"Teaching" an antenna

Discussion in 'General Science & Technology' started by Oxygen, Feb 21, 2007.

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  1. Oxygen One Hissy Kitty Registered Senior Member

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    There's this radio station down in Fresno I like, but the reception when it first went on the air a little over a year ago was lousy. It was like listening to old AM, with static underlining everything. When the weather was right, you couldn't get it at all. Still, I kept listening because the music selection was so awesome that I was willing to put up with the bad reception. As months went by and I listened every workday, the signal seemed to become clearer and clearer. Now it plays as clearly as any local station (except when I'm downtown surrounded by buildings). I checked the station and they haven't pumped up their signal.

    I'm in Modesto, California, and the weather is fairly stable. It's boring, even. I've been through all four seasons with this station in this same vehicle (I drive a delivery route for a dental lab), and when I tried recently to pull the station in on my personal vehicle's radio, it was back to square one.

    So, is it possible to "teach" an antenna to a signal over time? You know, have it "get used to it"? Or is there something in the atmosphere I'm forgetting?
     
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  3. spidergoat Valued Senior Member

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    Sunspots.
     
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  5. Roman Banned Banned

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    Neocons.
     
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  7. Baron Max Registered Senior Member

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    Global warming.

    And maybe the hot air thrown off by so fuckin' many liberals out there.

    Baron Max
     
  8. Fraggle Rocker Staff Member

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    West Oklahoma??? You poor thing!

    If there's any paranormal activity here, it's in your radio, not the antenna. The antenna brings in all signals indiscriminately, the receiver does the tuning.

    Well wait now... Your antenna is slightly more efficient at bringing in a signal whose wavelength is four times the length of the antenna. E.g, if you want an antenna perfectly tuned to a signal whose frequency is 100mHz, it should be 75cm long, since the speed of electromagnetic waves is 300 million m/sec. If you have accidentally extended or retracted your antenna a little bit, it's theoretically possible that you changed your antenna's tuning closer to the station of your choice. Calculate the quarter-wave of your station (divide 300 million meters by the frequency and then divide the result by 4) and adjust your antenna to that length.

    The difference in frequency from one end of the FM dial to the other is more than 20%, so I guess it's conceivable that this is what you're experiencing.
     
    Last edited: Feb 22, 2007
  9. Oxygen One Hissy Kitty Registered Senior Member

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    LOL! Nice answers, guys! Actually, Max, Modesto is pretty conservative as far as California towns go. Most of your liberals here are teenagers and college students who suddenly turn moderate to conservative when they get jobs. I know very few adult liberals in this town. I have no idea what that's all about. I'm sure the low liberal count among adults is because some of the kids leave town when they can, but the ones that stay, it's like the Stepford Constituency, I swear.

    Maybe it's the radio waves...

    Please Register or Log in to view the hidden image!

    :bugeye:
     
  10. Fraggle Rocker Staff Member

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    It's the demographics. The entire San JoaquĆ­n Valley was founded by Dust Bowl refugees. It really is West Oklahoma. Remember, Buck Owens was from Bakersfield.
     
  11. JohnMountain Registered Member

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    This is really cool!!
    So if your station is 103.7, then
    (300E6/103.7E6)/4 is 0.723 or 72.3 centimeters.

    If your station is 98.7, then
    (300/98.7)/4 is 0.76 or 76 centimeters.
    So make your antenna 76 centimeters long.

    If your station is 106.1, then
    300 divided by 106.1 and divided by 4 is 70.7 centimeters for antenna.

    Cool, man.
    So 7500 is the magic number for centimeters.
    7500 divided by the 90.5 frequency is 82.9 cm for antenna.
     
  12. Oxygen One Hissy Kitty Registered Senior Member

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    Interesting about the antenna length, Fraggle. I should have mentioned, however, that both of them are non-retractable. The radios in my house cannot pick up this station at all, no matter how hard I plead and beg, so I'm thinking maybe something is happening with the car itself (certainly not a static environment).
     
  13. leopold Valued Senior Member

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    no, a radio antenna doesn't learn.
     
  14. paulfr Registered Senior Member

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    Signal strength patterns do vary with terrain and man made objects like buidlings. Do you drive downtown a lot ? Near any mountain ranges often ?

    Whoever you talked to at the station may be a PR person just guessing. Even if you talked to the chief engineer, he may not be aware of actual signal strength as they are on tight budgets and only fix problems when "it is broken". So the most likely explanation is that they have an instability in the power amp driving the transmitter.

    Also, FM stations transmit with much less power [thus lower range] than AM stations, and what engineers call "threshold" is about 13dB to have clarity. This point drops off very quickly .... that is small changes seem to turn the reception on and off easily as it is very sensitive to signal to noise ratio.
    Driving away from Fresno [the station's transmitter antenna], the signal should go from clear to very noisy and then gone within a mile or two.
     
    Last edited: Mar 8, 2007
  15. Oxygen One Hissy Kitty Registered Senior Member

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    We're quite a ways north from Fresno, but not as far as we are south of Sacramento, and we can't get the Sacramento broadcast at all. I hadn't thought about instability on the transmitter. I did talk to an engineer. I used to work in radio as a board op, and making sure everything was transmitting properly was part of the job. However, it was a state of the art station with a juicy budget, so we had relay transmitters all over the place, back-ups for when earthquakes knocked the power out, etc. I guess I just got spoiled! I'm going to buy the squirrelly transmitter explanation and see how it behaves over the course of this year.

    As to where I drive, it's all over. Out in Oakdale it's pretty flat and open, so I normally hear it loud and clear, but yesterday I couldn't bring it in for the life of me. Today, it was clear all over, even downtown, where I normally don't get it very well. The only weather change was that we had some cloud cover in the morning. Today was bright and clear. Oddly, that's rarely made a difference. Days when I thought I wouldn't be able to get it, it would come streaming in. Maybe there was enough electricity in the clouds? When I lived in Arizona, I could hear the Las Vegas stations on thundercloud nights. Maybe some kind of link?
     
  16. Fraggle Rocker Staff Member

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    FM stations? The FM wavelength (100mHz --> 3m) is 100 times shorter than AM (1mHz --> 300m) so it's not generally affected by the atmosphere. If you can pick up an FM station 100 miles from the transmitter it's phenomenal. I lived in Tucson before it had TV and people put up tall antennas to bring in Phoenix TV, but it was basically animated snow with a soundtrack. (The FM band is right between two VHF channels, I think 5 and 6.)
     
  17. kevinalm Registered Senior Member

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    Between ch6 and 7. You can get ch6's sound on some fm radio's if you tune to the extreme low end of the fm band. The fm band as well as vhf and uhf tv are subject to reflection off cloud formations. It's called multipath reception and really screws up your tv picture. Doesn't do fm radio any good either. Also, attenuation by falling rain between transmitter and receiver increasingly becomes a problem with higher frequency, uhf can be virtually shut off by a light rain in the wrong place.
     
  18. Oxygen One Hissy Kitty Registered Senior Member

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    I'm pretty sure it was the FM band I was picking up. It was an FM station I had been trying to dial in (KDKB, I think it was out of Phoenix), when I started picking up this awesome oldies station that was playing some pretty rare stuff. I was waiting for the call letters (which never came), but all they'd say is they were "Las Vegas' best bet for oldies". I haven't been able to track down that line to tie it any station.
     
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