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Choosing uplink and downlink frequencies for 1 kg satelittes

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tempos

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About choosing uplink and downlink frequencies for CubeSat 1 kg, 1 cubic decimeter mini satellites

Why most of the cubesats use downlink frequencies around 436, 437 MHz in the UHF 70 cm band and uplink frequencies close to 144 MHz in VHF - 2 m band?
( see also the detailed explanations from here: **broken link removed** )
According to Friis formula this choice seems illogical but maybe there are good physical reasons for it.
 

Re: Choosing uplink and downlink frequencies for 1 kg sateli

tempos said:
About choosing uplink and downlink frequencies for CubeSat 1 kg, 1 cubic decimeter mini satellites

Why most of the cubesats use downlink frequencies around 436, 437 MHz in the UHF 70 cm band and uplink frequencies close to 144 MHz in VHF - 2 m band?
( see also the detailed explanations from here: **broken link removed** )
According to Friis formula this choice seems illogical but maybe there are good physical reasons for it.

not really illogical and the VERY GOOD reason for it is they are both our allocated amateur bands :)

another best reason is that we usually work full duplex we couldnt do that is we tx and rx on the same band a few 10's of kHz apart the TX would wipe out the RX and we wouldnt hear anything.

and another best reason is there aint any room on the satellites for physically huge cavity filters on VHF to keep the TX out of the RX and no other way of proving 60++ dB of isolation between a TX and RX on same band freq's
The best way to provide isolation is to use totally separate bands :)

in practice Friis formula isnt worth a lot. Amateurs are working satellites and moonbounce right up to 10GHz. It WORKS !!! :)

cheers
Dave
VK2TDN
 

Re: Choosing uplink and downlink frequencies for 1 kg sateli

davenn said:
not really illogical and the VERY GOOD reason for it is they are both our allocated amateur bands
So,
Reason 1: The bands are allocated to amateurs.
another best reason is that we usually work full duplex we couldnt do that is we tx and rx on the same band a few 10's of kHz apart the TX would wipe out the RX and we wouldnt hear anything.
Reason 2: Good separation for TX and RX bands. Good full duplex communication.
and another best reason is there aint any room on the satellites for physically huge cavity filters on VHF to keep the TX out of the RX and no other way of proving 60++ dB of isolation between a TX and RX on same band freq's
The best way to provide isolation is to use totally separate bands
Reason 3: Again, good separation between RX and TX (without using big and heavy electronic components on board the satellite) is obtained if the uplink and downlink are performed in two separate bands, VHF and UHF.
in practice Friis formula isnt worth a lot. Amateurs are working satellites and moonbounce right up to 10GHz. It WORKS !!!
Reason 4: Friis formula is not reliable.

Still not quite clear why the cubesats transmit in UHF and receive in VHF and do not do the opposite? Why they do not downlink in VHF and uplink in UHF?

Are radio amateurs more comfortable to transmit in VHF to satellites (not necessarily cubesats) and receive signals from them in UHF?
 

Re: Choosing uplink and downlink frequencies for 1 kg sateli

The main reason to have an UHF transmitter aboard of a satellite is that with a limited power and size of the antenna, the downlink signal is transmitted to Earth station.
On the Earth, we can use a high-power VHF transmitter and a big antenna to reach a satellite in orbit.
In principle, one can use the reverse frequency plan but the experience as explained above resulted in the actual frequency use.

Commercial communication satellites utilize higher frequencies to transmit many wideband channels; they also are typically larger, so they can carry larger microwave antennas and filters as well as utilize a higher power, for a typical life of >12 years.

Microsats should be economical, lightweight and still operate well; therefore, the radio amateurs' experience and equipment finds the best use.
 

Re: Choosing uplink and downlink frequencies for 1 kg sateli

tempos said:
Still not quite clear why the cubesats transmit in UHF and receive in VHF and do not do the opposite? Why they do not downlink in VHF and uplink in UHF?

Are radio amateurs more comfortable to transmit in VHF to satellites (not necessarily cubesats) and receive signals from them in UHF?

You are just looking at a single example, there is much more variation than just that. We, Amateurs, have many different up uplink and down link freqs spread throughout our VHF, UHF and microwave bands

Some of the first satellites I was working through in during the 1980's we uplinked on 145MHz and downlinked on 28MHz

have a look here for a list of uplink and down link freq's of the AO40 satellite
**broken link removed**

through telecommand from the ground station the command team could switch between modes .... ie. which sets of freq's would be uplink and downlink at any given time, and that goes for many of the satellites. Some 2m and 70cm only satellites could be switched between 2up 70down or visa-versa

Cheers
Dave
VK2TDN

one other really good reason to up on 2m and down on 70cm is that the bands are harmonically related ie. 3 x 144 = 432 MHz and transmitting on 2m is garanteed to induce additional noise into the 70cm receiver. But TX on 70cm and RX on 2m
within the satellite will not have that effect
 

Re: Choosing uplink and downlink frequencies for 1 kg sateli

The use of UHF downlinks is very strange indeed. It seems to me it's just a copy and paste design habit of many new cubesat teams. Please also consider that many teams don't have an RF specialist but rely on past designs which may not always be optimal.

Separation between the TX output and RX input is the only reason to do this.
But this is an argument of the past, as many new components are around to provide good filtering.

There are numerous advantages of using UHF (2m) uplinks and VHF downlinks (70cm), the so-called mode UV. The most important ones are:
- The IARU likes the UV mode (UHF up, VHF down), so coordination will be easier. VU mode is the recommended mode by the IARU.
- The VHF amateur band satellite segment has primary status all over the world. UHF 70cm does not.
- The downlink signal on VHF is better received than UHF (so friis formula is right!)
- The downlink signal on VHF has less doppler shift so its easier to tune
- COTS antennas and transceivers are available for mode UV
- The higher path loss on UHF is easily solved using more TX power on the ground side
- Transmitters are more simple to build on VHF (but the difference is not that great..)

An example UV mission is Delfi-C3 http://www.delfic3.nl.
If someone is planning a new cubesat mission, UV would be the preferred mode.

If there are good reasons to use mode VU, this can always be an option. Many cubesats sucessfully carried out their missions using the VU scheme, but once again, the recommendation of the IARU is still UV.

Regards,

Wouter Weggelaar
PA3WEG
 

Re: Choosing uplink and downlink frequencies for 1 kg sateli

There are numerous advantages of using UHF (2m) uplinks and VHF downlinks (70cm), the so-called mode UV.
...
If someone is planning a new cubesat mission, UV would be the preferred mode.
I have found this transceiver: **broken link removed** specially made for cubesats. It is an UV one.
Also there is an VU one, see:
**broken link removed**
which is more expensive.
 

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