Impact of having two antennas on link budget calculations

Question

I'm working on link budget of a cubesat mission that needs to have two dipole antennas that are perpendicular to each other. I want to know if having two antennas affect the link budget? losses, and other consideration. Thanks.

Answer 1

Wikipedia's Link budget; Equation shows:

Equation

A link budget equation including all these effects, expressed logarithmically, might look like this:

$$P_{RX} = P_{TX} + G_{TX} - L_{TX} - L_{FS} - L_{M} + G_{RX} - L_{RX}$$

where:

• $P_{RX}$ = received power (dBm)
  • $P_{TX}$ = transmitter output power (dBm)
  • $G_{TX}$ = transmitter antenna gain (dBi)
  • $L_{TX}$ = transmitter losses (coax, connectors...) (dB)
  • $L_{FS}$ = path loss, usually free space loss (dB)
  • $L_M$ = miscellaneous losses (fading margin, body loss, polarization mismatch, other losses...) (dB)
  • $G_{RX}$ = receiver antenna gain (dBi)
  • $L_{RX}$ = receiver losses (coax, connectors...) (dB)

If the spacecraft and ground station each used only a single linear polarization, then in addition to all of the other losses, the polarization mismatch loss component of the $L_M$ term would look something like

$$L_{M, Polarization} = -10 \log_{10}\left(\cos^2 \theta \right)$$

The minus sign makes the value positive, which is how the main equation wants it.

which looks like this:

where there can be deep losses at certain rotation angles. So in much of line-of-sight radio communications including links with spacecraft, circular polarization is used.

The ground station will either use pairs of crossed Yagi antennas or a dish with a crossed dipole inside the feed horn. Either way, these signals will be combined 90 degrees out of phase to provide either left hand or right hand circular polarization, which will make it insensitive to the rotation angle.

However, if your cubesat wants to receive a circularly polarized signal from Earth but it only has one dipole, there will always be a 3 dB loss because the dipole can only couple one of the polarization components. If your satellite has a crossed dipole it has the potential to recover that 3 dB loss if the two are phased properly.

If your cubesat is using a commercial RF system that has been designed to work with a pair of crossed dipoles, then it may have circuits which sense the state of the polarization and combine the two antennas correctly; it will "look" right hand or left hand polarized or linearly polarized to the cubesat depending on the specific 3D orientation (attitude) of the cube sat at each moment.

Recommendataion

1. Learn more about the RF system in the cubesat and understand how the signals from the two dipoles will be combined; passively or actively. Amend your link budget calculation accordingly.
2. In the mean time, either assume that the polarization loss is small because the RF system is smart, or throw in a fudge factor of 3 dB loss.
3. potentially helpful resource: Design, Development and Operation of a Student Ground Station
4. potentially helpful resource: Satellite communication; Construction of a remotely operated satellite ground station for low earth orbit communication

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Random examples of crossed-Yagi antennas used for cubesat ground stations

Sources: Left and Right click for full size.