Impact of having two antennas on link budget calculations

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.

• I think this question should be asked to engineers, for example, here community.libre.space/t/antenna-comparisons/4053 – A. Rumlin Jul 26 '19 at 17:40
• The perpendicular dipole antennas will help you avoid link budget losses due to polarization. – CourageousPotato Jul 26 '19 at 18:05
• @A.Rumlin there are currently 58 questions tagged antenna here. Questions about spacecraft antennas and link budget calculations for spacecraft are absolutely on-topic here, as are any other issues affecting design, building and communications with cubesats. – uhoh Jul 26 '19 at 21:54
• Can you give us some numbers to work with? What are the wavelength, length of the antennas, and the distance between the antennas? – DrSheldon Jul 26 '19 at 22:55
• @DrSheldon It's 436 MHz frequency (0.6876m wavelength), 17cm length of each dipole antenna, antennas are installed on the bottom face of 3U cubesat. – Behnoosh Jul 28 '19 at 13:39

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

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

Sources: Left and Right click for full size.

• Thanks for the explanation. Major challenge in the design is when two cubesats with crossed dipole want to talk to each other. So if I know the losses and blind spots of the crossed dipole, it helps me to specify the orientation of two satellites towards each other. I have considered polarization loss plus cross polarization power fraction (dB) which is around 6.02 dB (considering axial ratio of both Tx and Rx with 45 degrees of polarization angle) but I'm not sure in case of having crossed dipole, this loss would be less or not! Would appreciate your advice on this matter. – Behnoosh Jul 28 '19 at 13:57
• @Behnoosh This is a really interesting problem. What you've mentioned is more than just a comment used to clarify the answer, it's a new question. It deserves to be asked as a new question, and it should be 1) clearer and more specific about exactly what configuration you are talking about and what you want to know, and 2) probably asked in either Physics or Electronics Stack Exchange. Users on both of those sites will likely complain quickly if your question isn't clear though, so I'd recommend you try to do some calculations based on dipole radiation patterns first. – uhoh Jul 28 '19 at 14:15
• @Behnoosh each dipole on each spacecraft will have a radiation pattern roughly like this but then you have the option to phase each pair the way you like. This is now a serious engineering problem. – uhoh Jul 28 '19 at 14:16
• Do you know any software that I can use to simulate the antennas and observe the radiation pattern and their interactions? If it's an open source software it's much better. I know about HFSS but I don't have the license. – Behnoosh Jul 28 '19 at 14:21
• For myself I just use python and calculate whatever physics I need from first principles. You'll need to start doing some reading and research on your own, and then post a new question somewhere. Alternatively someone may post an additional answer here. – uhoh Jul 28 '19 at 14:24