Monday, December 22, 2008

Frequencies - transmitters

The World Above 30 MHz

Since VHF and UHF propagation is usually "line of sight," frequency allocations and usage are far more "localized" on frequencies above 30 MHz. However, there are some broad allocations for different purposes used in the United States and most of the rest of the Americas. The following is a summary of the main frequency bands found above 30 MHz. Please remember that listening to cellular phones, cordless phones and wireless intercoms is illegal in the United States.

30 to 50 MHz: This is known as the "VHF low" band. Most transmissions will be in narrow band FM with channels spaced at 20 kHz intervals. A wide variety of stations can be heard on this range, including businesses, federal, state, and local governments, law enforcement agencies, and various industrial radio services.

50 to 54 MHz: This is the six-meter ham radio band. The first megahertz is mainly used for USB, AM, CW, FSK modes, digital modes. The remainder of the band is used for narrow band FM, both simplex and through repeaters. 52.525 MHz is widely used as a simplex and calling frequency.

54 to 72 MHz: Television channels 2, 3, and 4 are located in this range. The video portions will sound like distorted noise on a scanner. The audio portions are in FM, but will sound "clipped" and "tinny" unless your scanner can tune this range in wide band.

72 to 76 MHz: This range is used for remote control signals for model airplanes and garage door openers, wireless microphones (including those used by law enforcement agencies), and two-way communications inside factories, warehouses, and other industrial facilities. Most channels are spaced at 20 kHz intervals.

76 to 88 MHz: This range is used for television channels 5 and 6.

88 to 108 MHz: This is where the FM broadcasting band is located.

108 to 136 MHz: This band is used for civilian aeronautical communications and all transmissions are in AM. Aeronautical beacons occupy 108 to 118 MHz; these continuously transmit a station identification and are used for navigation. The rest of the band is used for traffic between aircraft and air traffic control towers on channels spaced at 25 kHz intervals.

136 to 138 MHz: This segment is mainly used by weather satellites to transmit photographic images.

138 to 144 MHz: The various military services are the biggest users of this segment in the United States, with most transmissions in narrow band FM and spaced at 5 kHz intervals. You can also hear ham radio operators who are members of the military affiliate radio service (MARS).

144 to 148 MHz: This is the two-meter ham radio band. This is the most heavily used ham radio band in the United States. USB and various FSK modes are mainly used in the first 500 kHz, and the rest of the band is FM. Most activity is through repeaters, although simplex activity is found on frequencies like 146.52 MHz. For more information about this band, visit the ham radio section of this site.

148 to 150.8 MHz: The usage here is similar to the 138 to 144 MHz range.

150.8 to 174 MHz: This is known as the "VHF high" band, and it is used by the same wide spectrum of users as the 30 to 50 MHz band.

174 to 216 MHz: This range is used for television channels 7 through 13.

216 to 220 MHz: In the United States, this band is used by the automated maritime telecommunication system (AMTS) used on major inland waterways such as the Great Lakes and the Mississippi river. Communications are in FM on channels spaced at 12.5 kHz intervals. However, the 219 to 220 MHz range is shared with ham radio. On this range, ham stations can be used to relay digital messages to other hams, subject to a maximum power of 50 watts. Hams must first register to use their shared allocation, and cannot use it within range of maritime users.

220 to 222 MHz: This range was reallocated a few years ago from ham radio to land mobile radio. Frequency usage and modulation have not yet been finalized, although new narrow bandwidth modes are expected to be used.

222 to 225 MHz: This is the 1.25-meter ham radio band. It is mainly used for FM communication through repeaters, although it is much less heavily used than the two-meter band.

225 to 400 MHz: This very wide band is used for military aviation communications in AM. Most channels are 100 kHz apart.

400 to 406 MHz: This range is used primarily by government and military stations in FM.

406 to 420 MHz: In the United States, this band is used exclusively by the federal government. All transmissions are in FM, with most channels spaced at 25 kHz intervals.

