No frequency is ideal for all applications and the availability of having two models allows the user to choose the frequency that will best fit the application.

	Rosemount 5401, Low Frequency Transmitter (~6 GHz) The lower frequency of the 5401 is ideal for applications with turbulence, heavy vapors, foam or where there is a risk for deposits on the antenna.
	Rosemount 5402, High Frequency Transmitter (~26 GHz) The higher frequency of the 5402 offers a more narrow beamwidth and thereby making it suitable for tall or narrow nozzles and also less sensitive to mounting position thus potentially eliminating costly tank modifications.

There is no visible difference between the 5401 and 5402 transmitter heads but they are not interchangeable since the antennas are different.

The frequency selection matrix can help you choose between the two models.