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{ "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/HCQZOC2XXHBIODR26RVMQGEI53XFAXAU/?format=api", "mailinglist": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/?format=api", "message_id": "CADDbS5BOpR1q9QB83Z8PXNNxbD4-dUK-p01QHSL28OvZtWu4_A@mail.gmail.com", "message_id_hash": "HCQZOC2XXHBIODR26RVMQGEI53XFAXAU", "thread": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/thread/6VKSY2YUOJBL374MDTP74B2LXFYTBD44/?format=api", "sender": { "address": "k0jm.mark (a) gmail.com", "mailman_id": "4fa2bb5eca934ca4b14c4ede20ff0c9a", "emails": "https://mailman.amsat.org/hyperkitty/api/sender/4fa2bb5eca934ca4b14c4ede20ff0c9a/emails/?format=api" }, "sender_name": "Mark Johns, K0JM", "subject": "[AMSAT-BB] Re: Helix for 1.2/2.4 GHz", "date": "2021-02-28T22:18:00Z", "parent": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/B5YEHMQKGNQHUZCNWI5L6BINFJERIR6G/?format=api", "children": [], "votes": { "likes": 0, "dislikes": 0, "status": "neutral" }, "content": " Kent's analysis seems about right to me. I was asked, off list, if one\ncouldn't just buy or build two yagis for 1.2 or 2.4 GHz, mount them 90\ndegrees from each other, and build a phasing harness to make them circular.\nTheoretically yes, you could. But at those wavelengths phasing harnesses\nbecome really difficult to make. Tolerances get down to millimeters. Same\ndeal with the helix, I imagine, though I've never actually built one myself.\n\nCommercial products, of course, are available. And for builders, more\nrecently available test equipment certainly helps. Who could afford a VNA a\nfew years ago?\n\nBut consider why we use circular polarization in the first place: To be\nable to copy during cross-polarization fades. We can chase the\npolarization, or we can use raw gain to overcome those fades in order to\ncopy even when the signal is weak. At VHF/UHF, it is difficult and costly\nto get lots of gain, so we chase the polarization. At microwave, it's\ndifficult to manage polarization, but easier to get gain.\n\nBasically, it's just easier at microwave frequencies to build a 40-element\nbeam (or a dish) than it is to build the phasing harnesses or get the helix\nright. Whereas a 40-element beam or a 10-turn helix at 2 meters would be\nrather difficult, mechanically and financially. So it's just all about\nwhat's practical. And if it works, who cares if it's circular?\n--\nMark D. Johns, KØJM\nAMSAT Ambassador & News Service Editor\nBrooklyn Park, MN USA EN35hd\n-----------------------------------------------\n\"Heaven goes by favor; if it went by merit,\n you would stay out and your dog would go in.\"\n ---Mark Twain\n\n\nOn Sun, Feb 28, 2021 at 3:03 PM KENT BRITAIN <[email protected]> wrote:\n\n> Hi Jerry\n>\n> I have taken an antenna range to over 60 conferences. And done quite a\n> bit or work with Helix antennas at my lab. Of the 100 or so Helix's I have\n> tested, only about 10% were circularly polarized.\n> That's right, only about 1 in 10. I have proven many many times, HAMS\n> CANNOT BUILD CP HELIX ANTENNAS!\n>\n> It is amazing how a Helix Antenna can have an Axial Ratio of over 20 dB.\n> That's about the same axial ratio as you get from many Yagi's. The\n> few helix's that worked were usually built by a professional in the field.\n>\n> Unless you have actually tested your Helix on a range, the chances are\n> about 9 out of 10 is it not CP\n> (Yes, it is a valid statement that all antennas are ellipticaly polarized,\n> CP and Linear are just special cases.).\n>\n> As an example, the wave comes down the helix, hits the tip, them reflects\n> as a standing wave on the helix structure with an opposite CP sense.\n> You can put a helix antenna on an antenna range, then start trimming the\n> tip with some wire cutters a few millimeters at a time. The Axial ratio\n> will be constantly changing. You can, with some experimenting, find the CP\n> point, but statistics are against you without any testing.