Because most single antennas for FDX systems are not large enough to span an entire stream, instream monitoring applications nearly always require multiple antennas. In early stream monitoring systems, the only option for achieving sufficient directionality and redundancy was to use multiple transceivers, since each transceiver connected to a single antenna.
This was impractical for many applications, since additional transceivers increased the cost, complexity, and power requirements of the system—often to a point where installation of multiple transceiver systems was limited to sites with grid power.
Early model transceivers also suffered from regular detuning, even in small streams where a single antenna could be used. Detuning was even more pronounced when larger antennas were used, and it significantly deteriorated tag–reading range.
This combined problem of detuning and the need for multiple transceivers made the use of instream PIT–tag monitoring systems impractical for most research applications.
To broaden the availability of these systems, the National Marine Fisheries Service contracted a private engineering firm to co–develop the FS1001M transceiver (Downing et al. 2004).
Since its development, the FS1001M transceiver has been instrumental in expanding access to instream PIT–tag monitoring systems (Connolly et al. 2005, 2008; Zydlewski et al. 2006). The FS1001M transceiver:
One disadvantage of the multiplexing transceiver is that it lacks a backup system. If the transceiver fails, no data will be collected until it is repaired or replaced.
A second disadvantage is that antennas are activated in sequence. This means that a tagged fish could be missed if it passed an antenna during the inactive cycle.
There is a maximum inactive cycle of 500–msec for each antenna in a typical 6–antenna setup (often the cycle is only 150–200 msec). Fortunately, since most fish swim slowly in streams, the chance of a fish passing the entire electromagnetic field during the inactive cycle is small. Even more unlikely is the chance of a tagged fish passing multiple antennas during each of their respective inactive cycles.