Why does c-band still require huge dishes?

[daro2096]

Why is it that still to receive C-band signals you need a dish size minimum of 1.5 metres? KU band the dish can be as small as 45cm but 25 to 30 years ago you needed a 1 metre dish to receive Eutelsat II-F1 at 13 degrees east(I managed to get it with a 80cm dish in 1994 but was sparkly on some channels in good weather conditions) so on KU the signal strength has increased and dishes needed have become smaller so why hasn't C band not done the same?

 

[Captain Jack]

Main issue is that they are lower powered to cover much larger areas than Ku (global and hemi beams especially). If you're in the centre of a footprint, some can be obtained on smaller dishes - 40E can probably been had with an 80cm in the middle of its footprint.

 

[Channel Hopper]

There were reasonable powered C band broadcasts a few years back (the Gorizont satellites - 11 and 14W) where the main Russian state channel could be viewed with something as small as an 80cm dish in the UK.

The main limitation is the downlink power as CJ says, but additionally there is a limit of noise figure that can be reached with the electronics at the front end, plus the efficiency of a feed tuned to a specific frequency when coupled with a small dish.

 

[Terryl]

It also has to do with the transmit frequency used, a 3 to 5 GHz amp uses a larger wave guide(s) out to the transmit antenna(s), this takes up valuable room on the satellite, a 11 to 12 GHz amp uses much smaller wave guide(s) for that frequency, it's all about the size and weight of stuff on the satellites.

Also take into consideration the free space attenuation for those frequency's, at 22,500 miles the free space attenuation on a "C" band signal with a TX power of 10 watts (+40 dBm) and a dish that is 10 feet in diameter would be about a -114 dB.

Now using the same 10 watt power figures for "kU" band (a 10 foot dish at Ku frequency's has a gain of 48 d you get a -116.5 dB of signal loss.

So to compensate for the higher loss at the higher frequency's they use 200 watt power amps for "Ku" band, 200 watt power amps and wave guides to handle the power increase for "C" band would be a bit too heavy.

So using 200 watts for the TX side (+53 dBm) and a 1 meter dish with 38.5 dBi gain for "Ku" band you get a -113.5 dB loss, much better, and you can use a smaller dish.

You can get away with a lot at "Ku" band frequency's, bigger amps at the satellite, and smaller dishes on the ground.

For those that need to know, free space loss calculator....https://www.pasternack.com/t-calculator-fspl.aspx

 

[Huevos]

I don't understand these answers.

11.7 GHz (centre of Ku-band) has a wavelength of 2.6 cm.
3.9 GHz (centre of C-band) has a wavelength of 7.7 cm.

That is a ratio of 3:1. That means for the same dB gain on the ground a dish 3 times the size is needed.

A typical modern satellite like Astra 2E transmits on a 2.8 metre dish. To get the same amount of signal on the ground in C-band would require the satellite to carry an 8.4 metre dish. That is just not practical. This means the signal coming down from the satellite is much less directional and a lot of signal fails outside the target zone and that power is lost.

The same happens on the ground. For the C-band dish to have the same directional properties as a Ku-band dish it needs to be 3 times the size. If not it will not be as directional and will receive from a much larger segment of the sky and be much more subject to interference from adjacent satellites and other background noise such as the sun.

 

[Channel Hopper]

A reflector there times the diameter is not three times the size/area, nor does it give thr e times the gain.

A mental calculation would be a factor of 1.73 in this dimension.

 

[Huevos]

CH, I didn't say that.

You are talking about the gain at a certain frequency. i.e. if you are in Ku band and you maintain the same frequency and increase the dish size. I am talking about a different situation which is Ku versus C-band.

If you have a wavelength that is 3x longer you will need a dish 3x larger to achieve the SAME gain. I'm not talking about increasing gain. I am talking about having the identical focus as an antenna system operating at a different frequency.

 

[Huevos]

 

[Channel Hopper]

I did mention feed efficiency and smaller dish sizes earlier. Using the same beamwidth in your calculations is where the error lies

C band is not - three - times less efficient to broadcast (or receive), the wavelength has entirely different properties to Ku band, hence it has often been used to transmit over a much larger area using large continental, hemi and global beams.

The Kaur-3 spacecraft used for the Gorizont transmissions in the 80's and 90s had a reflector of no more than 1.8m, looking at the images. I also recall the specification for the spacebus mentioning 'horn antennas' though I don't have documentation to hand to confirm if this was for the 4/6GHz band

 

[Huevos]

CH, I am trying to stick to 101.

I mentioned 2 different cases...

On the satellite, dish size allows control of how big the satellite footprint will be on the ground.

On the ground, dish size allows control of how small the section of sky you are trying to isolate is.

Gone are the days of satellites spewing out high power signals in all directions. The ITU regulate that better now. But we can all reminisce.

 

[Channel Hopper]

On the ground, there is little to stop the uplink from 'spewing out high power in all directions' except by use of a larger reflector.

On the satellite, varying power of the TWTAs (plus dish shaping) determines how big the satellite footprint will be on the ground.

 

[Huevos]

On the ground, there is little to stop the uplink from 'spewing out high power in all directions' except by use of a larger reflector.

For the uplink there should be rules in force, enforced by the administration in charge of that location. e.g the GPO if you are reminiscing, But I thought the OP was asking about the dish for receiving the downlink (i.e. the only dish in the chain that he is in control of).

BTW, have you ever experimented with sound waves and a satellite dish? It is a practical way to learn about dish gain and be able to relate to it. Sound waves at the frequencies receivable by the human ear vary between 17mm and 17m. So just stick a microphone where the LNB goes and point it at different sound sources and listen to the output. This really gives good feedback how directional the dish is at higher frequencies and how inefficient it is at lower frequencies.

 

[Channel Hopper]

Yes, it's a hobby of mine (reminiscing) .

Rob Olins - Sculptor / Technical Artistry - Sat-Elite

But you have identified the one flaw, the humble reflector , revered among our brethren is not the greatest thing since sliced bread, though it does continue to serve us well.

Capturing audio in its original form is something for the birds , (the bats, people with big ears etc) not somebody with a parabolic reflector. A horn perhaps ?

 

[antrabe]

one doubt that i have its possible to increase by the sat operators the power of the sat?

 

[Terryl]

If the RF power amp for a transponder is rated at 12 watts your only going to get 12 watts, but if the power amp is rated out to a higher wattage then it may possible.

Good RF engineering practice would be to use a power amp that is rated to twice or three times the output power level required, this way the amp is not working as hard as possible to to put out the specified power levels, it would keep the heat and powe consumption down also.

 

[Huevos]

one doubt that i have its possible to increase by the sat operators the power of the sat?

Depends what you mean by operator? If you mean the broadcaster (uplink operator), then no. If you mean satellite owner/operator (e.g. Astra) then yes, but only up to the max safe output of the PA.