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It is possible to C band antenna for Ku band operation?
14th Jan, 2018 at 1:15pm
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I've just had an email query as above and thought it might help others to let you know my answer. More comment welcome please.

It is possible to use 2.4 meter dish C band antenna for Ku band operation?  How is the conversion?  Do other factor will affect the conversion such as the distance of feed to the center of dish?  size of feedhorn?

Why C band is using bigger antenna than Ku band and vice versa?  Can you explain how the wavelength affect it operation?
  

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Re: It is possible to C band antenna for Ku band operation?
Reply #1 - 14th Jan, 2018 at 2:13pm
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Dish surface and overall accuracy

You need to contact the manufacturer of the C band dish to ask if it is suitable for Ku band operation.

The quality of the surface and surface shape matters. Higher frequencies mean shorter wavelengths and thus need higher precision surfaces.

Poor quality surfaces mean lower gain and higher sidelobes and interference.
Poor quality overall shape causes the beamwidth and sidelobe patterns to be different in different directions and sometimes high sidelobes.

Many older C band white ‘plastic’ dishes will have an embedded wire mesh where the size of the mesh and its accuracy of shape is fine for C band operation, while being unsatisfactory for Ku band operation.

For Ku band operation a smooth metal film, possibly with fine perforations, is better.

For Ka band a solid metal surface is essential.

I think a surface shape accuracy of 1/10th wavelength is about right.  1/8th is acceptable while 1/16th is probably too expensive. It all depends on what you want.

Wavelengths:
5 GHz      60mm    
10 GHz    30mm
15 GHz    20mm
30 GHz    10mm

Feed systems

If a manufacturer makes a reflector suitable for multiple bands then they will also supply suitable feeds that match the F/D ratio.
Dishes with relatively short F/D ratios need small diameter feeds to spread the power over a wider angle.
Dishes with relatively long F/D ratios need wider diameter feeds to spread the power over a narrower angle

The feed will distribute power across the dish so that the power level is about –10 dB to –16 dB lower at the edge to get a good compromise of gain and sidelobe levels.

If you change the feed system from one frequency to another then you need to position the new feed system so that the phase centre (which is the apparent point source of the signals) is at the same focal point of the dish.
The phase centre may be at the aperture or down inside the horn, depending on the feed design.

The above assumes a true parabolic dish.

Large antennas, with Cassegrain (hyperbolic) or Gregorian (elliptical) sub-reflectors, tend to have modified shapes to optimise gain and sidelobes. In these cases it is essential to have the manufacturers feed designs.   

Size of antenna

C band dishes tend to be larger for two reasons:

1. At C band, traditional satellite beams have been large, covering whole countries or ocean regions, thus the power flux density down onto the earth surface tends to be low, so you need as bigger dish to receive unless an exceptionally high power downlink transmitter is used on the satellite.

2. Regardless of frequency, a dish needs to have good discrimination between on-axis gain and off-axis side-lobe gain to avoid interference to or from adjacent satellites. Since the off-axis gain of all dishes sizes is about the same (29-25log(theta) dBi) this means that to get sufficient discrimination there is a minimum gain requirement.  Gain reduces with lower frequencies so lower frequency dishes must be larger.

Spacing between satellites also matters.  There is much pressure to squeeze more satellites into the 360 deg of the geo-orbit, thus reducing satellite spacings and increasing interference unless earth station on-axis-gain versus sidelobe gain is improved.
  

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