rf preamp distance

Discussion in 'General Electronics Chat' started by yourownfree, Jun 4, 2014.

  1. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    just a thought will it work?
    I am going to use this as example as it will be easiest to understand.
    I want to increase reception of a broadcast radio .54 MHz - 1.710 MHz.
    I could wind a coil say 12 inch in diameter and attach a variable capacitor. I know it works without power. I could make bigger loops etc. not what I am looking for here.
    If I take the same 12 inch loop, same variable capacitor and attach it to a an rf amplifier to increase the signal. easy enough. But what if I put the loop outside and preamp outside the house and use coax to run inside and then attach another loop inside the house attached to the output of the rf amplifier? I know it will work. But what if you beef up the preamp even more say to 100 mw (just using this value for conversation) of output considering you will have to do a lot of amplification to get there I suppose, but the final thought is how much of a distance can I get between the radio and the loop by doing this. The idea is to cover the whole house as you walk around with the radio inside the house. This is in the country so no houses next door. Or is it just a useless idea? maybe best just to make a front end receiver and re-broadcast the signal. It would be a fixed signal 1.460 MHz am. The idea behind the idea is to increase the range of the output from coil to radio. So you dont have to be close by the output loop to make it function correctly.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    A coil and a capacitor won't do anything to raise the signal level available at your location. The only thing it will do is attenuate some signals away from the passband, thus reducing the bandwidth of the signal passed to the next stage. AM receiver chips in most radios are so good at what they do, it is unlikely that any stuff you cobble together will have any effect on your ability to receive weak stations at great distance. Fading and static crashes will obliterate most AM weak signals from low power stations. On a good night you'll be able to hear the 50,000 Watt clear channel stations when propagation conditions favor it.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    As Papabravo alluded to, the sensitivity of most standard AM receivers is mostly limited by atmospheric and other noise sources if you have a good antenna. The trick is getting a good antenna at such a low frequency since the wavelength is so long. There are special ferrite antennas specially designed for that purpose and you might try something like one of these if you are interested in long distance AM broadcast reception.

    When I was a kid my parents has an AM table radio connected to about a 75 foot of wire suspended between the house and a nearby tree as an AM antenna (with a screen-door spring in series to absorb any movement of the tree). Worked fairly well, although it was still well short of a quarter-wavelength (about 240' at 1MHz).
     
    Last edited: Jun 4, 2014
  4. Papabravo

    Expert

    Feb 24, 2006
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    I did my share of AM DXing when I was a kid in New Jersey. I remember picking up WLS Chicago, WWL New Orleans, and KTLA Los Angeles with an All-American 5 table radio. Love the soft purple glow of a vacuum tube.
     
  5. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    I think you are missing the point. Indeed a loop and a capacitor does work to resonate at the frequency in question, and a preamp does work. The radio is an older transistor radio not an ic unit, and the person likes to listen to a certain station. With just a loop coil of about 12 inches in diameter and a variable cap you can actually create a preselector by this method. Papabravo is correct how it works but this does work great. The radio needs to be in close proximity to the loop in order for it to work. So I have not tried this, but I have planned on beefing up the output just to see what kind of distance I can get ( notice) "between the loop coil and radio" not distance of radio stations etc. Sure there are noise factors when you amplify a signal especially if you amplify it broadband. Using the coil and cap method tend to alleviate a lot of the noise. If you were to use a broadband amp and no tuning sure there will be a lot of noise. Its impressive to hear the difference when you hear it work but real noisy. So stay away from broadband amps using no tuning for this.
    Papabravo I have been building projects not from kits either, from scratch for over 45 years. I am a good cobbler, so yes I can cobble up something that will work. In this case we are talking about an old transistor radio the old man does not want to part with. Its like a radio used to hear the ball games in the old days. about a 8-10 inch in size. So I asked the question here I wonder what if, since I have not tried it, to increase the distance at which you have to have the radio in proximity to the output coil. I will use two coils. The output of course will have to be either tuned off slightly or just a coil or else I could get oscillation. Now back in the tube days a 6AK5 was an excellent rf amplifier for this. But for today's electronics I will be using a mars chip with a low noise figure. Don't want to make any more noise than the old radio already makes. I will then look for an output device that has a low figure as well. I really dont have to have any distance as in this question, it was just a thought what if? and of course since I dont have anybody to talk to about it except you folks I tossed it out there for discussion.
    Thanks for the replies.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    You can do anything you want to try to improve your situation. Some things may work and others may not. In trying to listen to weak signals from distant stations raising the RF gain on the front end may not do much for you if you overload the following stage. I often reduce the RF gain on the receiver front end or even put in some attenuation and raise the audio gain for better copy and lower noise.

    I understand that you've been doing this for a long time, but so have I. We have this in common.
     
  7. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    I will change my agree or disagree with myself. Actually I do believe there is voltage increase using a loop. Sure it acts like a filter but in tuned circuit you cancel out the reactance and end up with supposedly a pure resistance. In which case there is a voltage across this value where as you would see much less on a non tuned circuit. At the frequency in question. You know I love to see how many stations I can pull in. Even for myself it is still a challenge. DX is fun.even with all the tricks.rf up or down for overloading etc. But I love to push the limits.
     
  8. bertus

    Administrator

    Apr 5, 2008
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  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    there is a loop on the market that is fairly large in diameter with a tuning cap that yo set near a transistor radio to increase its range, but has no preamp. coupling the output of an amplified loop inductively to the radio might lead to self oscilation, due to coupling from loop to loop. larger loops do recieve better, just as larger antennas recieve better, more micro volts per meter when there are more meters.
     
  10. Doktor Jones

    Active Member

    Oct 5, 2011
    57
    1
    I'm not too knowledgeable about RF... but what if you set up a receiver near a rooftop antenna at 1.46MHz, then retransmitted the signal on a different unoccupied frequency at a lower power? Within/near the house, I imagine it should come across clear and strong over any background noise, and avoid some other potential issues?
     
  11. davebee

    Well-Known Member

    Oct 22, 2008
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