I am now the proud owner of a very 'second-hand' 3m mesh-covered skeleton dish.
The mesh covering is metaphorical, rather than actual. I watched the dish on its back while as a group of small children rolled the dish back and forth and trampled holes in the mesh. That's the reason I was able to ask for it. "Please take it away," was the manager's reply to my query.
Now, I need to re-mesh the dish, but I need to decide what 'flatness' ('true-ness') the surface should be. There has to be a formula, that encompasses things like: frequency of operation, curvature or focus distance, etc.
I've done some looking online, but can't find it readily.
Suggestions, and discussion, please?
Please note: If you're going to also make product, brand name, or supplier suggestions, then be generic as suppliers and products in VK (Australia) will differ from where most list members will be sited.
With thanks, Peter VK1KEP
The mesh covering is metaphorical, rather than actual. I watched the dish on its back while as a group of small children rolled the dish back and forth and trampled holes in the mesh. That's the reason I was able to ask for it. "Please take it away," was the manager's reply to my query.
Now, I need to re-mesh the dish, but I need to decide what 'flatness' ('true-ness') the surface should be. There has to be a formula, that encompasses things like: frequency of operation, curvature or focus distance, etc.
Peter,
I'm sure there some on here than can answer your questions accurately.
But I can tell you my experience...I acquired an open framework dish, It's about 35". I cut gores the size of the framework, with a slight overlap, and wired them as tightly as I could to the frame....I cut them about 2" longer than the radius, and the extension conformed nicely to the curve of the dish...In an attempt to get a 39" dish...
I homebrew all my feeds, and it worked very successfully on AO40 at 2.4 gigs...You did not mention a frequency, and of course that is very important..
73, Dave, WB6LLO dguimon1@san.rr.com
Disagree: I learn....
Pulling for P3E...
Hi Peter:
I assume that the dish is a paraboloidal reflector. Can you determine the focal length and the f/ ratio? What was the original size of the mesh holes?
I checked with Skolnick's Radar Handbook, and there was no information with regards to your question, just equations on how to create the paraboloidal form. From my experience with optical paraboloids I can state that it is widely expected that such mirrors have a deviation over their diameter of no more than 1/4 of a wave (hmm, where have we seen that number in antenna design before?). Due to the Rayleigh limit, little gain is added if the accuracy is increased beyond 1/8 of a wave.
The ARRL Antenna Handbook 19th edition states on page 19-19 regarding "Parabolic Gain Versus Error": "Each time all the errors are halved, the frequency of maximum gain is doubled".
Hope this helps. Tom W1TFD Peter Ellis vk1kep@gmail.com wrote: I am now the proud owner of a very 'second-hand' 3m mesh-covered skeleton dish.
The mesh covering is metaphorical, rather than actual. I watched the dish on its back while as a group of small children rolled the dish back and forth and trampled holes in the mesh. That's the reason I was able to ask for it. "Please take it away," was the manager's reply to my query.
Now, I need to re-mesh the dish, but I need to decide what 'flatness' ('true-ness') the surface should be. There has to be a formula, that encompasses things like: frequency of operation, curvature or focus distance, etc.
I've done some looking online, but can't find it readily.
Suggestions, and discussion, please?
Please note: If you're going to also make product, brand name, or supplier suggestions, then be generic as suppliers and products in VK (Australia) will differ from where most list members will be sited.
With thanks, Peter VK1KEP _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
At 03:00 PM 7/25/2008, Peter Ellis wrote:
Now, I need to re-mesh the dish, but I need to decide what 'flatness' ('true-ness') the surface should be. There has to be a formula, that encompasses things like: frequency of operation, curvature or focus distance, etc.
Focal length, f/d, etc are irrelevant. The thing that is relevant is wavelength of your signal. You want the variations in the dish's surface to be small relative to a wavelength. 1/4 wavelength is ok. 1/8th wavelength is better. Better than that matters not much.
Its not to hard to get an intuitive feel for what is going on. Suppose you have a dish that is a perfect paraboloid, except for one little dent that's a little farther from the focus. Suppose the dent is 1/2 wavelength farther from the focus than the rest of the dish. The signal that reflects off of that dent will be 1/2 wavelength late, in other words 180 degrees out of phase with the rest of the signal, and will therefore SUBTRACT from the rest of the signal. That isn't good. That's a clue that 1/2 wavelength is too much variation. You can go thru the same sort of thinking with a 1/4 wavelength dent and see that the effect is much smaller, etc.
Surely a signal that reflects off a half wavelength dent will be one wavelength late, half a wavelength getting to the dish and half a wavelength coming back. So a dent (or bump) 1/4 wavelength will cause a 180 deg shift. I would suggest that your dish needs to be better than (my rule of thumb) 1/10 wavelenth accuracy.
