Each of these designs is rectangular, although circular antennas have been constructed for applications such as culverts. Antenna size depends directly on the transceiver and tag being used; antennas used with the 12.5–mm FDX–B PIT tag are normally 1.8–6 m long with an opening of 46–122 cm. However, larger tags allow for larger effective antennas.
For most of our instream monitoring projects, we have constructed antenna housings from 10.2–cm diameter PVC pipe (Downing et al. 2008). Antenna wires inside the pipe are kept dry and insulated, and this diameter provides the minimum air gap needed for optimal performance of the antenna (5 cm).
A pass–by design is typically used in streams with high gradients, dramatic changes in water depth, and high debris loads. Generally, these antennas are secured directly to the stream channel and lie in a horizontal position.
A pass–by antenna is sometimes partially buried in the substrate to reduce turbulence and scouring. Whether buried or not, a solid top can be installed on the pass–by configuration to provide rigidity and potentially affect flow over the antenna.
In slow–moving, deep–water conditions, some researchers allow the pass–by antenna to float slightly above the stream bed. This allows tags to be read both above and below the antenna.
In a normal pass–by configuration, fish are interrogated as they pass over the antenna. Debris generally passes the antenna without snagging, and this configuration can withstand high water velocities if properly anchored.
One disadvantage of the pass–by design is that vertical read range is limited (about 50 cm for the 12.5–mm SST tag). Given the orientation of tags inside a fish body, it is not likely that they will pass over the antenna in an optimal position for reading (i.e., with tag perpendicular to antenna opening).
Pass–through antennas are typically used in streams with low gradients, deep channels or pools, and minimal debris loads. This type of antenna is often attached to a bridge or other structure spanning a stream.
The pass–through antenna is installed perpendicular to water flow, in a vertical position. The bottom is attached to the stream channel, and additional support is supplied from an overhead or side bracing system. Fish are interrogated as they pass through the antenna.
This design generally optimizes tag–reading range; however, it is highly vulnerable to damage from debris. Furthermore, transceiver tuning is more affected by changes in water level under this configuration. A transceiver with dynamic tuning capability may be needed when using the pass–through configuration, even if only a single antenna is installed.
The hybrid antenna is essentially a combination of the pass–by and pass through designs. It is attached to the stream bed in a manner that allows the upstream end to pivot on its moorings while the downstream end floats freely.
Hybrid antennas are typically used in streams with low gradients, medium–to–large depth changes, and medium debris loads. One advantage of this design is that the likelihood of damage to or removal of the antenna from debris is significantly reduced in comparison to the pass–through design.
The hybrid configuration also provides better reading range than the pass–by design, since its antenna field is strongest in the area of the stream where fish typically pass. Because it adjusts automatically to changing water depths and velocities, the hybrid design allows for consistent high reading efficiency under changing stream conditions.