Advanced RFID & PIT Tag Technology

Radio Frequency Identification (RFID) is a technology that uses electromagnetic waves to automatically identify and track objects. Just as visible light allows us to see and identify objects around us, RFID uses radio waves—another form of electromagnetic radiation—to detect and read information from tags attached to objects. An RFID system consists of three main components: tags, readers, and antennas.

The tags, also known as transponders, are small devices that store data, typically a unique identification number. The reader, or interrogator, sends out electromagnetic waves through its antenna to power and communicate with the tag. The tag's antenna receives this energy and uses it to transmit its stored information back to the reader. RFID systems operate across various frequency ranges, each with unique properties affecting read range and data transfer rates. Low-frequency (LF) systems, operating around 125–134.2 kHz, are commonly used for animal tracking, including PIT tags in fisheries.

Passive Integrated Transponder (PIT) tags are a specialized type of RFID tag that are passive—they have no internal power source. They rely entirely on the energy transmitted by the reader's electromagnetic field, making them lightweight and durable. PIT tags were developed in the 1980s to provide a reliable method for long-term identification of individual animals. Advances in microelectronics and materials science have led to the miniaturization of PIT tags, enabling their use in small fish species without impacting their health. Modern PIT tags are encapsulated in biocompatible materials like glass or medical-grade polymers to prevent adverse reactions when implanted.

Operating at 134.2 kHz, a low frequency that allows effective communication even in challenging environments like water, PIT tags can be detected at ranges up to 1 meter (approximately 3 feet). This range is practical for monitoring fish movements in natural habitats.

In fisheries science, PIT tags are extensively used for monitoring and studying fish populations. They allow researchers to individually identify fish over long periods, facilitating studies on growth rates, survival, migration patterns, and habitat use. By implanting PIT tags in fish, data can be collected without the need to recapture or handle the fish repeatedly, minimizing stress and potential harm.

For example, researchers studying salmon migration install fixed detection stations along rivers to automatically record tagged fish as they pass. This data provides valuable insights into migration timing, route selection, and survival rates. Such information is crucial for understanding the impacts of environmental changes, habitat alterations, and conservation measures on fish populations.

Electromagnetism is the branch of physics that deals with the study of electric and magnetic fields and their interactions with matter. Electromagnetic waves encompass a broad spectrum, including visible light, radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays. All these are fundamentally the same phenomenon—oscillations of electric and magnetic fields—but differ in frequency and wavelength.

The electromagnetic spectrum can be thought of as a rainbow that extends beyond the colors we can see with our eyes. Visible light occupies a small portion of this spectrum, with wavelengths ranging from approximately 400 to 700 nanometers (nm). Radio waves, used in RFID systems, have much longer wavelengths, ranging from millimeters to kilometers.

In PIT systems, the reader emits electromagnetic waves that interact with the tag. The behavior of these waves is governed by Maxwell's equations, which describe how electric and magnetic fields are generated and altered by each other and by charges and currents. Faraday's Law tells us that when a magnetic field changes over time, it generates an electric field. Ampère's Law says that a changing electric field produces a magnetic field. When these two principles work together, they create self-propagating electromagnetic waves. As the electric field changes, it creates a magnetic field, and as the magnetic field changes, it creates an electric field. This process keeps repeating, allowing the wave to travel through space without needing any material to move through (like air or water). This is how electromagnetic radiation (like light, radio waves, and microwaves) can travel through the vacuum of space.

Imagine a stadium filled with people sitting in a large circle, representing points in space. Each person symbolizes the electric and magnetic fields at their location. Instead of physically moving, the people oscillate by standing and sitting, representing the changing electric field, while waving their arms side to side represents the magnetic field. As the wave passes, the electric field rises (standing) and falls (sitting), while the magnetic field oscillates perpendicularly (waving arms). The wave moves through the stadium, but the people (fields) stay in place—just as electromagnetic waves propagate through space without the physical movement of matter. The oscillating electric and magnetic fields sustain each other, allowing the wave to travel continuously through space. The frequency of the wave depends on how quickly people stand and sit (oscillate), while the wavelength relates to the spacing between them.

