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weird signal pattern

Discussion in 'Home Brew' started by ab77, Oct 4, 2009.

  1. ab77

    ab77 Member

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    i have a homemade FM transmitter consisting of a belkin tunecast II with the attenuator bypassed, connected to two signal amplifiers, connected to two antennas, mounted about 30 feet up in the air.

    the signal seems to work fine for a pretty good distance if i go in one direction away from the transmitter, but if i go in the opposite direction, the signal still goes pretty far, but it goes in and out every couple feet. there's basically nothing in the way that would be blocking the waves.

    the two antennas are only one foot apart from each other. would this be causing the problem? and if so, how far apart would the antennas need to be for it to work properly?

    i'm also unsure if using two amps and two antennas is even the way to go with this project. i'm hoping that it will yield a better reception area than with just one antenna, but does anyone know if i could just connect the amplifiers together in some way instead?


     

  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Go with one antenna. The two signals are adding together in some cases and cancelling out in others. The reason it seems to fade out every couple feet is because you are changing the relative phase angles as they combine in your receiver. Anytime a source of light,sound, or RF is emitted from two differant sources interferance patterns are created by this adding and canceling.It makes for some pretty weird patterns. Have you ever listened to a dual speaker sound source and noticed that if you stand in just the right place and with your head turned the right way the sound disappears? Move slightly or turn your head the sound is there. Same thing only this involves sopund waves instead of radio waves.
     
  3. ab77

    ab77 Member

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    thanks. i switched it over to one antenna and one amplifier, and it's working great now. there is no longer any dropping in and out, and the signal seems to go the same length, if not further. for some reason i thought 2 would be better.

    but now that i have an extra amplifier and antenna laying around, does anyone know if it would be possible to use either of those components to boost up the signal strength even more? as far as i'm aware, each amplifier outputs 18db, and i have no idea about the antennas. one is a telescopic metal antenna taken from an old radio, and one is a length of wire strung through some glued-together coffee stirs...

    i've never noticed anything like this, but i can definitely imagine how such a thing would happen. i find it really interesting that waves can actually effect each other like that.
     
  4. Beetle

    Beetle Sr. Member

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    Unless you have an LPFM license from the FCC, you want to be very careful about the range of equipment operating within the AM and FM broadcasting bands. If you're ten miles from the nearest town, that's one thing. In a more urban setting, however, you can find yourself in some pretty serious trouble.

    A lot of businesses use unlicensed LPFM stuff to attract customers. Some of them aren't too careful about the frequencies they select and they interfere pretty severely with the licensed broadcast station on that frequency. Finding the bad guy isn't hard at all.
     
  5. ab77

    ab77 Member

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    thanks for the warning. i don't have a license, and i'm also not worried about the FCC. the FCC pisses me off immensely. it seems like any time i turn on my radio, it's the same few songs, over and over again, with way too many commercials. so many of the radio stations are owned by conglomerates, and it really drives variety down while driving the amount of commercials up. and i blame that at least for the most part on the FCC.

    anyways, i've checked the frequencies of nearby radio stations, and the frequency i'm using is .6 MHz away from the next closest stations, so it doesn't interfere at all. and i'm around 3 miles from the nearest urban area, which is (unfortunately) significantly further than my signal reaches, so i think i'm safe.
     
  6. C2

    C2 Sr. Member

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    I think those large commercial conglomerates are just as fond of the FCC as anyone. I would not blame the FCC for too many commercials and bad programming, unless your not getting enough F-bombs...

    You cannot gain anything with using the second amp. If you fed the output of one into the second, the second would probably be overdriven and it would not be able to put out any more than the first one anyway.

    BUT

    It depends on how hard the first one is being driven. If it is underdriven, you might be able to use it as a first stage amplifier. It might be OK inline or it might need its output padded down to drive the second one at optimum level.

    I'd need to know more about the amplifiers (input/output specs) and the actual output level of the source FM signal.
     
  7. Beetle

    Beetle Sr. Member

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    FCC has nothing to do with any station's playlist. If you're unhappy with what you hear, turn the station selector OR write directly to the station and let them know. Personally, I agree with your comment about way too many commercials. However, putting a commercial broadcasting station on the air and keeping it there isn't exactly cheap, whether AM or FM.
     
  8. ab77

    ab77 Member

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    i realize that the FCC has nothing to do with the playlists or content in any way, other than creating laws and punishing violators regarding what they consider to be "Obscenity, Indecency and Profanity". but one of the reasons that i've speculated that most radio stations, typically consisting of the conglomerate-owned ones, lack variety and have an overwhelming presence of commercials is the way the FCC licenses stations and doesn't license stations. like Beetle said, "...putting a commercial broadcasting station on the air and keeping it there isn't exactly cheap...". the FCC does not license stations under 100 watts, yet the only legal way to have a station under 100 watts is if the station is used for "noncommercial educational broadcasting only". this leads to conglomerate-owned radio stations, which support their mandated-to-be-high-powered stations with a myriad of commercials, fueled by playing the same few songs that will be sure to lure listeners in long enough to hear the commercials. if there was no FCC, or a less strict FCC, more and more people would make smaller radio stations, which would result in a wider array of variety, and less commercials. that's how i see it, at least.

    in response to C2's "unless your not getting enough F-bombs...", i'm not getting enough, in the sense that i am adamantly against all forms of censorship, especially censoring specific words, such as swear words. censorship is really not accomplishing anything. if someone can't handle hearing a swear word or seeing someone flip the bird, maybe they just shouldn't be listening to that particular song or watching that particular show. i compare it to the fact that people can buy movies, music, magazines, books, etc. with "Obscenity, Indecency and Profanity" in them, so why shouldn't those same things be available on broadcasts? just because 5 bureaucratic commissioners decide that something is too much for people to handle doesn't mean that it actually is, especially at the level they currently enforce.


    i have absolutely no idea about the FM signal source, other than that without the amplifier, it goes about 40 feet at the most. the amplifiers both just say that the output is "54-1000MHz +18dB" and were originally meant for stringing long lengths of television cable to prevent signal degradation.
     
  9. packrat

    packrat Active Member

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    in response to C2's "unless your not getting enough F-bombs...", i'm not getting enough, in the sense that i am adamantly against all forms of censorship, especially censoring specific words, such as swear words. censorship is really not accomplishing anything. if someone can't handle hearing a swear word or seeing someone flip the bird, maybe they just shouldn't be listening to that particular song or watching that particular show. i compare it to the fact that people can buy movies, music, magazines, books, etc. with "Obscenity, Indecency and Profanity" in them, so why shouldn't those same things be available on broadcasts? just because 5 bureaucratic commissioners decide that something is too much for people to handle doesn't mean that it actually is, especially at the level they currently enforce.


    That is a very shortsighted view you have there. What about all of the little kids around,(just as an example), who should have their innocence protected as long as possible? And how would you feel, if you got your way, about these innocent persons seeing and hearing all of the stuff that is now censored. How about Stern and Springer, and company on school buses? Hell, I have a problem with the condom and tampon and personal lubricant, etc... ads on the air now. You have thre right to buy whatever trash you like now, so why impose your tastes on others who have the right to not be assaulted by it. I would rather go back in time to when there were cigarette and booze commercials. Lots less intrusive!

    PR
     

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