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{
    "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/TKFQB5R66FTXEUQZ2H6P4TL2INHKW25G/?format=api",
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    "message_id": "693682E7994243418EFDBB1692138DAC019B124B202D@venus.rfdesign.co.za",
    "message_id_hash": "TKFQB5R66FTXEUQZ2H6P4TL2INHKW25G",
    "thread": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/thread/PXCPRAVCCAG6HIRXD2MPJ4SPOBSKZJAL/?format=api",
    "sender": {
        "address": "allan (a) rfdesign.co.za",
        "mailman_id": "df41e8d85c7b48b7be90951a75107b15",
        "emails": "https://mailman.amsat.org/hyperkitty/api/sender/df41e8d85c7b48b7be90951a75107b15/emails/?format=api"
    },
    "sender_name": "Allan Saul",
    "subject": "[amsat-bb]  A word of caution - Hygain DB-218SAT Sat Antenna's",
    "date": "2009-05-11T05:31:48Z",
    "parent": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/PXCPRAVCCAG6HIRXD2MPJ4SPOBSKZJAL/?format=api",
    "children": [
        "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/CJSB5U2VYZMZBFREXSLLJGS4GJQMCC4G/?format=api",
        "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/MNYDQ25IW6DAFNT42GY6LKIXIYGCFS5P/?format=api"
    ],
    "votes": {
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        "status": "neutral"
    },
    "content": "Hi Norman,\n\nYou mention the use of the Hy-Gain Sat Antennas\nin you setup. I was foolish enough to purchase this\nOscar Link combination of the VB-216SAT 2m 16 element\ncross Yagi and the UB-7030SAT 70cm Cross Yagi. \nThe antennas themselves are basically OK, but there\nare serious technical issues with the \"Polarization Switch\"\nthey supply for changing from LHCP to RHCP. \n\nFirst of all, as supplied by Hy-Gain, these switches \ncould never work by design. \n\nSome technical explanation - As the T-Match they use on the \nHorizontal and Vertical driven element presents an impedance\nof 200 ohms this needs to be transformed at the feed point\ndown to 50 ohms using a 4:1 balanced to unbalanced balun using\na 1/4 wavelength piece of co-axial. These seem cut to the\nright length using the thin RG-316. \nAs per the Satellite Experimenters Handbook and RSGB VHF/UHF handbook, the idea is then to transform this 50 ohms\nat the feedpoint up to 100 ohms at the switch using a 1/4 wavelength\nrun of 75 ohms co-ax (normally RG-6/U). The two 100 ohms from the\nV and H will them be in parallel and be 50 ohms again.\n\nSo what's wrong with the Hy-Gain switches you ask ?\n\n1: They use RF-303/u 50 Ohm cable from the feedpoint baluns to the\nPolarization switch, not 75 Ohm cable. This means that at best\nyou will get a 2:1 VSWR through the switch, even if the antenna\nis perfectly matched. This was confirmed by putting the switch\non a Agilent Network Analyzer which I have at work.\n2: The cables they use are of equal length. The problem with this\nis that when you open the switch up, the designers neglected to \ntake into account the extra phase delay through the cheap Tyco\nDc relay they use. This meant the phase match between the H and V\nwas off by at least 40-90 degrees from the ideal 180. This means\nthose guys using the switch as it exists will have anything but\ncircular polarization.\n\nTo fix the problem I had to practically rebuild the Switching Network by replacing the RG303 with RG-6. The 1/2 wavelength coax\ninside the switch used to add 180 degrees phase also has to be replaced with RG-6. The 1/4 wave legs then needed to be adjusted by measuring the phase delay on the Agilent Network Analyzer until you get a perfect 180 degree switch between RHCP and LHCP.\nThis a way beyond the capabilities of the average Radio Amateur. Emails sent to Hy-Gain (MFJ) about this issue went un-answered off course.\n\nOther issues :\n1: I found that the 70cm antenna had a VWSR dip at around\n440 MHz and nothing I did with the match could get it any lower\n2: On one of the feed point baluns the \"Red\" marking on the \nsolder lugs was different from the other 3. This means if assembled\nas instructed, you would never get circular polarization. There \nis no way I could have know this. It was only discovered when I picked out the Silicone RTV from the balun and saw the\nmanufacturing mistake on their part.\n3: I seriously question the Hy-Gain claim on the data Sheet\n\"True RF, 50 Ohm switching relays are rated for 200W PEP and contribute to improved VSWR\". They use cheap Tyco DC switching\nrelays in the design and they are definitely not RF Relays.\n\nMy intention is to replace the antennas with the M2 versions\nin the summer. From the data sheets and correspondence I have\nhas with them it looks like they have done the job properly.\n\n73's\nAllan Saul\nZS1LS\nSouth Africa.\n\n\n",
    "attachments": []
}