Electromotive Force (EMF): The voltage generated by a change in magnetic flux, which drives current in a circuit. This is what powers the PIT tag when it enters the magnetic field of the reader.

Inductance (L): A property of a coil (or inductor) that quantifies its ability to store energy in a magnetic field as current flows through it. The more turns in the coil and the larger its area, the higher the inductance.

Capacitance (C): Capacitance is the ability of a component, known as a capacitor, to store electrical energy in the form of an electric charge. It is defined as the amount of charge a capacitor can store per unit of voltage applied across its plates. The unit of capacitance is the farad (F).

Resonance: When a system oscillates at a particular frequency, known as its natural frequency. In the case of PIT tags, resonance occurs when the inductive and capacitive components of a circuit work together to maximize energy transfer.

Mutual Inductance (M): The ability of one coil to induce an EMF in another coil when their magnetic fields interact. This is the principle that allows the reader to wirelessly transfer energy to the PIT tag.

LC Circuit: A circuit consisting of an inductor (L) and a capacitor (C) that can resonate at a specific frequency, defined by the values of L and C. This is the backbone of both the reader and PIT tag’s operation.

LC circuits oscillate by transferring energy between the electric field of the capacitor and the magnetic field of the inductor. When the capacitor discharges, it sends current through the inductor, generating a magnetic field. As the magnetic field collapses, it induces current back into the capacitor, recharging it. This repeated energy transfer creates a continuous oscillation, with the frequency determined by the values of L (inductance) and C (capacitance).

The resonant frequency of an LC circuit is given by the formula:

Where:

In the LC circuit, the inductor and capacitor perform complementary functions:

• Inductor (L): Stores energy in a magnetic field when current flows through it. As the current decreases, the inductor releases the stored energy to maintain the current flow.
• Capacitor (C): Stores energy in an electric field when current charges its plates. As the capacitor discharges, it drives current through the inductor.

This interaction creates a "counterbalance" effect:

• The capacitor charges up, storing energy.
• The capacitor discharges, sending current through the inductor, creating a magnetic field.
• The inductor stores this energy, and as the current drops, it sends the energy back to the capacitor.

This back-and-forth energy exchange causes the circuit to resonate at its natural frequency. In PIT tag systems, this oscillation at 134.2 kHz is essential for efficient energy harvesting from the reader’s magnetic field.

Both the reader and the PIT tag contain LC circuits tuned to the same frequency of 134.2 kHz. When the reader emits a magnetic field, the tag’s LC circuit resonates with this field, maximizing energy transfer. This allows the tag to harvest enough energy to power its microchip, enabling it to transmit data back to the reader.

Resonance is vital in passive PIT tags because it enables the tag to function without a battery, relying entirely on energy harvested from the reader.

In RFID and PIT tag technology, the antenna coil serves as the inductor in the LC circuit. A capacitor is added to form a resonant circuit, tuned to the operating frequency of 134.2 kHz.

By adjusting the values of inductance and capacitance, the circuit can be fine-tuned to resonate at this frequency. Resonance enhances energy transfer between the reader and the tag, maximizing the current flowing through the antenna coil and ensuring effective communication.

Changing the values of inductance or capacitance alters the resonant frequency of the circuit:

• Increasing Inductance (L): Lowers the resonant frequency. A higher inductance slows the oscillation of the circuit, similar to how a heavier pendulum swings more slowly.
• Decreasing Inductance (L): Raises the resonant frequency, as the circuit oscillates more quickly with less opposition to current changes.
• Increasing Capacitance (C): Lowers the resonant frequency, as a larger capacitor takes longer to charge and discharge, slowing down the oscillations.
• Decreasing Capacitance (C): Raises the resonant frequency, allowing the circuit to oscillate faster.

