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Monitoring and recording a two-way radio system was a relatively easy task until the introduction of trunked radio presented new challenges by
Reprinted with permission from January 1996, Communications magazine. Copyright 1996, Intertec Publishing Corporation, Overland Park, KS, All Rights Reserved |
| Until the introduction of
trunked radio, monitoring and recording a two-way radio system was a relatively
easy task. Adapting a multi-track or multi-channel communication recorder
to capture conventional two-way radio conversations is almost trivial -
even though the recorder's original purpose was to record multiple telephone
lines. Recording and recovering conversations from a trunked radio system
presents new challenges.
To make matters worse, trunked radio systems can also do "private" conversations which are ad-hoc two-party talk groups. The trunked system controller puts these two-party "private" talk groups together, and they exist only as long as the parties are talking. Trunked radio systems can also do radio-to-telephone interconnects. Radio-to-telephone interconnects differ from normal trunked transmissions because they typically tie up one voice channel while connected and do not trunk. Simplex Radio Simplex radio is the original, genuine, two-way radio as shown in Figure 1. Simplex radio uses the same frequency for both transmit and receive. Contrary to popular opinion, simplex radio is not dead and will not die for some time to come. Today, simplex radio is primarily used for applications that need to go beyond "line-of-sight". Simplex radio is also used for aviation and marine applications where repeaters would not be practical. Monitoring and recording transmissions on a simplex radio system are elementary. A single frequency receiver or a hard wire connection and a clock connected to a recorder will accomplish the purpose. Recovery of a radio conversation consists of locating the time period of interest on the tape and playing it. Duplex Radio Duplex radio was a later development in two-way radio. Duplex operation provides separate talk paths (frequencies) for transmit and receive. In practice, duplex radio is sub-divided into two categories: full duplex for simultaneous transmit and receive (telephone) and half-duplex, where transmit and receive occur sequentially even though there are two talk paths. While duplex radio is possible
on the lower frequencies, it typically has used only on the higher VHF
and UHF frequencies with repeaters to extend the "line-of-sight" range
of a system. A radio system that uses repeaters must be a duplex
system where the mobile radios transmit on one frequency ("up" frequency)
to a stationary repeater receiver and listen to the repeater transmitter
on another frequency ("down" frequency). The repeater is usually
located on high ground with a tall antenna as shown in Figure 2.
Monitoring and recording transmissions occurring on a half or full duplex radio system are identical to the method used for simplex radio systems. A single frequency receiver tuned to the "down" frequency (which has both sides of the conversations) and a clock connected to a recorder will accomplish the purpose. Recovery of a radio conversation consists of locating the time period of interest on the tape and playing it. How trunked systems work The name "trunked" comes from the telephone industry. A better name for trunked radio would be "computer aided radio". Many people have heard of telephone trunk lines, but they usually do not know how they differ from the phone lines that runs to their houses. The line that runs to your house is yours and yours alone, unless you are on a party line. Part of your phone bill each month is to pay rent on your line. |
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