AM Antenna

Discussion in 'Wireless & RF Design' started by PRS, Apr 9, 2014.

  1. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    I've just rediscovered my interest in designing radios. I plan to start with an AM radio. I'm wondering about the antenna. In the past I used a long wire -- usually just a strand of magnetic wire strung over a distance. But I am now in a mobile home park and space is limited. I'm thinking of running a cable from my shop to the roof through a PVC pipe and fastening a wound ferrite rod at the top.

    Does this sound like a good plan? Does anyone have a better one? After the AM radio I will be making a short wave receiver using the same IF strip and audio amplifier and hopefully, antenna. So this antenna must be versatile and sensitive.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    A long horizontal wire antenna free from obstructions and high off the ground gives good results.

    I should also add, don't forget a good ground connection.
     
  3. #12

    Expert

    Nov 30, 2010
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    I wonder...TV frequencies escape if the twin lead wires run along a ground plane, like a mobile home, or just the planet. Is the AM band low enough that loss by parallel capacitance is negligible?

    You don't have to answer that. Just be aware that tying the wire around a tree trunk can diminish your signal.:)

    and, ps, a good ground is more important than you would expect.
     
  4. jmoffat

    Member

    Jul 18, 2012
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    You may want to read "Build your own transistor radios" by Ronald Quan. Lots of relevant information.
     
  5. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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  6. MrChips

    Moderator

    Oct 2, 2009
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    There are hundreds of designs for various loop antennas.
    My opinion is go with a simple straight wire antenna.
    And don't forget the ground.

    [​IMG]
     
  7. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Thanks. I realize the importance of the ground. The transmitter has an earth ground and so the signal is with respect to earth ground. Even though an arbitrary common can be used, as with a battery driven radio, I don't think the input to the antenna is as strong as when a true earth ground is present.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    I think you may be missing the importance of the ground. It is not as simple as having a common voltage reference. The ground is an integral part of the electromagnetic wave transmission system. You can say that the ground is a part of the antenna system that sends and receives the radio waves.
     
  9. bertus

    Administrator

    Apr 5, 2008
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  10. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    35
    Hello Bertus, long time no see. Your link is exactly the antenna I had in mind. I've seen it before and it is the one I want to build. I'm making a modularly constructed AM radio for broadcast AM and HF up to 30 MHz. So I want an antenna that can handle the whole range. I'm planning to make an LO that can mix it all into a 455 kHz IF strip having a maximum gain of 1500 v/v with an attenuator at its front end. The IF strip has a regular AM broadcast band type of detector (the simple diode followed by an RC filter) and with a BNC output before this detector so as to accommodate a single sideband detector as a separate module. Does this all make for a sound design in your opinion? I'm just an experimenter, and versatility is the name of the game.
     
  11. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Did I miss something? Are we not talking about the need for an honest earth ground via a rod driven into the earth verses a simple common so-called ground as in a portable radio? As I said before, I realize the distinction. Radio waves are sent out by transmitters that are referenced to true earth ground. Therefore if one is to receive them at full strength one must tie the ground of his receiver into true earth ground. Is this what you're talking about or am I missing something?
     
  12. k7elp60

    Senior Member

    Nov 4, 2008
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    Paul,
    I have an interest in building an AM broadcast receiver just like you. I have built various IF amplifiers as experiments and I haven't decided the one to use yet. I once built an active AM amplifier that used a short vertical whip antenna to a rf amplifier then a emitter follower for a 50 ohm output.
    It worked very well. I once lived in a mobile home that had metal siding
    on it so the rf penetration inside was very poor. I think going with a loop
    antenna is the best idea. The only problem is it has to be rotated to get then maximum signal as you know.
    Grounding is important. If you are using an isolated power supply for your
    circuits you can connect an earth ground to the ground of your circuits.
    Most of what I build is designed to work on +12V DC. I have some solar panels and some batteries that are charged by the solar panels and I have a ground rod connected to the - side of the batteries.
    I would like to stay in contact with you, as I may be able to help you with
    circuits and possibly parts as I have collect some over the years. If you would like to stay in touch please send me a private message.
    Ned
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The loop antenna is a tuned antenna.
    If you want to cover the range from 500 kHz to 30 Mhz, it will not work with one coil.

