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GET /hyperkitty/api/list/[email protected]/email/YIZONSDTD5Y4O6I4VCOOU4XG6VKR3BD2/
{ "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/YIZONSDTD5Y4O6I4VCOOU4XG6VKR3BD2/", "mailinglist": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/", "message_id": "CAJdc4v7f1er5a8LCQrCPNrmv_kHqLOTR8mW5wJie+Qgf0+O2jQ@mail.gmail.com", "message_id_hash": "YIZONSDTD5Y4O6I4VCOOU4XG6VKR3BD2", "thread": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/thread/HR6ESX6W7O62TJNYCCN3WTCEHHZXOZXH/", "sender": { "address": "rico.van.genugten (a) gmail.com", "mailman_id": "64397f008c1d4d7a8656e4f690b21e7f", "emails": "https://mailman.amsat.org/hyperkitty/api/sender/64397f008c1d4d7a8656e4f690b21e7f/emails/" }, "sender_name": "Rico van Genugten", "subject": "Re: [amsat-bb] Moxon vs. Turnstile", "date": "2017-12-11T12:32:22Z", "parent": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/HR6ESX6W7O62TJNYCCN3WTCEHHZXOZXH/", "children": [], "votes": { "likes": 0, "dislikes": 0, "status": "neutral" }, "content": "Hi,\n\nIt depends on the type of turnstile and your application. I'm assuming that\nyou want to use the antenna in a fixed position pointing upwards, e.g. not\non a rotor. In this case you want the gain in upward direction to be as\n_low_ as possible, since this means that you will have more gain at lower\nelevations, where you need it most.\n\nA turnstile antenna consisting of crossed dipoles and crossed reflector\nelements has maximum gain in upward direction, but the gain and impedance\ndepend on the distance between radiating and reflecting elements. At about\n37 cm or 14,5 inch spacing the impedance is 50 ohm, but the gain is way too\nhigh to use as a non-directional antenna. Increasing the distance means\ndecreasing the gain, but also increasing the impedance, so you need some\nkind of match to get a good swr.\n\nA turnstile antenna consisting of crossed dipoles over a ground screen\ngives a better pattern than one that uses reflector elements, but can be\nmore impractical. The gound screen needs to be fairly large in terms of\nwavelengths (about 2 minimum), but if you can mount the antenna on the\nground this might not pose a problem. At a dipole hight of about 70 cm\n(27,5 inch) above the ground screen you can get an impedance of about 100\nohms per dipole and a good pattern (yes, I did quite some simulations in my\nquest for the ultimate stationary antenna. :)), which with some trickery\ncan give you a good 50 ohm match when the dipoles are combined, see the\neggbeater design which also shows an impedance of 100 ohm per loop.\n\nA moxon antenna has quite some gain in upward direction and therefore less\ngain at lower elevations, therefore I think it is unsuitable for stationary\nuse unless you are only interested in passes above 50 degrees, which rarely\nhappen and if so, for only a very short time. It is more suitable to be\nmounted at a fixed elevation as a small directional antenna if you have\nazimuth control but no elevation control. I have recently built a crossed\nmoxon mounted at 25 degrees on a simple azimuth-only rotator. Tests\nconducted from the yard are promising so far. I'm putting it on the roof\nwhen the weather improves, I will send an update to this list when I have\nsome results.\n\nThen there is the option of building/buying an eggbeater antenna, but in my\nopinion it is bulky, difficult to build and does not offer many advantages\nover a turnstile.\n\nI've experimented a lot with stationary antennas, but didn't have a lot of\nsuccess so far. The following properties of most stationary antennas are\nhard to overcome:\n - Most of the satellites currently active are linearly polarized and\nunpredictable in orientation, so a ground-station antenna should be\ncircularly polarized or have changeable/switchable polarization. Most\nstationary antennas are circularly polarized in upward direction, but\nhorizontally polarized at the horizon, resulting in deep signal fades when\nthe sat is at low elevations where the signal is lowest to begin with.\n - Intelligibility of a signal depends on the SNR or Signal to Noise Ratio.\nOmnidirectional antennas have lower gain and therefore lower signal levels,\nbut due to the omnidirectionality also have a higher noise floor,\ndecreasing the SNR even more than gain figures alone would indicate.\n\nThat doesn't mean stationary antennas are not useful:\n - On linear sats you don't need the SNR required for FM-sats, so you might\nhave more luck with those.\n - VHF downlinks are much easier to hear than UHF ones (for this and the\nabove reason you can forget about SO-50)\n - As an uplink antenna they could be fine, usually hearing the sat is much\nmore difficult than getting in.\n - FO-29 has circularly polarized antennas and therefore does not give as\nmuch fading with an antenna that is linearly polarized at low elevations\n - AO-85 and AO-91 have pretty strong downlinks, at higher elevations you\nwill hear them, at lower elevations you might too if you are lucky with\ntheir orientation.\n - Use good quality coax and connectors, preferably the least possible.\nDon't use RG-58 for anything over 1 meter (40 inch).\n\nPhew, maybe that is a bit more information than you ask for, good luck\nexperimenting. :)\n\nRegards,\nRico van Genugten\nPA3RVG\n\n\nOn Mon, Dec 11, 2017 at 3:37 AM, KD4ZGW <[email protected]> wrote:\n\n> What would be the better antenna: moxon or a turnstile?\n>\n> _______________________________________________\n> Sent via [email protected]. AMSAT-NA makes this open forum available\n> to all interested persons worldwide without requiring membership. Opinions\n> expressed\n> are solely those of the author, and do not reflect the official views of\n> AMSAT-NA.\n> Not an AMSAT-NA member? Join now to support the amateur satellite program!\n> Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb\n>\n", "attachments": [] }