For its size, the Rafsec 3000518 was clearly an excellent performer in terms of read distance. In some orientations, we had strong reads at 4 feet, and in other tests, we read the tag as far as 7 feet away. It does have two large null zones centered at 105 degrees and 285 degrees in the E-plane. The nulls in the radiation pattern are considerable and must be managed. In other tests, we found that performance did degrade slightly at the higher frequencies used in the United States, and barely worked at all at 953 MHz, a frequency used in some parts of Asia. Rafsec does not market this tag for Asian companies.

The Symbol I1030 had the worst overall performance, barely readable at 3 feet, and the radiation pattern (similar to the Rafsec) showed large null zones centered at 90 degrees and 270 degrees in the E-plane. It is probably using the oldest and least efficient microchip, which might account for the poor relative performance.

Alien and Avery tag results: The Alien ALL-9334, the largest tag tested, was the best performer. The read distance of the Avery AD-010 was good, not great. The E-plane of the Alien ALL-9334 (top left) has a nearly perfect radiation pattern with very small null zones; the H-plane (top right) is a perfect response. With the Avery AD-010, a unique radiation pattern shows several small null zones in the E-plane (bottom left); the H-plane (bottom right) shows one large null zone.

HF vs. UHF for Tagging Items
HF stands for high frequency, which for passive RFID systems represents a region centered at 13.56 MHz. UHF stands for ultra high frequency; for passive RFID, it's the various open bands worldwide between 860 MHz and 960 MHz. (In the United States, the UHF interrogators operate between 902 MHz and 928 MHz.) Different radio frequencies have different characteristics and different properties. Over short distances, HF systems are much more robust in that they are less affected by water, metal and densely packed tags. But after a certain distance, HF performance drops off precipitously.

UHF performance also drops off with distance, but not as rapidly. Some larger passive UHF tags can be read under ideal conditions from almost 35 feet away, for example, while HF tags are often limited to about 3 feet.

But UHF suffers from a number of problems. Highly dielectric materials (liquids) and conductors (metal), even in small amounts, can drastically change the properties of a tag antenna, reducing efficiency and shortening the read distance, sometimes to the point of becoming completely unreadable at any distance. Also, two UHF tags that are too close can interfere with each other. This is especially evident when small tags are stacked closely, such as when numerous tagged items are in a case.