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{
    "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/PVKHGYPHA7IEECCSCIPC7FX7GI4TJIMP/?format=api",
    "mailinglist": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/?format=api",
    "message_id": "000301c8ef9f$e3d75e00$0200a8c0@kc6uqh",
    "message_id_hash": "PVKHGYPHA7IEECCSCIPC7FX7GI4TJIMP",
    "thread": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/thread/5EAUU3WYN2OSQV22V6CAZTXPUBWU7SF4/?format=api",
    "sender": {
        "address": "kc6uqh (a) cox.net",
        "mailman_id": "d9201820d18047e4a2772c86bda5e111",
        "emails": "https://mailman.amsat.org/hyperkitty/api/sender/d9201820d18047e4a2772c86bda5e111/emails/?format=api"
    },
    "sender_name": "Art McBride",
    "subject": "[amsat-bb] Re: AMSAT-BB Digest, Vol 3, Issue 375",
    "date": "2008-07-27T04:18:57Z",
    "parent": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/5EAUU3WYN2OSQV22V6CAZTXPUBWU7SF4/?format=api",
    "children": [],
    "votes": {
        "likes": 0,
        "dislikes": 0,
        "status": "neutral"
    },
    "content": "Peter,\nA dent in dish parabola is just one factor.\n You also need to consider\n1. Overall parabola shape.\n2. Feed choice for best illumination.\n3. Feed Alignment. \nIn a well designed dish antenna the efficiency rarely is greater than 50% \n(-3 dB from ideal) Spending a little time learning how a dish works and all\nthe considerations will give you a far better antenna.\n\nArt,\nKC6UQH \n\n-----Original Message-----\nFrom: [email protected] [mailto:[email protected]] On\nBehalf Of James Whitfield\nSent: Saturday, July 26, 2008 3:05 PM\nTo: [email protected]\nSubject: [amsat-bb] Re: AMSAT-BB Digest, Vol 3, Issue 375\n\nPeter Ellis, vk1kep:\n\nI hope that you will get \"better\" answers from others of the group that have\npractical rather than theoretical experiences with large parabolic\nreflectors.  My only qualifications on this subject come from experience as\na Geometry teacher and limited amateur astronomy, but for what it is worth,\nhere are my thoughts.\n\nThe simple answer to the necessary \"flatness\" is better than a quarter\nwavelength error.  The general idea is that the radio waves you want to\ncapture come to the \"ideal\" reflector and all of them bounce back to a focal\npoint, and all of the waves reaching that point at the same time, that is\n\"in phase\".  The error of the real surface translates into a phase shift.\nIf a substantial surface area is a quarter wave behind the ideal and if an\nequal area is a quarter wave in front of the ideal, then the waves from the\ntwo areas will be half wave apart, causing full cancelation.  The quarter\nwave error is used to indicate that the maximum error difference from a high\npoint, relative to the ideal, to a low point is no more than a quarter wave.\nThat way the wave error causes phase shift rather than cancelation.\n\nA greater error probably will provide some \"gathering\" of the signal, but if\nyou want to have a parabolic reflector that all of its aperture is\neffective, then you need the surface to meet the quarter wave flatness.  As\nwith optics, the less the error, the better the \"image\" that can be\nresolved.  As a practical matter on radio frequencies, trying to exceed\nquarter wave is not worth the effort.\n\nYou did not state if the framework was in good enough shape to serve as a\ntemplate for the ideal parabola.  If it is not, I can give you a formula,\nbut I will need more information about the dimensions of the dish.\n\nBest Wishes.\n\nJames\nn5gui\n\n\n\n\n\n> Date: Sat, 26 Jul 2008 08:00:26 +1000\n> From: Peter Ellis <[email protected]>\n> Subject: [amsat-bb]  Dish surface 'flatness'\n> To: [email protected]\n> Message-ID: <[email protected]>\n> Content-Type: text/plain; charset=ISO-8859-1; format=flowed\n>\n> I am now the proud owner of a very 'second-hand' 3m mesh-covered\n> skeleton dish.\n>\n> The mesh covering is metaphorical, rather than actual. I watched the\n> dish on its back while as a group of small children rolled the dish back\n> and forth and trampled holes in the mesh. That's the reason I was able\n> to ask for it. \"Please take it away,\" was the manager's reply to my query.\n>\n> Now, I need to re-mesh the dish, but I need to decide what 'flatness'\n> ('true-ness') the surface should be. There has to be a formula, that\n> encompasses things like: frequency of operation, curvature or focus\n> distance, etc.\n>\n> I've done some looking online, but can't find it readily.\n>\n> Suggestions, and discussion, please?\n>\n> Please note: If you're going to also make product, brand name, or\n> supplier suggestions, then be generic as suppliers and products in VK\n> (Australia) will differ from where most list members will be sited.\n>\n> With thanks,\n> Peter\n> VK1KEP\n\n\n\n_______________________________________________\nSent via [email protected]. Opinions expressed are those of the author.\nNot an AMSAT-NA member? Join now to support the amateur satellite program!\nSubscription settings: http://amsat.org/mailman/listinfo/amsat-bb\n\n__________ Information from ESET NOD32 Antivirus, version of virus signature\ndatabase 3230 (20080701) __________\n\nThe message was checked by ESET NOD32 Antivirus.\n\nhttp://www.eset.com\n\n \n\n__________ Information from ESET NOD32 Antivirus, version of virus signature\ndatabase 3230 (20080701) __________\n\nThe message was checked by ESET NOD32 Antivirus.\n\nhttp://www.eset.com\n \n\n",
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}