Its not to hard to get an intuitive feel for what is going on. Suppose you have a dish that is a perfect paraboloid, except for one little dent that's a little farther from the focus. Suppose the dent is 1/2 wavelength farther from the focus than the rest of the dish. The signal that reflects off of that dent will be 1/2 wavelength late,
Peter, I suggest that the surface irregularities be less than 1/8 Wave length. Parabolic shape is important. I recommend some excellent practical advice and charts in the VHF/UHF Manual published by the RSGB. It is easy to understand and well written.
Art, KC6UQH
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Peter Ellis Sent: Friday, July 25, 2008 3:00 PM To: amsat-bb@amsat.org Subject: [amsat-bb] Dish surface 'flatness'
I am now the proud owner of a very 'second-hand' 3m mesh-covered skeleton dish.
The mesh covering is metaphorical, rather than actual. I watched the dish on its back while as a group of small children rolled the dish back and forth and trampled holes in the mesh. That's the reason I was able to ask for it. "Please take it away," was the manager's reply to my query.
Now, I need to re-mesh the dish, but I need to decide what 'flatness' ('true-ness') the surface should be. There has to be a formula, that encompasses things like: frequency of operation, curvature or focus distance, etc.
I've done some looking online, but can't find it readily.
Suggestions, and discussion, please?
Please note: If you're going to also make product, brand name, or supplier suggestions, then be generic as suppliers and products in VK (Australia) will differ from where most list members will be sited.
With thanks, Peter VK1KEP _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
__________ Information from ESET NOD32 Antivirus, version of virus signature database 3230 (20080701) __________
The message was checked by ESET NOD32 Antivirus.
__________ Information from ESET NOD32 Antivirus, version of virus signature database 3230 (20080701) __________
The message was checked by ESET NOD32 Antivirus.
__________ Information from ESET NOD32 Antivirus, version of virus signature database 3230 (20080701) __________
The message was checked by ESET NOD32 Antivirus.
I believe you should be looking at no bumps, holes etc of more than one tenth (1/10) of a wavelength. Obviously better than that will benefit performance.
Peter Ellis wrote:
I am now the proud owner of a very 'second-hand' 3m mesh-covered skeleton dish.
The mesh covering is metaphorical, rather than actual. I watched the dish on its back while as a group of small children rolled the dish back and forth and trampled holes in the mesh. That's the reason I was able to ask for it. "Please take it away," was the manager's reply to my query.
Now, I need to re-mesh the dish, but I need to decide what 'flatness' ('true-ness') the surface should be. There has to be a formula, that encompasses things like: frequency of operation, curvature or focus distance, etc.
I've done some looking online, but can't find it readily.
Suggestions, and discussion, please?
Please note: If you're going to also make product, brand name, or supplier suggestions, then be generic as suppliers and products in VK (Australia) will differ from where most list members will be sited.
With thanks, Peter VK1KEP _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
----- Original Message ----- From: "Peter Ellis" vk1kep@gmail.com To: amsat-bb@amsat.org Sent: Saturday, July 26, 2008 12:00 AM Subject: [amsat-bb] Dish surface 'flatness'
I am now the proud owner of a very 'second-hand' 3m mesh-covered skeleton dish.
The mesh covering is metaphorical, rather than actual. I watched the dish on its back while as a group of small children rolled the dish back and forth and trampled holes in the mesh. That's the reason I was able to ask for it. "Please take it away," was the manager's reply to my query.
Now, I need to re-mesh the dish, but I need to decide what 'flatness' ('true-ness') the surface should be. There has to be a formula, that encompasses things like: frequency of operation, curvature or focus distance, etc.
I've done some looking online, but can't find it readily.
Suggestions, and discussion, please?
Please note: If you're going to also make product, brand name, or supplier suggestions, then be generic as suppliers and products in VK (Australia) will differ from where most list members will be sited.
With thanks, Peter VK1KEP
Hi Peter, VK1KEP
To answere your question I have sent to you in two separate email the TECHNICAL REPORT # 5 of the Crawford Hill VHF Club - W2NFA with the following subject:
"A PARABOLOIDAL REFLECTOR ANTENNA FOR 1296 MHz"
This paper covers the following matter :
Basic Designe Considerations Feed Antenna The Parabolic Reflector - Error Tolerances Surfacing Material Paraboloidal Construction Bandwidth Radiation Pattern Maximum Gain Phase Center Noise Temperature Polarization
The above article is currently used by all EME'rs and was written with the contribution of many authors of the Crawford Hill VHF Club - W2NFA
The article is 10 pages long and it was necessary to divide it into two zipped files each about 2 MB long
If some other people is interested on it pleas let me know
Best 73" de
i8CVS Domenico
participants (7)
-
Art McBride -
Dave Guimont -
Franklin Antonio -
i8cvs -
Nigel Gunn G8IFF/W8IFF -
Peter Ellis -
THOMAS F DAVIS