915 MHz, 250 mW transmit power. Obsolete, but still in use in certain areas.
  • Aironet (now Cisco) ARLAN. 902-928 MHz with 12 channels, 450 mW transmit power. Obsolete, but still in use in certain areas.
  • Alvarion BreezeACCESS 900. 902-928 MHz, 200 mW transmit power. Still being sold and in use in certain areas as of 2008.
  • A now defunct commercial user of 900 MHz wireless networking was Ricochet. They used 900 MHz for the "last mile" connection to the user in an early wide area mobile wireless network.
  • Motorola Canopy is also used by WISPs depending on the Line-of-sight propagation

Walkie Talkies

MotoTalk or with Nextel, DirectTalk, uses the 900 ISM band. It uses FHSS and employs 10 "channels" and 15 "privacy codes". This is available as a feature on several of the walkie-talkie phones, for "off network" simplex communications.

Motorola makes a line of walkie-talkies (the DTR family) which are FHSS digital units. They are very similar to the DirectTalk-capable iDEN cellphones, even sharing some accessory items, but it has not been confirmed that they can interoperate with DirectTalk units.

Trisquare makes a line of walkie-talkies (the eXRS family) that are also license free FHSS units, operating in the 900 MHz band. They are similar to DirectTalk but not compatible.

Amateur Use of 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 contestant more points than if the contact was made on another band.

FM Repeaters

No amateur radio equipment manufacturer has ever made an FM repeater for the 33 centimeter band. Amateurs who wish to build a repeater and those who wish to use that repeater must do so using modified 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.

A number of resources are available for amateurs to build repeaters that can be used on the 33cm band. Some of those resources are:

http://www.repeater-builder.com/motorola/msf/msf-index.html - for use with conversion of the Motorola MSF5000 and PURC stations;
http://www.repeater-builder.com/motorola/nucleus/nuc-article.html - for use with adding analog modulation to the Motorola Nucleus paging station;
http://www.repeater-builder.com/motorola/gtx/gtx-index.html - for use with conversion of the Motorola GTX-series radio;
http://www.repeater-builder.com/maxtrac/maxtrac-900mhz-vco.html and
http://www.repeater-builder.com/maxtrac/maxtrac-900-t2c.html - for use with conversion of the Motorola MaxTrac radio.

Resources are also available via several e-mail discussion groups (such as the Yahoo Groups: "AR902MHz" and "900mhz") where information regarding conversion of other brands commonly used (such as Kenwood, EF Johnson, and GE/Ericsson) can be found/discussed.

The advent of issues involving interference to the PAVE PAWS RADAR systems located on the East and West coasts of the United States has pushed many amateur repeater operators to vacate the 70cm band in favor of 33cm, and proliferation of 33cm repeaters has understandably increased exponentially in the past few years.

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 or no circuit modification depending on the choice of radio. For instance, the Motorola model GTX, mobile and handheld versions do not require any hardware modifications at all.

However, since almost all radio equipment used on the 33 cm band amateur band was previously designed for and internally programmed for frequencies outside the 33 cm band edges, reprogramming is always necessary to get them working properly between 902-928 MHz after any physical modifications have been made and this includes the GTX.

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 signal would if transmitted on the 70 centimeter band. The reader is advised to note that receiver front-end noise figure and antenna gain are the defining factors in line-of-sight signal propagation in the local area.

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.

Popular Equipment Used on the 33 Centimeter Band

External links

Very high frequency (VHF) and ultra high frequency (UHF) amateur radio bands
VHF UHF


6 m 4 m 2 m 1.25 m 70 cm 33 cm 23 cm 13 cm
50 MHz 70 MHz 144 MHz 219–220 MHz 420 MHz 902 MHz 1.24 GHz 2.3–2.31 GHz
54 MHz 70.5 MHz 148 MHz 222–225 MHz 450 MHz 928 MHz 1.3 GHz 2.39–2.45 GHz



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