AboutLuis Emiliani Expertise Strengths
- Satellite link design and analysis
- Vsat network design and analysis
- Propagation aspects for terrestrial and satellite links
- ITU-R P recommendations
Can not help with:
- Questions related to TV receivers, TV kits, or specific networks such as DirecTV or DISH
- Antenna pattern synthesis, RF MoM analysis, S-parameters.
Experience My experience is in satellite communication networks and terrestrial microwave links.
Expert: Luis Emiliani Date: 6/2/2008 Subject: satellite communication
Question why is uplink frequency kept larger than downlink? is it related to wavelength and directivity?
Answer Hi,
there are various reasons why today the uplink frequency is larger than the downlink, but these reasons are the consequence of the studies developed in the early days of bidirectional satellite communications
A fundamental issue in satellite payloads is that there is coupling between the transmit and receive antennas due to the limited size of the satellite platform (in some cases the transmit and receive antennas are one and the same). that means that some signals from the transmit chain will couple with the receive side . Seeing this possibility, and also seeing that the gain of a chain in the satellite is very large (around 100 dB overall from receive antenna to transmit antenna), a positive feedback of this coupled signal would be very detrimental. Hence the decision to use different frequencies for the uplink and downlink. Now, this does not explain why downlink is lower than uplink. The main reason i have been given is related to the power spectrum of noise. it was better for the downlink to use the lower frequency as that would improve its overall noise performance.
Nowadays, one can argue that other reasons exist, such as
1. allowing lower propagation losses on the satellite/earth path, where it is more difficult to compensate dynamically. This argument is losing weight with the size of spot beams today and with the advent of multiport amplifiers that enable the dynamic allocation of power among beams
2. allowing for lower free space losses on the downlink path
of course the interference issue is always there. you can not use the same frequency for transmit and receive as they would interfere each other.