Proper tuning of the LC circuit is essential for optimizing the performance of PIT tag systems. Deviations from the resonant frequency can lead to:

• Off-Resonance: Reduced energy transfer between the reader and tag.
• Reduced Read Range: Poor resonance leads to weaker signals, reducing the effective communication range.
• Signal Distortion: Mismatched resonance can cause errors in data transmission.

Maintaining precise values for L and C ensures efficient operation in the field, enabling the PIT tag to harvest enough energy and function properly.

The reader generates an alternating electrical field which passes into the antenna, subsequently creating a magnetic field, resulting in an electromagnetic field.

The LC circuit in the antenna amplifies this electromagnetic field and causes it to resonant at 134.2 kHz. The self-propagating wave, known as electromagnetic radiation, leaves the antenna and can interact with nearby conductive loops (like the PIT tag’s antenna coil).

Inductive Coupling: Once the PIT tag enters the reader’s magnetic field, it becomes inductively coupled. Because the tag has encountered an alternating magnetic field, it therefore experiences an alternating electric field (Fardays Law) across the circuitry of the PIT tag, powering it.

The circuitry of the PIT tag varies its internal impedance (electrical resistance), which because they are inductively coupled, creates tiny perturbations in the reader's electromagnetic field.

The reader detects these disruptions as slight variations in the magnetic field strength or phase and translates it into binary data (0s and 1s).

The reader’s internal circuitry decodes the modulated signal into binary data. This data is then processed, revealing the unique ID or other information stored on the PIT tag.

Modulation is the process of varying a carrier signal to transmit information. In RFID systems, modulation techniques are essential for encoding data onto the electromagnetic waves emitted by the tags and readers. Two primary communication protocols used in PIT tag systems are Full Duplex (FDX) and Half Duplex (HDX).

Full Duplex (FDX) communication allows simultaneous transmission and reception of signals. In FDX systems, the tag does not generate its own signal. Instead, it modulates the reader's magnetic field by varying its impedance, causing perturbations, typically using amplitude-shift keying (ASK) modulation in the reader's field that the reader detects and decodes. The tag relies entirely on the energy in the reader's field for communication and doesn't transmit its own electromagnetic wave. This is a passive process.

Half Duplex (HDX) communication involves alternating periods of transmission and reception. In HDX systems, the tag actively transmits its signal during the reader's listening phase. After accumulating enough power from the reader’s magnetic field (via inductive coupling), the tag generates its own signal, typically using frequency-shift keying (FSK) modulation. The reader then listens for this signal after it temporarily stops transmitting its own field. This is an active process.

Why FSK is Used in HDX:

The tag transmits at 123.2 kHz to represent a 1 and 134.2 kHz to represent a 0

Robust against noise: FSK is less sensitive to noise and interference, which makes it a reliable choice for HDX systems where the reader has to detect a signal during a quiet period.

for long distances: Since HDX tags transmit their own signal, FSK helps maintain data integrity over the longer read ranges typically associated with HDX systems.

Why ASK in FDX

The tag varies its internal impedance to change how much energy it absorbs from the reader’s field. This change in absorption results in small amplitude variations in the reader's magnetic field, which can be detected by the reader.

For example, when the tag's impedance is high, it absorbs more energy from the field, reducing the amplitude of the signal at the reader. When the impedance is low, less energy is absorbed, and the amplitude is stronger. These amplitude changes represent the binary data (0s and 1s).

ASK works well in FDX systems, where the tag is passively modulating the reader’s magnetic field without transmitting its own signal.

ASK and FSK Modulation Visualizer

Press the 0 on the key to simulate modulatiion.

• Antenna Windings: The construction of RFID antennas frequently involves the use of three loops of Litz wire to optimize inductance. Larger antennas typically require fewer windings because the overall inductance is determined by the total length of wire used. Smaller antennas need more loops to increase inductance and ensure the resonant circuit functions effectively.
• Winding Marking: During antenna construction, individual wires should be marked with tape to prevent inadvertent creation of closed loops. Post-tinning (as Litz wires are insulated to mitigate the skin effect), a continuity test should be conducted to verify appropriate connections.
• Connecting Litz Ends: To ensure robust connectivity when joining two ends of Litz wire, a ring terminal, speaker lug, or another secure connection mechanism should be employed. This mitigates the risk of poor electrical contact.

