125 kHz (LF) Antennes

Antenna types and setups will depend on your project and your application. Selecting the right antenna for the project is a very important part of the development process. A LF antenna must be resonant at the operating frequency, match the output impedance of the transmitter, and have the proper bandwidth to pass the data signals placed on the carrier wave. If the bandwidth is too narrow the data signals will be cut off. If the bandwidth is too wide though noise will be able to pass through to the reader and interfere with the reception. The optimum bandwidth defines the quality factor of the antenna.

There are two main types of LF antennas, gate antennas and stick antennas. Gate antennas will provide a more broad read pattern and are used in door ways, tabletops, portals, etc. and generally provide greater read distance than stick antennas. The size of the gate antenna does not impact the read range proportionally however will usually yield a slightly longer read range and will increase the read zone. Stick antennas have a narrow and focused read zone therefore having a greater ability to discriminate between adjacent transponders. Read ranges also depend on the reader that is used, so take great consideration as to the reader being selected.

LOW FREQUENCY ANTENNA CONSIDERATIONS

INTRODUCTION
One of the most critical elements of any RFID installation is the performance characteristic of its antenna system. The antenna is the main component for transferring energy from the transmitter to the passive RFID tags, receiving the transponder's replying signal and avoiding in-band interference from electrical noise and other nearby RFID components. Long wave radio transmitters and nearby PC monitors are common sources for disturbances.

We supply "Off-The-Shelf" antenna components that can offer sufficient performance for most applications. Custom antenna design may be required to optimize performance where the radio field must cover large areas or be focused into a local area. The application may also require a special shaped antenna that needs to be built around, or into, an existing space. Special field patterns may also be desired to avoid nearby sources of electrical noise.

ANTENNA PERFORMANCE
The reading range of an antenna is dependent on many variables. These include the quality of the earth ground connection, the antenna size, the tag size, the tag's orientation with respect to the transmitting antenna, the antenna location with respect to other materials and the ambient electrical and magnetic noise within the band of interest.

LOW FREQUENCY TRANSMISSION
TIRFID low frequency tags operate on a carrier frequency of 134.2 kHz. The "uplink" from the RFM Reader (Radio Frequency Module) to the RFID Tag Transponder is a Frequency Shift Keyed (FSK) transmission with a bandwidth of 25 kHz. The "downlink" from the RFID Tag Transponder to the RFM is an Amplitude Shift Keyed (ASK) transmission replying with identification and stored data information.

Classical radio transmission is comprised of a combination of an electrostatic field and a magnetic field component. These fields are depicted as orthogonal vectors that propagate a transverse electro-magnetic (TEM) disturbance that can be received at a distance from the source. Although Low Frequency RFID Tags are generating TEM waves their magnetic component becomes most the significant source of energy transfer for the link of the near field.

As the energy is coupled from the RFM to the antenna, magnetic flux waves extend into the space surrounding the coils. Similar to a transformer coupling energy from its primary winding to its secondary winding, the expanding field from the transmitter antenna coils can induce a voltage in a second coil in its proximity, such as an antenna coil within an RFID tag present in the field. The induced voltage in the tag's coil is utilized to charge a capacitor. Acting as a temporary battery, the capacitor then powers a chip that provides the data and intelligent protocol for transponding back to the RFM.

The ratio of turns between the RFM antenna coil and the RFID transponder coil determines the maximum voltage that can be induced by this transformer action. By increasing the turns ratio of the antennas it may increase the induced voltage linking in one direction but decreases the induced voltage linking in the opposite direction. Just adding more turns to the RFM antenna does not increase performance. In fact it may decrease the performance.

BIGGER IS NOT ALWAYS BETTER
Reading performance does not necessarily increase when using a larger antenna. Although larger loops tend to yield wider coverage areas for the transponder tags, received noise from the environment may result in obtaining a worse "Signal-To-Noise Ratio" at the receiver. A careful balance must be attained between the coverage area required and the reliability of the reception. A 6 dB difference between signal and noise levels must be maintained.

ANTENNA TYPES
Antennas come in all shapes and sizes - "one size does not fit all!". When selecting the antenna type which will optimize your performance take into account size, shape, proximity to other materials, field pattern, cost and perhaps a number of other concerns. Also, consider utilizing a multiple array of smaller antennas that may operate better than one larger antenna.

LOOP ANTENNAS
Small RFM antennas have less area than large antennas to transmit and capture energy. By increasing the number of windings of the smaller antenna more energy can be captured. However, as mentioned previously, the induced voltage at the RFID transponder is somewhat dependent on the turns ratio between transmitter and tag coils. More turns on the transmitter coil induces a stepped down voltage at the tag. Also, as the distance between the coils increase, less lines of flux are available to cut through each other. The loss of voltage due to the step-down ratio and loss of field lines can be offset by increasing the "permeability" of the core of the couplings.

Also, the number of turns of the loop is determined by the overall size of the antenna and how tightly the wires are packaged together. The optimum number of turns for maximum range is also affected by the close proximity of metal. Metals having different magnetic permeability will exhibit different achievable ranges.

