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UHF links typically connect voting receivers, usually from a different frequency band, and sometimes provide a pathway to controlling remotely located transmitters. These links function as replacements for leased telephone circuits, at times to remote locations, where telephone or Internet-based circuits may require trenching or stringing aerial cable. These may also be deployed tactically through mobile systems, such as adding an ad hoc voting receiver in a command post vehicle brought to the scene, or may be used as part of a permanent cost reduction strategy.
While the economics of using UHF links are sound, they are accompanied by a number of drawbacks. These include secondary status, inefficient use of spectrum, and audio characteristic issues. Operational fixed systems are authorized under 47 CFR 90.261, which allows for use of frequencies from 450-470 MHz on a secondary basis to land mobile operations. From the coordination standpoint, a frequency coordinator may have the need to assign a UHF frequency to a land mobile system (i.e., a community may want to put up a repeater). If all other frequencies are consumed, the coordinator may select the frequency the operational fixed system functions on. The system which employs the link then becomes secondary to the new repeater system and: 1) must then not interfere with the land mobile system; and 2) must accept whatever interference the new neighbor may provide.
UHF links may consume an inordinate amount of UHF spectrum to support one conventional system, usually on another band. For example, a repeater system in a large county with 8 voting receivers may have one “in cabinet” receiver co-located at the repeater site, while 7 other receivers are linked from other parts of the county. In this configuration, the one VHF system would consume 7 different UHF frequencies to link the voting receivers back to the comparator. In some cases, the transmitter may be locked into continuous transmit with a 2175 Hz tone (Motorola/Raytheon analog voting systems) or a 1950 Hz tone (GE/Ericsson/M/A-Com/Harris systems) in between transmissions. This tone is called a “status tone” and indicates that the link is functional between the remote site and the voting comparator. When the proper CTCSS or CDCSS is detected, the status tone drops and the voice traffic is heard coming through. This mode of operation is technically disallowed unless the power level is extremely low (below 100 mW), although in practice, this is often ignored. The limitations of frequency availability often mean that UHF links are deployed outside of metropolitan areas or in places where UHF use is not as popular as other bands, such as VHF or 800 MHz.
When UHF link systems comply with the material intent of channel occupancy, they may introduce a chain of system access delays into the system. For example, the voting receiver at a remote location may be a low band mobile and a UHF mobile connected together using the CTCSS detect logic line and an audio matching network. The low band receiver detects a signal with the proper CTCSS tone, then keys the UHF transmitter, which then must be received by the UHF receiver, and sent to the comparator. These may occur rapidly, but the sum of delays may mean that a system built entirely from remote receivers configured this way clips the first few syllables of a message. The local receiver co-located with the comparator would usually provide the first opening and then, when the other receivers have activated, there may be voting for the best signal. In these configurations, UHF links are not immune from interference and may present with system-wide problems during Tropospheric ducting, particularly if the offending signal uses the same coded squelch system.
UHF links are also dependent on the audio characteristics of the link transmitter. Receiver deemphasis, transmitter preemphasis, and compression levels must be carefully examined and matched with local receivers or receivers brought through telephone lines. Sometimes equalizer cards are necessary to make the transitions from one receiver to another sound the same. In essence, the system cannot sound better than the audio characteristics of the worst audio path.
UHF links consist of a variety of equipment and may be mobile transceivers, repackaged portable transceivers with heatsinks or active cooling, or specialized original equipment manufacturer (OEM) transmitter and receiver modules. Best practice is for these links to use directional (beam/Yagi) antennas and no greater power than to reliably be received at the required site. For more elaborate systems and for microwave and satellite systems, the net gains and losses, including free-space loss, are calculated to derive a link budget, or effectively how reliable the system can be.
UHF links are usually characterized as “operational fixed” and are provided with the station class code of FXO for a point to point link, or FX2 as an in-between point repeater. There is contention in whether the O is the letter O or a zero with some interpretations in either direction. Most license applications use the letter “O.”
Other uses for FXO systems include SCADA (Supervisory Control And Data Acquisition) telemetry, control of tornado sirens, site alarms, etc. UHF links are predominately analog, although a 9.6 kbps or higher link may also be capable of linking digital communications system components.
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