420 to 450 MHz: This is the 70-centimeter ham radio band, second in popularity to the two-meter band on VHF/UHF. The 420 to 444 MHz range is used for USB, digital modes, ham television, and ham communications satellites. The 444 to 450 MHz range is used for FM, mainly in conjunction with repeaters.

450 to 470 MHz: This is the "UHF" band on most scanners, used for many of the same purposes as the 30 to 50 and 150.8 to 174 MHz bands.

470 to 512 MHz: This is known as the "UHF-T" band, and covers the same frequency range as television channels 14 to 20. This band is used for many of the same purposes as the "UHF" band in areas of the country without television stations on those channels.

512 to 825 MHz: This range is where television channels 21 through 72 are located.

825 to 849 MHz: This range is used for cellular telephone service, with cellular units transmitting here. Listening in this range is prohibited.

849 to 851 MHz: This band is used to provide telephone service from aircraft in flight. SSB is generally used here. Listening in this range is prohibited.

851 to 866 MHz: This is used by many of the same users as the 450 to 470 MHz band, with channels spaced at 25 kHz intervals.

866 to 869 MHz: This allocation is used by public safety and law enforcement agencies.

869 to 894 MHz: This range is used for cellular telephone service, with cells transmitting here. Listening in this range is prohibited.

894 MHz and above: These higher frequencies are where new communications technologies, such as wireless local area networks, spread spectrum telephony, and direct satellite broadcasting are being implemented.


Terminology and Specifications

CTCSS: Continuous Tone Coded Squelch System (CTCSS, sometimes called .PL. for Motorola.s .Private Line. trademark) is a method of including a low-pitched tone in the transmission which trips the squelch on the intended recipient.s receiver(s), thus excluding the undesired signals from other users sharing the frequency. Some scanners now include the CTCSS capability so the listener can choose which service to monitor on a multi-user frequency.

Dynamic Range is the ability of a scanner to handle extremely strong as well as extremely weak signals without distortion or overloading. Since most scanners do this poorly, this specification is virtually never given.

Intermodulation ("intermod') is produced in the scanner circuitry when strong signal frequencies mix together, producing spurious signals on seemingly-unrelated frequencies. It is often recognized by its distorted sound, often containing mixed audio from multiple signals. Intermod is a consequence of inadequate dynamic range.

Images are produced in every receiver. They are recognizable as a duplicate of the legitimate signal, typically offset by 21.4, 21.6, or 21.7 MHz (twice the intermediate frequency--I.F.) from the actual frequency, depending upon the scanner. With up-conversion design, image frequencies may fall outside the tuning range where they aren't a problem.

Memory: The ability of a scanner to store favorite frequencies for scanning.

S.A.M.E. (Specific Area Message Encoding) Weather Alert: Your National Weather Service will alert you of weather and other emergencies for your county if programmed.

Scanning: The rapid sampling of memory-stored frequencies to find active frequencies. When the transmission ends, the scanning cycle resumes until another active frequency is found.

Searching: An automatic sweep between an upper and a lower frequency limit in a search for activity. When an active frequency is found, the search stops until the activity ends, then resumes.

Selectivity: The radio's ability to reject adjacent frequency interference; the sharper, the better, especially in strong-signal conditions. Average selectivity for a scanner is typically 30 kHz (+/- 15 kHz) at -50 dB.

Sensitivity: The weakest signal voltage that the radio will respond to audibly; the lower the number, the better. Good sensitivity is about 0.3-0.5 microvolts.

Priority Channel: Assigning a specific memory channel to be automatically sampled every few seconds for activity regardless of other functions currently running on the scanner.

Trunking: The automatic, equal sharing by several departments of a pool of frequencies. These frequencies often change between transmissions as the number of users increase or decrease.

VFO Control: A variable tuning control to allow continuous manual tuning up and down in frequency found on some scanners.

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posted by u2r2h at Monday, December 22, 2008


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