STICK ANTENNAS
Ferrite rods are used to increase magnetic flux density without appreciable energy losses at the transmission frequency. The concentrated flux lines at the ends of the ferrite rod focus the field pattern. This effect tends to increase the transmission distance while in this main lobe of the beam. The field outside of this main lobe decays rapidly with distance.

Ferrite rods are utilized in both RFM antennas and within transponder tags. Examples can be seen in our P-7558 "Stick Antenna" and can also be seen in the glass tube RFID transponder series, P-7521, P-7518, etc. and our long range "Cylindrical TRP" transponder, P-7531.

RESONANCE
In order to transfer maximum energy from the RFM to its antenna system the antenna circuit (tank) should be tuned to resonate at the carrier frequency of 134.2 kHz. At resonance the capacitance of the circuit is balanced out by the inductance of the antenna coil. Antennas having a nominal inductance of 27 uH is ideal for standard low frequency RFMs. On-board variable tuning components should properly resonate antennas having from 25.5uH to 28.5uH.

QUALITY FACTOR - "Q"
dimensionless figure of merit called the "Quality Factor", or simply "Q" represents the relationship between effective impedance caused by the inductance of the coil at the frequency of transmission and the resistance of the antenna wire. The lower the resistance of the conductor - the higher goes the Q.

A high Q antenna not only transfers maximum energy at resonance, it also has a narrow bandpass limiting out-of-band interference. Keeping the resistance of the coil approximately 0.3 Ohm will yield a Q typically near 100; offering increased performance and maximum immunity to noise.

PROXIMITY WITH METALS
Altered performance of the RFID system can be expected when metals are in close proximity of the radiation field. Proximity to metals effectively lowers the antenna's inductance. Lower inductance causes an increase in resonant frequency and also a reduction in "Q". Designers may consider starting with a higher-than-needed "Q" "in the lab" to start with, expecting it to be lowered when it is installed in its intended location. External capacitance may be required to tune the modified antenna system back to resonance. These types of situations can be experienced around conveyor belt structures and also embedding antenna loops on concrete driveways where metal re-bar absorbs some of the radiated energy.

SKIN EFFECT
"Skin Effect" is the tendency of alternating currents to exist in the area of a conductor approaching the surface, rather than in the entire cross-sectional area of the conductor. At radio frequencies the moving charges in the conductor cause a self-induced magnetic field which in itself generates a counter voltage. This self-inductance is greatest at the center of the conductor and thus limits the current in that area. Electrical currents move toward the outer surface of the conductor where the counter emf is minimum.

To limit losses due to skin effect the conductors chosen should have a maximum surface area for its volume. Multi-stranded, insulated, fine wire has more surface area than does a single solid wire of the same overall gage.

ANTENNA ARRAYS
Multiple antennas may be desired to cover a larger area or to alter the polarization characteristics. Multiple loops may simply be directly connected together or "multiplexed" through intelligent antenna switches and matching networks to alter the tag detection patterns.

An analogy can be realized by connecting audio speakers to a stereo amplifier. Caution must be taken when attaching "+" and "-" leads to be sure the sound produced by each speaker is "in-phase" with each other.

If parallel antenna loops are connected "in-phase" a strong field is produced between them. This field is ideal for operating tags that are oriented parallel to the loops. If they are connected "out-of-phase", also referred to as "anti-phase", the field is rotated and is ideal for a tag oriented at right angles to the loops. Anti-phase connection is used where noise reduction by phase cancellation is required.

Consideration must be given to the effective inductance of two individual loops connected in parallel. Note: Two 54uH loops connected in parallel will have a combined inductance of 27uH.

13,56 MHz (HF) Antennes

A High Frequency Antenna for RFID is generally a device that receives an electrical energy wave from an “RFID Reader” and consequently emits a magnetic wave into space, or vice versa. These antenna devices could appear as simple as a paper clip or become as complex as a multi-element array.

The three most important aspects of the High frequency Antenna which will allow it to act as an efficient radiator in an RFID application is the following:

1.) The center frequency tuning should be resonant at 13.56 MHz. This resonance is obtained by adjusting capacitors and inductors such that one cancels the other’s effects. When the capacitive reactance equals the inductive reactance only the real, or resistive, part of the impedance remains. This phenomenon should be forced to occur only at 13.56 MHz.

2.) The output terminals from the RFID Reader are typically driven from a 50 ohm, real source impedance. The transmission cable is also usually a 50 ohm impedance cable. In order to get maximum power transfer of the energy available into the antenna circuit, and possibly prevent damage to the Reader, the input impedance of the antenna should also be adjusted to match the 50 ohm transmission system.

3.) The antenna circuit should allow the energy of the data signals, as well as the carrier signal centered at 13.56 MHz to be emitted into space. The data energy is in the form of an upper and lower sideband, centered about the 13.56 MHz carrier. These sidebands are accountable for the “Bandwidth” of the system.

SUMMARY
RFID antenna design is a complex issue. Whenever possible, try to utilize standard antenna products that have a predictable design characteristic. The environment that the proposed antenna will occupy should be looked at for sources of electrical noise, proximity to nearby materials, especially liquids and metals.

EasyLogic has a number of proven antenna products for sale. Whether you chose a stick antenna, gate loop antenna or multiple element array, we recommend seeking expert advice which will probably save you from reinventing the RFID wheel. In any case, we are here for you when you need assistance.