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{
    "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/6HSLT73IOS6QBBBO5F2TUETF5UTPWM4N/?format=api",
    "mailinglist": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/?format=api",
    "message_id": "Pine.SOL.3.95.1060827201452.19017A-100000@bookworm",
    "message_id_hash": "6HSLT73IOS6QBBBO5F2TUETF5UTPWM4N",
    "thread": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/thread/53ASRSXGBSWDFG2DPQG2R2VVXKRGRESG/?format=api",
    "sender": {
        "address": "tmcgrane (a) suffolk.lib.ny.us",
        "mailman_id": null,
        "emails": null
    },
    "sender_name": "McGrane",
    "subject": "[amsat-bb] Re: front-end overload  was: echo",
    "date": "2006-08-28T00:19:44Z",
    "parent": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/JSZNJ5DTOMHFHQD4TI5MMWYNNITGRBXX/?format=api",
    "children": [],
    "votes": {
        "likes": 0,
        "dislikes": 0,
        "status": "neutral"
    },
    "content": "\nHi andy- thanks for the education but I'm using narrow fm for the phone\ndownlink , not packet. I did try a diplexer with a .25 db loss between the\nantenna and the mast mounted preamp but noticed significantly lower\nrecieve at or near fringe.\nI'm in a rural area. Big power stations are not a problem. Thanks for\ninfo, pat\n\nOn Sat, 26 Aug 2006, Andrew T. Flowers, K0SM wrote:\n\n> McGrane wrote:\n> \n> >the radio set in the Narrow FM mode for greater sensitivity.\n> >\n> Two things come to mind, and I want to make sure anyone reading this \n> thread doesn't get confused:\n> \n> The first is to make sure that the NBFM bandwidth is wide enough to pass \n> 9600 baud transmission.  Doesn't one need about 20 KHz?  I'm sure \n> someone out there knows.  Don't forget that you have to account for \n> being off frequency due to doppler, particularly if you are using 5-kHz \n> steps.\n> \n> Secondly, the mode setting on the radio is something that affects a \n> filter in the IF chain, not the front-end of the radio.  I think the \n> issue with the FT-8800--and generally any other transceiver designed to \n> double as a wider VHF receiver--is that the the front end amplifier is \n> overloaded by a strong signal at *any frequency*.  The classic receiver \n> design has a (preferably low-noise) amplifier as the first thing in the \n> receive chain, which boosts the incoming signal enough to overcome the \n> following mixer losses.  That ampflifier is being bombarded by \n> everything from DC-GHZ, and will amplify whatever the transistor is \n> capable of.  (There is usually some sort of filter ahead of it in \n> commercial equipment, but in the case of wide-band VHF receiver it's \n> likely to be pretty wide).\n> \n> Now as we all know, amplifiers have a limit as to how much output they \n> can supply, and after an incoming signal gets too loud it will become \n> distorted.   This is familiar to many of us in urban areas when we \n> suddenly hear a pager on the frequency of our favorite repeater.  That \n> is often an effect of that very strong signal being clipped and \n> introducing new frequency components.  Another effect is that a strong \n> incoming signal to that front end--not matter what frequency it is \n> on--effectivly swamps the amplifier so that the weaker signals one \n> desires to listen to are not amplified faithfully.  This often manifests \n> itself as \"deafness\", but it is really because the front end is spending \n> what juice it can supply on that big signal 50 MHz away from where you \n> are trying to listen.  No changes to IF filters are going to make that \n> signal go away.  When this happens one usually has to use some sort of a \n> filter ahead of the amplifier, either to notch out the offender, or pass \n> only the band of interest.  This will allow the front-end amplifier to \n> run in its linear region, and thus amplify all the incoming signals \n> without (significant) distortion.\n> \n> The lab specifications for front-end overload can generally be figured \n> from the thrid-order intercept point (IP3), which essentially tell you \n> how much distortion one gets for an input signal of a given amplitude.  \n> The \"sensitivity figure\" doesn't tell you anything about how the radio \n> will perform in a high-RF environment.\n> \n> I don't mean to lecture, but rather clear up some possible confusion.\n> \n> -Andy K0SM/2\n> \n> _______________________________________________\n> Sent via [email protected]. Opinions expressed are those of the author.\n> Not an AMSAT-NA member? Join now to support the amateur satellite program!\n> Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb\n> \n\n",
    "attachments": []
}