Nothing is easier.
In other words: ask Yourself what really need.
E.g. the total power radiometer do not require a sophisticate simulator.
A very simple, cascade simulator, wrote in MS Excel is often enough.
One of the key is the input power. The out voltage is:
Vout = K(Tobs+Te)B G Gamma
K : Boltzmann Constant
Tobs: the brilliance of the target
Te: Input Noise Temperature of Your Radiometer
B: Noise Bandwidth
G: overall RF gain
Gamma: the sensitivity of the square law detector
Do at least two simulation: one for Tobs=3K and the second with Tobs=the maximum expected temperature. The latter case should shows a voltage less than the non linearity knee ( say less than 5 mV).
Another key is the radiometer sensivity.
The sensivity equation (less the drift and 1/f noise) is:
DT = (Tobs+Te)/(B*tau)
DT : smallest brillance observable
B : Noise Bandwidt
Tau : post-detection time constant.
enjoy