RFID tags orientation issue...how to fix?
RFID coils usually line in a single plane - ie they are essentially 2D on shape but will hav 3D thickness.
The coil at the bottom of the net could be wound in 3 dimensions rather than 2 to allow it to be less &quot;null sensitive&quot;.
Or two windings could be used that are either processed separately or combined in some analog manner.
An extremely good guide to RFID aerial design is given in
Microchip application note AN710 
Antenna Circuit Design for RFID Applications

(Maintenance page here )

Also liable to be of significant assistance is
Microchip AN680 
microID® 13.56 MHz RFID System Design Guide

Two &quot;loop&quot; aerials which are essentially 2D in shape will have a very distinct null when aligned orthogonally (ie at right angles). If the fixed antenna is on a flexible substrate then simply bending it in a curve will reduce the tendency to form sharp nulls, but will also reduce the peak response when the aerials are optimally aligned. In this application optimal alignment is unlikely to be important.

For $ IEEE paper RFID high frequency 3-dimensional loop antenna analysis and design

ABSTRACT:  This paper presents an analytical model for a single folded loop antenna with detection coverage in three dimensions of space. Based on the antenna theory, the inductance and impedance of the loop antenna is investigated. The electromagnetic simulation is used to determine proper antenna topology for optimal performance. The proposed design is implemented as a RFID reader antenna on the industry inventory control and supply chain application.

Similar  Simulation and design for 3D RFID application

ABSTRACT: Based on transformer antenna theory, this paper first presents an analytical model for near-field magnetic coupling incorporating transformer inductance coil system. Formulae are derived for the self inductance and mutual inductance characteristics at the reader antenna. The paper then describes how this model and electromagnetic simulation are used for the design of a prototype three dimensional (3D) RFID antenna application as well as the results achieved.

For $ IEEE paper 3-D and package-conformal UHF RFID antennas with enhanced performance

New UHF Tag Uses 3-D Antenna

... The tags are made with Atmel's ATA5590 chips, which have 1 kilobit of read-write memory and come housed in a shock-resistant thermoplastic casing that resembles a small chocolate bar and measures 135 by 30 by 8 millimeters. The interior of the tag's injection-molded housing has been given a laser-structured metal coating to create a three-dimensional conductive structure that works as an inverted-F antenna and is assembled with an RFID chip.


Phliips app note - 2D focus mifare
®  (14443A) 13.56 MHz RFID Proximity Antenna - 2002

Questionable patent for 3D spherical field with two antennas here
Another3D patent

ADDED:
Coupling -
If the master and slave (= transmit-receive coil and tag) coils are  flat coils that are essentially 2D with some thickness, then
Optimum coupling occurs when

their planes are parallel,

they are on the same central axis and

close together.


You will get decreased coupling if you move them

further apart along the central axis (planes still parallel)
OR

if you slide one across its plane in any direction so they are not on the same axis


OR

if you rotate one around  its X and/or Y axis

OR

Any combination of the above three.

If the plane of your tag coil is almost perpendicular  to the plane of your &quot;pickup&quot; coil you have near worst case orientation.
ie the magnetic field vector is at right angles to the plane of the coil so to maximally couple the two coils the sens and receive coils need to be parallel.