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Dish replacement

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wucherer
Ex Member


Dec 20th, 2012 at 1:26am  
I am looking into making the actual dish itself more portable.  Have seen military installations where their dishes are mesh instead of the regular fiberglass or compostive material.  Any resources avaliable?  Figure with the possible hoping to another base, it would make it easier and possibly cheaper than having to throw away the dish and keep the feedhorn assembly.  I'm still looking.

Thanks.
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USN - Retired
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Reply #1 - Dec 21st, 2012 at 12:23am  
I doubt you'd want a mesh dish to be any smaller, so I'm having a hard time understanding why you think it would be any easier to ship. A mesh dish by the way, will be considerably more expensive. And the size of the openings in the mesh have to be consistent with the wavelengths it will be reflecting. Too wide for example, and the radio waves will pass right through.

//greg//
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wucherer
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Reply #2 - Dec 21st, 2012 at 6:13am  
Thank you.  As you ask why i would want to have a mesh one verses the one they give in the package?  My thought was it was almost difficult to move my dish system with me to another fob, if it wasn't for my site lead sending my personal things including the ku-band dish i had bought figured to lose at least the dish being the feedhorn assmbly and satellite modem would be easier to ship being they were small items and the dish would be a major issue.  I figured finding an easier alternative of having to lose the dish reflector, having to purchase another one to replace it and waiting for it to be shipped and deliveried.  At this point i have 3 dishes, 2 are bw ka-systems and 1 is a ku-system which i am trying to sell off to lighten to load.  Being i might return if there is no work back in the states satellite internet is my choice verses the ones already in place which at any time can be shutdown for red river.  My choice is a bit pricey for the average person but that's the choice i perfer while i am over here in afghanistan.  Thank you for giving me an explaining of the difficult of the comparing between the metal dish reflector verses the mesh version.  The cost might be about the cost of having to replace the dish plus the shipping cost of sending it here to me in afghanistan.  Mesh would, my thought simply fold up and store easily and if i had to move suddently to another fob it would be easier as i find it to setup the dishes i have already in place with me at my pod quarters and work site so i can keep in touch with my family and friends back home.
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USN - Retired
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Reply #3 - Dec 21st, 2012 at 12:12pm  
Still not clear, unless you're specifically looking for a small mesh reflector to do the job of your current reflectors. Whereas they are in fact far less common, it's also a matter of parabolic (antenna) gain versus transmitter power and receiver (LNB) sensitivity. You're going to have to ascertain the parabolic gain figures for all 3 of your reflectors, transmit and receive. You can often find that data on the manufacturer websites. Then you shop for a Ka/Ku mesh reflector that meets or exceeds the gain of your best reflector. And note that I specified Ka/Ku, as one replacement dish will have to be designed to operate in both segments of the RF spectrum. Otherwise you'll have to get two small ones; one Ka, one Ku. Short of that, you'll also need a more powerful transmitter and more sensitive LNB as well.

Now that's just theory, with a little technical jargon sprinkled in. Actually finding one will not only be difficult, it will be expensive. The most common consumer mesh reflectors are for C-band use, the smallest of which is typically a 10 foot diameter. But assuming I'm correct that portability is more important to you than mesh, you should have better luck searching for a smaller high gain solid dish. Try ViaSat, General Dynamics, Global TT. And another good search phrase to use would be "fly-away internet". Here's an example: http://www.mobilsat.com/Flyaway-transportable-satellite-internet/

//greg//
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Reply #4 - Dec 21st, 2012 at 2:25pm  
The surface accuracy of a dish, for receive only quality, should be about 1/8th wavelength.
4 GHz (C band receive only, e.g. TV)  wavelength = 300/4 = 75 mm  Surface tolerance = 9.4 mm
11 GHz (C band receive only, e.g. TV)  wavelength = 300/11 = 27 mm  Surface tolerance = 3.5 mm

The surface accuracy of a dish, for transmit gain and sidelobe quality, should be about 1/10th wavelength or better.
4/6 GHz (C band VSAT tx/rx) wavelength at 6 GHz = 300/6 = 50 mm  Surface tolerance = 5 mm
11/14 GHz (Ku band VSAT tx/rx) wavelength at 14 GHz = 300/14 = 21 mm  Surface tolerance = 2 mm
20/30 GHz (Ka band VSAT tx/rx) wavelength at 30 GHz = 300/30 = 10 mm  Surface tolerance = 1 mm

If you have a distorted dish, the gain can be much lower (e.g. 4.5 dB low) compared with what you expected. Sidelobe performance will be appalling and you will cause unacceptable interference into services on nearby satellites.

Open wire mesh dishes will work at C band for receive only TV.
At Ku band you need a perforated metal sheet.
At Ka band you need a solid metal surface.

'Plastic' dishes have a fine wire mesh just beneath the surface or a metalised film layer under the front surface. 'Plastic' dishes with wire mesh hidden inside may only work acceptably at C band.

The feed horn, the feed support arm and the dish are designed as a whole and all three parts should be kept together.  The feedhorn has a radiation pattern of its own and 'illuminates' the main reflector surface. The illumination pattern is important to obtain the best compromise of both good enough on-axis gain and good enough off-axis sidelobe performance. The feed must aim accurately at the dish so that the edge illumination, typically -16 dB down,  is the same all around, otherwise sidelobes start coming up. The system needs to be in focus with the feed phase centre at the focal point of the dish. Rely strictly on use of the manufacturers feed support arm and assembly instructions.

At Ku band, a number of manufacturers make easy-to-carry 'flight-case' antennas, in segments that clip together.  These are high precision products with accurately aligning clips. They are often made of carbon fibre and are very expensive.

Best regards, Eric.
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« Last Edit: Dec 28th, 2012 at 3:56pm by Admin1 »  
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Maxim Usatov
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Reply #5 - Dec 26th, 2012 at 8:38am  
Merry Christmas!

I thought I'd note that Eric's mention of 1/8 to 1/10 lambda means surface tolerance RMS, so all the reflector surface roughness adds up and produces noise. This means that if you are going to actively transmit to space, just using any dish that matches the gain is not going to be enough. You also need an accurate reflector shape. If you are producing too much noise then the satellite operator will ask to shut down your terminal in order to avoid service degradation for other customers. The same thing may happen if your feed has a different focal point/radiation pattern than the reflector was designed for. The best option is indeed to go with a fly away antenna from a reputable vendor. It's expensive, but it's the only way you can be sure you are not going to cause troubles on the satellite, unless you want to build your own full blown lab to test your antenna (and probably certify it with the satellite operator as it may be required.)
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