Why is the Noise Figure (NF) measurement of a Combiner 3dB higher than the expected value?
This is neither a bug nor a problem. You would notice this behavior if you have two sources pumping noise into the combiner.
The Noise Figure/ Noise Temperature is something of a fiction. One measures the output noise spectral density of a system then this is divided by the device's gain to get an equivalent input noise power spectral density. This noise is added at input of the now noiseless system to get the observed output noise. Assume that there are two identical noise sources pumping noise into the combiner. Since the output noise is combined, there is twice the noise power at the output than each input noise source. Noise Figure/Noise Temperature measurement is defined as all the noise coming from the one input source, thus the input noise is doubled so there is a 3.01 dB increase in NF.
This is exactly the same as the situation with mixers, where there are effectively two inputs, the RF and the image. When the output noise is due only to the RF input, it results in the single-side-band noise figure. When the output noise is due to both, the RF and the image, it produces the double-side-band noise figure, which is 3.01 dB higher.
Solution for Cascaded Noise Figure (C_NF)
The default setting for the "TYPE" of the cascaded noise figure measurement is "Ideal, Circuit and Channel". This can cause the noise figure through a branched path; a splitter and combiner to be higher than expected. Changing the "TYPE" to "Operating Point, Circuit and Channel" gives the desired results.