# UHF transceiver for cubesats network

I already asked things related to this topic. But this time I will try to divide it by points to make it more understandable.

Features of my cubesats:

1. Would have 8 cubesats 3U in Earth's orbit
2. The cubesats will be at 850 km altitude

Communication details:

The idea is that they communicate with Earth by UHF and send it through a 75cm diameter deployable parabolic antenna via X-Band to the moon.

On the moon there will be 4 cubesats 3U orbiting that receive with a 75cm diameter deployable parabolic antenna and transmit to the surface in UHF.

The UHF antennas will be deployed from the corners of the cubesat, being able to deploy antennas up to 60cm long

The connection must reach 2Mbps at all times to receive and send commands, telemetry data and photos.

I honestly do not know the typical reception cups and I say 2Mbps because it seems reasonable to me, sorry if it is an unthinkable number for an Earth - Moon communication

My questions:

1. What power should the UHF transceiver have? and what powers the X-Band?
2. What equipment to have on board the cubesat to receive by X-Band and send by UHF? And what equipment should you have to receive by UHF and send by X-Band?
• Much better; your questions are improving. Welcome to Space! Apr 4, 2020 at 3:25
• This answer has everything you need. Why don't you try the calculation out and if you can get a result post it as an answer to your own question. It will be a great exercise for you. It's always okay to post an answer to your own question.
– uhoh
Apr 4, 2020 at 10:29
• When you try the calculation linked by @uhoh , you may start small and extent your answer stepwise. You may ask intermediate questions and remove them later. The antenna gain will be a good step to start with. If there is an error we will tell you. Just start!
– Uwe
Apr 4, 2020 at 13:01
• Nice paper about a cubesat parabolic antenna.
– Uwe
Apr 4, 2020 at 13:45
• When doing the math just insert the used frequency for calculation of the wavelength to insert into the formula for antenna gain.
– Uwe
Apr 4, 2020 at 18:17

The question asks for a design of a complete system and that's more than I can answer, however I've worked out a rough link budget calculation for your deep space X-band link between Earth orbit and Moon orbit. You can use the same math for each of your UHF links, but if you use a different kind of antenna than a dish you'll have to look up the gains for your UHF antennas elsewhere, the equation below applies only to a circular dish.

For the question of the deep space link we can do that easily by using the math expained in much more detail in this answer:

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

• $$P_{RX}$$: received power
• $$P_{TX}$$: transmitted power
• $$G_{TX}$$: Gain of transmitting antenna (compared to isotropic)
• $$L_{FS}$$: "Free space Loss" but really $$\lambda^2 / r^2$$ because of the way gain is defined
• $$G_{RX}$$: Gain of receiving antenna (compared to isotropic)

where

$$L_{FS} = 20 \times \log_{10}\left( 4 \pi \frac{R}{\lambda} \right)$$

and

$$G_{Dish} \sim 20 \times \log_{10} \left( \frac{\pi d}{\lambda} \right).$$

We have antenna diameters of 0.75 meters from the question and let's use 8 GHz as a ballpark/typical deep space X-band frequency. $$\lambda = c/f$$ gives 0.0375 meters, and that makes the gain of each antenna about 36 dB. The distance to the moon is about 4E+08 meters so $$L_{FS}$$ is about 223 dB, making $$G_{TX} - L_{FS} + G_{RX}$$ about -151 dB. That means that for every 1 W of transmit power there will be 8E-15 W of received power.

For an effective receiver temperature of say 300 Kelvin the noise equivalent power or NEP will be about $$k_B T \times \Delta f$$ where $$k_B$$ is the Boltzmann constant which is about 1.38E-23 J/K. The required bandwidth will be of the order of the bits per second though the details depend on encoding schemes and error correction outside the scope of this answer.

So with $$\Delta f$$ of 2E+06 Hz we get an NEP of a 300 K receiver front end of 8E-15 W, which is surprisingly just the same as the received power. That makes the signal to noise ratio $$S/N = 1$$ and according to the Shannon-Hartley theorem this suggests that yes indeed a bandwidth $$BW$$ of 2 MHz just barely allow your data rate of 2 Mbit/sec with only 1 Watt of transmit power!

If you use instead 10 Watts and everything else is perfect, you should be okay.

$$C = BW \ log_2 \left(1 + \frac{S}{N}\right)$$

where $$C$$ is the theoretical maximum possible data rate.

• so, if i use 10w transmitter and a 75cm diameter antenna i can reach 2MBit/s. and very thanks for the help, but if i use 50cm diameter antenna(icubesat.org/wp-content/uploads/2014/06/…)? Apr 5, 2020 at 1:33
• realy thanks for the help Apr 5, 2020 at 1:33
• @ValentinoZaffrani the closer you get to the Shannon-Hartley theoretical limit the more every other factor has to be perfect. Leave yourself some safety margin, make sure that you can switch between different levels of compression so you can maximize your data's quality as other things vary. You need to consider things like unexpected interference as well.
– uhoh
Apr 5, 2020 at 2:01
• yes, i know in my cubesat i have 30w so i can let 15 for the transiver. so if i use the 0.5m parabolic antenna linked in comments, what power i need? 15w? Apr 5, 2020 at 3:53
• @ValentinoZaffrani that kind of specific question can not be correctly answered in Stack Exchange. You have the information you need to start evaluating the tradeoffs. If you don't know what S/N ratio you need then you can't go any further until you do. The details of a telecommunications link, all the small losses challenges with signal integrity is outside the scope of Space Exploration SE. You're going to have to dig in and learn some electrical engineering and signal processing yourself.
– uhoh
Apr 5, 2020 at 13:08