Why does the same model of portable radio deliver 5W on VHF yet only 4W on UHF?

Looking at the DP4000e specification sheet, one can see that on VHF (136-174MHz), the maximum transmit power is 5W. Yet, on UHF, the maximum transmit power is 4W. But why is this?

For people like me, this is just an arbitrary number. You could, in theory, make a UHF portable that went up to 10W. In fact, if you looked on Alibaba etc. I'm sure there would at least be one manufacturer who say they do this.

The reason for the lower power on UHF and above, is user safety!

The safe power limit on both bands is actually defined by the International Commission on Non-Ionizing Radiation Protection (ICNIRP for short), a non-profit organization; formally recognized by the WHO; ILO; industry and government bodies.

 

The ICNIRP's aim is to protect people and the environment against adverse effects of non-ionizing radiation by developing and distributing freely available; science-based advice on this. This organization is made up of scientists in the fields of biology; epidemiology; medicine; physics, and chemistry.

The ICNIRP Guidelines for Limiting Exposure to Electromagnetic Fields are for the protection of humans exposed to radio frequency electromagnetic fields between 100 kHz and 300 GHz. The guideline sets limits for the Specific Absorption Rate (SAR) for devices in different settings.

SAR is a measure of the rate, at which energy is absorbed, per unit mass by a human body, when exposed to a radio frequency (RF) electromagnetic field. It is measured in watts per kilogram of tissue.

The guidance provided by ICNIRP is used as a basis for many international standards and regulations. For example, EU directive 2013/35/EU references ICNIRP Guidance. In order for a two-way radio to be CE, it has to comply with this directive.

Based on the ICNIRP Guidance, 5W below and 4W above 300MHz has been determined to be within safe limits for hand-held radio transmitters. In North America, where they can use 800MHz for LMR, the safe limit at that frequency is 2,5W.

The maximum TX power on the SL4000e series is limited by what is practical with a small battery.

The maximum TX power on ATEX models is in part limited by the EU ATEX directive (more specifically by the maximum current the radio can consume).