What are the differences between the 433MHz and 868MHz antennas?

What are the differences between the 433MHz and 868MHz antennas?

1. Frequency: Obviously, these two antennas are for different frequency bands, 433MHz and 868MHz, therefore, their antenna lengths are also different.

2. Wavelength: Due to the different frequency, the wavelength of these two antennas are also different. In 433MHz band, the wavelength is 69.24cm, while in 868MHz band, the wavelength is 34.54cm.

3. System requirements: Due to the difference of frequency and wavelength, the system requirements of these two antennas may also be different. For example, in 868MHz band, the wavelength is shorter, so it is more suitable for small size electronic devices in some application scenarios.

4. Design: Although both antennas are made with spline form of conductor, their designs may be slightly different due to the different wavelengths. For example, for the 868MHz band, the antenna length is shorter, so the design of their antennas needs to be more compact.

When conducting experiments with different node setups, I needed multiple antennas. I found that the information provided on the internet regarding the length of the 868MHz antenna is not accurate, so I have provided the formula for calculating the antenna length for LoRa applications in the 433MHz and 868MHz bands to facilitate a complete understanding of this information. The antenna is typically a conductor in the form of a spline and is connected to the communication module cable via a transmission line. The diameter of the antenna does not affect its effectiveness; the key is that the shape of the antenna must remain in spline form. The length of the antenna is the same as the wavelength used, usually using half or a quarter of the wavelength length. Most LoRa antennas use a 1/4 wavelength.

To calculate the wavelength of the frequency, the formula is 869v/f, where v is the transmission speed and f is the (average) transmission frequency. In a gaseous medium, the transmission velocity v is equal to the speed of light at 299792458 meters per second c. Therefore, the wavelength for the 868 MHz band is 299.792.458/868.000.000 = 34.54 cm, half of which is 17.27 cm and a quarter of which is 8.63 cm. For the 433 MHz band, the wavelength is 299.792.458/433.000.000 = 69.24 cm, half of which is 34.62 cm and a quarter of which is 17.31 cm.

This gives a wire length of 8.6 cm is required as an antenna for LoRa applications in the 868 MHz band. The exact length of the antenna is a major factor in the quality of the antenna. Unless the antenna is soldered directly to the LoRa module, any transmission line needs to be a 50 ohm cable with certified connectors to ensure signal quality.