Can you please identify some factors I'll need to consider?
Passive ultrahigh-frequency (UHF) tags and readers form a data-capture system, so issues that affect tags also affect reader performance. But for the sake of answering your question, I will focus on those issues that affect the reader. Before you deploy a UHF system, you should have a systems integrator conduct a site survey to determine any potential problems. Most can easily be resolved once properly identified. Here are some of the most common issues, in no particular order, and their solutions.
Noise from machines: In a factory or warehouse, machines can emit electromagnetic interference, or noise, that can make it difficult for a reader antenna to pick up a signal from a passive UHF tag. The solution is usually to add some shielding, such as a sheet of Mylar, around the machine to block that inference.
Interference from other RF devices: If you have an older Wi-Fi system, or a cordless phone or other device that operates at 915 MHz, these can interfere with a reader's ability to pick up a tag's signal. You should upgrade your Wi-Fi system and shield any devices emitting RF that could interfere with your readers. Depending when and where you are reading tags, you might be able to isolate the interrogators from other signals in the environment.
Multipath: This occurs when signals bounce off metal shelving, rebar in concrete floors, diamond plate or anything else. The signals can wind up canceling out those from the reader, thereby preventing tags from being interrogated. If in phase, they can also boost the read range of tags. Repositioning the antennas is often the solution. In extreme cases, you might require mats that will absorb the RF energy and prevent signals from bouncing off the floor.
Water: If you are reading tags within an area containing a lot of liquid—puddles on the floor, sprays for cleaning, and so forth—then that liquid can absorb RF energy and prevent it from reaching the tag.
Those are the major issues. Other potential problems are antennas not being aligned properly to cover the area where tags might be located, readers not being oriented correctly to interrogate tags (linear-polarized readers and dipole tags must be aligned the same way to achieve a successful read), antenna failure and staff sabotage. (Regarding the latter problem: Employees sometimes think they are being tracked, but have not been properly educated about the purpose of the RFID system. And workers sometimes just do silly things. I've heard that forklift truck drivers sometimes challenge each other to swing their vehicles close enough to a portal reader to knock an antenna off.)
—Mark Roberti, Founder and Editor, RFID Journal