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Generally, it's a choice of the transceiver designer and overall radio system architect. In the TACAN systems, the TX/RX frequency pairs can be in either orientation (TX above RX, or vice versa), depending on what mode you're in, and what channel you're on.
It's not a hard and fast rule that RX must be above TX, just a common practice. The background on why that is the case has probably been lost to the ages...
In the old ages, say for mobile user, if the mobile user use higher freq, for the same antenna, the user can get higher gain. That is important for limited dimension and battery of the user mobile.
The frequency separation in using a higher frequency for uplink and a lower one for down link is historical in origin.
The reason was that on Earth it has been easier to generate a high power density while generating RF power aboard a geostationary satellite is dependent on solar power and antenna size, among other conditions.
Today with better technology a satellite system can be designed differently, so both directions can even share the same frequency. Satellite transponders originated with the "bent pipe" concept now operate in a digital mode and can receive and transmit with time division as well.
Power Consumption is proportional to frequency of use higher the freq power required to drive the radiation is more. Thus in cell phones mobile to BTS freq is low and BTS to mobile is high. Whereas in SATCOMM satellite to station is low and station to sat is high freq.
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