Mesh dish and Cband
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C-band should be really close. I know PSB has a 6 foot mesh dish and his numbers are comparible to my 6 foot solid dish
KU is where the numbers chnage
KU is where the numbers chnage
A solid reflector will always perform best, though surface accuracy is something that lower cost reflectors lack, and do not hold up well when compared to the quality ones.
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A surface with voids less than 1/10 the wavelength of the signal in question can be considered to be solid.
C band wavelength is approx 12.5 centimeters or 5 inches.
Ku band wavelength is approx. 2.5 centimeters or 1 inch.
All other things being equal, holes in the reflector of a C band dish .5 in or less in diameter can be ignored (1/10 inch for a Ku-band dish.)
Of course a reflector which is 50% holes is not going to be as physically rigid as a solid surface. Any portion of the dish not aligned to reflect signal to the LNB can be considered non-existent when computing gain (although such a bent panel will still serve to shield the LNB from the hot and noisy Earth.)
C band wavelength is approx 12.5 centimeters or 5 inches.
Ku band wavelength is approx. 2.5 centimeters or 1 inch.
All other things being equal, holes in the reflector of a C band dish .5 in or less in diameter can be ignored (1/10 inch for a Ku-band dish.)
Of course a reflector which is 50% holes is not going to be as physically rigid as a solid surface. Any portion of the dish not aligned to reflect signal to the LNB can be considered non-existent when computing gain (although such a bent panel will still serve to shield the LNB from the hot and noisy Earth.)
Half an inch openings acceptable for C band??? I hope not....the textbooks as well as everyone that ever spoke with eloquence about tolerances on C-band preferred numbers somewhere between 1/8 and 3/16 inch as the maximum hole size. And depending who you talk to....1/32 to 1/16 inch seems to be the consensus on Ku-band.
Large mesh antennas are really not a great idea for Ku-band, because even if you can somehow get acceptable gain when installing a brand new antenna, a season or two will usually cause noticable losses in efficiency on Ku-band.
It's the nature of the beast, because unsupported mesh tends to move around.
Solid or perf would be much better choices in material.
Just my two cents' worth.
Large mesh antennas are really not a great idea for Ku-band, because even if you can somehow get acceptable gain when installing a brand new antenna, a season or two will usually cause noticable losses in efficiency on Ku-band.
It's the nature of the beast, because unsupported mesh tends to move around.
Solid or perf would be much better choices in material.
Just my two cents' worth.
C / F = W
I took a moment to run the numbers and yes, I do need to revise my statement about C band reflectors. Using the high end of the C band downlink band 4.12 GHz come up with a wavelength of 7.14 cm, and 1/10 of 7.14 cm is closer to .3 inches, not .5
Double checking my statement regarding Ku, I come up with .098 inches.
A half inch for C band sounded plausible to me, since my very first C band setup employed plain old chicken wire as a reflector ... and it worked ... not particularly well, but it worked!
The supporting frame was made of common lumber. The reflector shape was not parabolic but rather a section of a sphere measuring about 10x25 feet. It remained stationary. Moving between satellites was accomplished by turning the "low noise amplifier", which was mounted garden variety TV antenna rotor.
I took a moment to run the numbers and yes, I do need to revise my statement about C band reflectors. Using the high end of the C band downlink band 4.12 GHz come up with a wavelength of 7.14 cm, and 1/10 of 7.14 cm is closer to .3 inches, not .5
Double checking my statement regarding Ku, I come up with .098 inches.
A half inch for C band sounded plausible to me, since my very first C band setup employed plain old chicken wire as a reflector ... and it worked ... not particularly well, but it worked!
The supporting frame was made of common lumber. The reflector shape was not parabolic but rather a section of a sphere measuring about 10x25 feet. It remained stationary. Moving between satellites was accomplished by turning the "low noise amplifier", which was mounted garden variety TV antenna rotor.
I used to build 18 and 20 foot spherical antennas myself--in Alaska during the early 1980s. Did you ever run across a company in Cleveland called Tri Star General (1981 or 1982) that made a spherical feed? My best luck was setting up Chaparral's spherical device with a 1.25 f/d ratio. Earlier feeds such as the rectangular ones put out by McCullough in Arkansas were specified for 1.5 f/d ratio, which meant you were 27 feet in front of the antenna with an 18 foot model.
Quite a trick to reach the feed location when your average elevation angle was between ten and 15 degrees above the horizon. My first screening job used "regular" aluminum mesh, and required 7000 staples to attach to an 18 x 18 foot redwood frame. Subsequent antennas used a special 1/8 inch expanded mesh, which had an interesting "spring" to it, and only averaged 3000 staples to attach. The good old days!
Quite a trick to reach the feed location when your average elevation angle was between ten and 15 degrees above the horizon. My first screening job used "regular" aluminum mesh, and required 7000 staples to attach to an 18 x 18 foot redwood frame. Subsequent antennas used a special 1/8 inch expanded mesh, which had an interesting "spring" to it, and only averaged 3000 staples to attach. The good old days!
About 10 years of my background was devoted to the design and manufacture of mesh and perforated dishes for high wind load applications (military and maritime)
Antenna design is nothing but one big compromise in that antenna gain is traded for survivability. Ones first consideration is the operating frequency, which establishes a nominal opening size for the dish surface. Second consideration is designed wind load.
Openings in the dish surface are sometimes much larger than you might imagine or calculate because the signal loss (signal passing thru openings) can be offset by making the dish larger in size.
Dishes that I'm experienced with for Ku (10 to 12 Ghz.) often had openings slightly larger than a No.2 pencil. That has always been my rule of thumb as a good compromise between gain and survivability. For FTA the same rule would apply. Openings larger than a No.2 pencil won't work very well but anything less than that will work fine.
Antenna design is nothing but one big compromise in that antenna gain is traded for survivability. Ones first consideration is the operating frequency, which establishes a nominal opening size for the dish surface. Second consideration is designed wind load.
Openings in the dish surface are sometimes much larger than you might imagine or calculate because the signal loss (signal passing thru openings) can be offset by making the dish larger in size.
Dishes that I'm experienced with for Ku (10 to 12 Ghz.) often had openings slightly larger than a No.2 pencil. That has always been my rule of thumb as a good compromise between gain and survivability. For FTA the same rule would apply. Openings larger than a No.2 pencil won't work very well but anything less than that will work fine.
Electromagnetically speaking, what is the theory regarding the thickness of a passive reflector at various frequencies? Is a flash plating thin layer of conductor as efficient a reflector as an inch thick slab of the same material? If there is any difference does the effect increase with frequency?
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