• Loop Separation: The physical separation of antenna coils can extend the duration a tag remains within the electromagnetic field, thus enhancing detection opportunities. However, this practice weakens the overall field strength and increases the risk of tag collisions, which occur when multiple tags are in the field simultaneously.
• Flat Plates vs. Passthroughs: The deployment of flat plate antennas demands considerably more time for anchoring compared to passthrough antennas, often due to their structural configuration and the need for precise placement.
• Turbulent Areas Setup: In high-turbulence environments, such as spillways, two antennas can be deployed with reversed polarity on one unit. This configuration can improve tag detection efficacy when tags are oriented suboptimally.

• CANBUS Cable: The CANBUS cable connecting the power source to the IS1001 should not exceed a length of 300 feet. In multi-reader configurations, the aggregate length of the system should be maintained below 500 feet to minimize signal degradation.
• Pigtail/Exciter Cable: The waterproof cable (commonly referred to as the "pigtail") connecting the IS1001 to the antenna must be kept under 6 feet in length to optimize performance and reduce signal loss.
• Ferrule Crimper: For all connections entering the IS1001, use of a ferrule crimper is recommended to ensure secure and reliable terminations.

• Noise Sources: Environmental noise can emanate from diverse sources, including power lines, transmission lines, and even remote submarines. Utilizing a portable loop to test environmental noise or deploying Software Defined Radios (SDR) can facilitate noise detection and assessment.
• Cable Management: Ensuring all cables are elevated off the ground is a simple yet effective strategy for mitigating noise-related issues.
• Firefly Testing Tool: The Firefly—a diagnostic tool consisting of an LED attached to a wire coiled around a ferrous core—serves as a visual aid for assessing electromagnetic fields. Unlike PIT tags, which may have diminished detection capabilities in high-noise environments, the Firefly will still illuminate, providing a realized representation of the potential detection range even in noisy conditions. This allows users to determine if poor detection is due to noise interference, and the actual detection range should subsequently be verified with PIT tags.
• Grounding Practice: Grounding is not implemented in these RFID systems—just like on naval vessels—because who needs ground loops making your life harder? Besides, if it's good enough for the Navy, it's good enough for us.

• Solar Controller Connection: When connecting a solar controller, always connect the battery first, followed by the solar panel. Many solar controllers shut off power to the load side below a set voltage—often higher than the requirements of the IS1001. To circumvent this, it is advisable to run power directly from the batteries to the IS1001.
• Solar Charge Controllers: Solar charge controllers come in two main types: Maximum Power Point Tracking (MPPT) and Pulse Width Modulation (PWM). MPPT controllers are generally superior for RFID applications. If using an MPPT controller, disable PWM modulation to prevent interference with antenna operations.
• Power Consumption: At its highest setting, the IS1001 uses approximately 1 amp per hour, which equates to about 25.2 watts per hour. This consumption rate can vary slightly depending on external factors, such as exciter volatage level (AVE) and perhphial devices (modems).
• Idle Time: You can adjust the idle time to affect the duty cycle of the antenna. A value of 750 is equivelent to 750 ms of off time. This can greatly increase the battery life while sacraficing detection speed.
• A 300W panel should be suffecient in most cases
• Daily Energy Consumption=25.2watts×24hours=604.8watt-hours (Wh)
• Daily Energy Production=300watts×5hours=1500watt-hours (Wh)