\n>\n> And the Kraus Formulas are about 3 dB Optmistic.\n>\n> Winding the helix on a plastic pipe creates all sorts of problems. In\n> free space the radio wave travels along the wire at about 95% the speed of\n> light. When you wrap the wire on a plastic tube, the velocity factor is\n> now about 60%.\n> The effective length and turns ratio is completely different from the\n> calculations. And darn near impossible to even simulate What is the\n> dielectric constant of that plastic you got at the hardware store? And\n> If you did look up the Er, that value was measured at 1kHz, not 1269 MHz!\n> So unless you actually measured the Er at 1269 MHz, your CST, HFSS,\n> OPENEMS, etc simulation is not valid.\n>\n> Sam, G4DDK was the first to measure the effects of using just one\n> dielectric boom with the helix winds attached directly to the boom. i.e.\n> one boom and the helix turns attached to the boom at their 12 O'clock\n> position.\n> The dielectric effects of the boom tilts the beam in the direction of the\n> boom.and really reduces the gain of long helix antenna.\n>\n> The NASA way, central boom and spokes to support the helix is by far the\n> best way to go.\n>\n> 73 Kent WA5VJB\n>\n>\n>\n> On Sunday, February 28, 2021, 01:39:25 PM CST, <[email protected]> wrote:\n>\n>\n> Build one myself for 23cm years ago on 1268Mc. 2*19el fixed RHCP. The\n> tricky part was the DR environment because spacings are small on 23cm.\n> Made it with 1/4L mechanical spacing. It never lead me dwn, worked beter\n> then my 30t helix.\n> I hope my 4pol switch for this crossyagi will be finished in a month or 2,\n> but busy with building µwave EME setup.\n> Maybe i will try it first with my 70cm 4pol switch, but this will have\n> more loss.\n>\n> I've never seen commercial crossyagis for 23, but WIMO used to have helix\n> antennes for 23c,and 13cm, or try to make them yourself.\n>\n> best 73's\n>\n> Jerry,ON4CJQ\n>\n> ----- Oorspronkelijk bericht -----\n> Van: \"amsat-bb\" <[email protected]>\n> Aan: \"amsat-bb\" <[email protected]>\n> Verzonden: Zondag 28 februari 2021 11:41:35\n> Onderwerp: [AMSAT-BB] WTB: Circular Polarized Yagis for 1.2/2.4 GHz\n>\n> Trying to locate a good source for these. M2 doesn't have these. Ideas\n> pls?\n>\n> 73/Ed NI6S\n>\n> -----------------------------------------------------------\n>\n> Sent via AMSAT-BB(a)amsat.org. 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>\n> View archives of this mailing list at\n> https://mailman.amsat.org/hyperkitty/list/[email protected]\n> To unsubscribe send an email to amsat-bb-leave(a)amsat.org\n> Manage all of your AMSAT-NA mailing list preferences at\n> https://mailman.amsat.org\n>\n>\n> -----------------------------------------------------------\n>\n> Sent via AMSAT-BB(a)amsat.org. 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>\n> View archives of this mailing list at\n> https://mailman.amsat.org/hyperkitty/list/[email protected]\n> To unsubscribe send an email to amsat-bb-leave(a)amsat.org\n> Manage all of your AMSAT-NA mailing list preferences at\n> https://mailman.amsat.org\n>\n> -----------------------------------------------------------\n>\n> Sent via AMSAT-BB(a)amsat.org. 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>\n> View archives of this mailing list at\n> https://mailman.amsat.org/hyperkitty/list/[email protected]\n> To unsubscribe send an email to amsat-bb-leave(a)amsat.org\n> Manage all of your AMSAT-NA mailing list preferences at\n> https://mailman.amsat.org\n\n\n", "attachments": [ { "email": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/HCQZOC2XXHBIODR26RVMQGEI53XFAXAU/?format=api", "counter": 2, "name": "attachment.html", "content_type": "text/html", "encoding": "utf-8", "size": 10720, "download": "https://mailman.amsat.org/hyperkitty/list/[email protected]/message/HCQZOC2XXHBIODR26RVMQGEI53XFAXAU/attachment/2/attachment.html" } ] }