    A simple active antenna can be made using an unbuffered cmos chip.
    Here is the schemaic:

    [​IMG]

    Bertus
     
    Last edited: Apr 13, 2014
  14. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Ditto. I did pretty much the same thing back in the early 90's, except I just used a long enamel coated wire across the roof, then down through the window into the basement where my shop was. That was my first experience designing a radio. It made me heady after I watched the audio signal jumping around on my old navy HP oscilloscope ... and to listen to it play!

    That's exactly my present situation. I'll go with a long wire at first, but Bertus posted a link to an antenna I plan to make. I'll put the antenna on a pole above the roof, run down a piece of coax into my room and have the variable capacitor in a box on my desk.

    I, too, favor 12 volts DC. I first built an IF strip with 2 common emitter stages, wound my own toroids with primaries center-tapped and connected to Vcc there, and, of course tuned to 455 kHz via variable capacitors. It has a gain of 1500 with an attenuator to make it adjustable. I use a small Marshal practice amp for the audio amp. I just built the mixer with a dual gate MOSFET and I've painted the board (in preparation for etching solution) for the rf amp at the front end -- a common base amplifier with 50 ohms input resistance.

    I'd like to stay in touch with you, but I don't know how to instant message.
     
  15. k7elp60

    Senior Member

    Nov 4, 2008
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    Just click on my screen name on one of my posts, then select send a private message.I just sent you a private message.
     
    Last edited: Apr 13, 2014
  16. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    989
    35
    Back in the early 90's I made an antenna out of a straight enamel coated wire that picked up AM radio and CB radio. I did not employ a tuned circuit at the receiver's front end is why, I suspect.
     
  17. k7elp60

    Senior Member

    Nov 4, 2008
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    I have used dual gate mosfets as the first rf amplifier and I find they work very well and are pretty immune to rf overloads. I also have been using SA602 mixers that work very well. I want to use some varactor diodes like the MVAV108 to tune both the rf and the oscillator. I haven't quite got the frequency differential between the two tuned circuits done right with the different diode voltages yet.
    I one built the signal magnet antenna that is offered by Ramsey Electronics. It worked very well and was pretty immune to interference. Using that I could consistently receive AM radio signals in the daytime several hundred miles away.
    I remember reading an article once on the ferrite rod antenna for the broadcast band and it was interesting about the sensitivity of the antenna. Course is was bidirectional also.
    Ned
     
    Last edited: Apr 15, 2014
  18. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    989
    35
    It sounds like you're ahead of me in the game. I quit experimenting back in the early 2000's, but I'm picking it back up again and just getting my feet wet. Based on past experience I realize from the outset that I don't want to built a complete radio. I put the IF amplifier in a small aluminum hobby box with IF input, amplified IF output -- both BNC jacks, plus a detector output to an RCA jack. The input has a variable gain pot so that I can dampen signals that are too large.

    I have now built two circuits to go into another small aluminum hobby box. One is the rf input fed by a coaxial F jack, the other a mixer with a BNC-connected local oscillator, and a 455kHz output BNC jack. This, of course will connect to the input of my IF strip. And, combined I should be looking at broadcast AM pretty soon. My audio amp is a 30 watt Marshal practice amp. Perhaps someday I will build my own.

    I plan to use both of these hobby box units as modules for other modules to connect to -- such as a BFO at the end of the IF strip. I'll probably make a 10.7 MHz IF strip with an FM discriminator. That's the plan, vaguely. I plan to just putter till the day I die. I was 61 yesterday and got a new toy -- a Dremmel 4000! Oh, boy! :)
     
    Last edited: Apr 15, 2014
  19. MrChips

    Moderator

    Oct 2, 2009
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    Happy belated birthday, Paul!

    For your next birthday, ask for an Emco Unimat.
     
  20. Art

    Distinguished Member

    Sep 10, 2007
    785
    61
    I've been experimenting very recently with varying success.
    My first oldschool indoor AM broadcast antenna was a bit of a failure,
    but before I understood resonance, and I now suspect it's a simple matter as already stated in the thread.
    One coil can't cover the entire AM broadcast range because of the huge variance of wavelength across the dial 200 - 550 meters.
    You need to do a big coil tuned to the big end of the band with plenty of taps to get full coverage tuned to resonance.

    [​IMG]

    Since then, also been playing with HF dx, and can tune my shortwave wire in multiple
    harmonics up to 30MHz which is the limit of the receiver anyway.

    [​IMG]

    This one was supposed to look made in the 50's but don't ask me where the extruded aluminium angle came from! :D

    Have not yet tried this with a ferrite loop, but it's on the list of things to do :)
     
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