When it comes to building a simple layer antenna coil to be resonant to 134.2kHz, one of the most important considerations is the number of turns. While there is no one-size-fits-all answer to this question, there are some general guidelines that can be followed to ensure the best results.

Firstly, it's important to understand what is meant by resonance. When an antenna coil is resonant at a particular frequency, it means that the coil is able to efficiently receive and transmit signals at that frequency. In other words, the coil is tuned to the frequency in question.

To achieve resonance at 134.2kHz, the antenna coil needs to be designed with a specific number of turns. The optimal number of turns will depend on a number of factors, including the diameter of the coil, the thickness of the wire used to make the coil, and the intended application for the coil.

In general, a coil with more turns will have a higher inductance and will resonate at lower frequencies. Conversely, a coil with fewer turns will have a lower inductance and will resonate at higher frequencies.

For a 4.5cm diameter coil made with 1mm insulated copper wire, a good starting point might be around 100-150 turns. However, it's important to note that the actual number of turns required may be higher or lower depending on the specifics of the design.

One way to determine the optimal number of turns is to use a tool like an LC meter or an impedance analyzer to measure the inductance of the coil at different frequencies. By experimenting with different numbers of turns and measuring the resulting inductance, it's possible to dial in the resonance frequency to the desired value.

Another consideration when building an antenna coil for RFID applications is the number of coil circles. For best results, it's often recommended to use a coil with three circles. This allows for optimal coverage and sensitivity when reading RFID tags.

In conclusion, when building a simple layer antenna coil to be resonant at 134.2kHz, it's important to consider factors such as the diameter of the coil, the thickness of the wire used, and the intended application. By experimenting with different numbers of turns and using tools to measure inductance, it's possible to dial in the optimal number of turns for the best resonance at the desired frequency. Additionally, using a coil with three circles can help to optimize coverage and sensitivity for RFID applications.