• Amps and Sensitivity: The current (amperage) directly influences the sensitivity of the antenna, enhancing both tag detection and susceptibility to environmental noise. Amperage is primarily dictated by the voltage settings (AVE 1-5) and the physical attributes of the antenna, such as shape, size, and inductance, rather than by altering capacitance.
• Current Bell Curve: When tuning (AFT, NFT), amperage behavior should resemble a bell curve. If amperage starts low, spikes suddenly, and then drops off sharply, the antenna lacks sufficient operational area under the curve, thereby impairing tuning effectiveness. Also if you get a bimodal peak, you see the amps increasing, then starting back from a lower value, the capacitors on the IS1001 are likely damaged. Replace the board.
• Capacitor Adjustment: In tuning scenarios where the antenna capacitance values fall within the low range (100-500), decrease the installed capacitance. Conversely, if capacitance values are high (500-1024), increase the capacitive load. A phase reading that remains at zero typically indicates a disconnection within the circuit.
• Detection Range and Amperage: Detection capability is significantly constrained at a current of 1.1 amps, whereas the upper operational limit is 11 amps. A smaller antenna (approximately 20 feet in diameter) should ideally operate at around 9 amps, achieving a detection range of roughly 3 feet.
• Water Intrusion: A gradual decrease in amperage over time may indicate water intrusion within the antenna assembly, warranting inspection.
• Syncing Antennas: When synchronizing multiple antennas, designate them as "master" and "secondary master." The use of "slave" settings is discouraged due to its limitations in providing consistent communication.

Recent advances in RFID technology have revolutionized fish tagging. Full‐duplex (FDX) PIT tags, which are significantly smaller than traditional HDX tags, are now the preferred choice. Their reduced size enables implantation via hypodermic needle injection—eliminating the need for surgical incisions and sutures. This minimally invasive technique not only speeds up the tagging process but also minimizes stress and potential health risks for the fish. Common implantation sites include the dorsal musculature and the body cavity, making it possible to safely tag even juvenile individuals.

Fixed antenna arrays installed in rivers and coastal environments detect tagged fish as they pass, providing essential data on migration routes and timing. Complementary mobile readers allow researchers to pinpoint fish locations in specific habitats, yielding detailed insights into habitat use and behavioral responses to changing environmental conditions.

Data collected from PIT tags is analyzed using advanced statistical models and GIS mapping to elucidate survival rates, movement patterns, and habitat preferences. This analytical framework supports fisheries management and conservation efforts—for example, by using survival estimation models to evaluate conservation measures and habitat utilization studies to inform restoration projects.

The adoption of non‐surgical, needle‐based implantation methods has considerably reduced the physical stress and health risks associated with tagging. By forgoing invasive surgery, fish recover faster and quickly resume normal activities. In parallel, ongoing innovations in RFID technology are leading to even smaller, more efficient PIT tags with improved microchip design and antenna performance. Multi‐frequency and universal reader–compatible tags are enhancing data reliability and integration across various monitoring systems.

Future research opportunities include ecosystem‐level studies that explore interspecies interactions and the broader environmental impacts on fish populations. Additionally, advancements in automated data analysis—including artificial intelligence and machine learning—promise to streamline the processing of large datasets, revealing patterns and trends that might not be evident through traditional analysis methods.

The PIT Tag Information System (PTAGIS) is a centralized database used to manage and analyze data from PIT-tagged fish. It plays a crucial role in mark–release–recapture (MRR) studies by recording key event types that help researchers assess population dynamics and survival rates. These events include:

• Mark: When a tag is first implanted in a captured fish and that fish is released.
• Recapture: When a PIT-tagged fish is recaptured, processed, and re-released.
• Observation: When a PIT-tagged fish is detected by automated detection equipment at an established interrogation site.
• Passive Recapture: When a PIT-tagged fish is detected by an ad-hoc antenna or other equipment outside of an established interrogation site, without being physically handled.
• Recovery: When a PIT-tagged fish is sampled after death or when a bare tag is recovered following release.

By compiling these events, PTAGIS facilitates robust MRR studies that enable researchers to estimate population size, survival rates, and movement patterns—critical information for effective fisheries management and conservation.